The abstract keyword enables you to create classes and class members that are incomplete and must be implemented in a derived class.
The sealed keyword enables you to prevent the inheritance of a class or certain class members that were previously marked virtual.
Abstract Classes and Class Members
Classes can be declared as abstract by putting the keyword abstract before the class definition. For example:
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideInheritance/CS/Inheritance.cs13
using System;
using System.Drawing;
using System.Windows.Forms;
namespace Inheritance_ProgGuide
{
class TestPaint : Form
{
//<Snippet35>
protected override void OnPaint(PaintEventArgs e)
{
// Perform custom drawing
System.Drawing.Graphics gfx = e.Graphics;
gfx.DrawString("Hello, World!",
new System.Drawing.Font("Arial", 16),
new System.Drawing.SolidBrush(System.Drawing.Color.Blue),
new System.Drawing.Point(10, 10));
}
private void InitializeComponent()
{
SuspendLayout();
//
// TestPaint
//
ClientSize = new Size(284, 264);
Name = "TestPaint";
Load += new EventHandler(TestPaint_Load);
ResumeLayout(false);
}
private void TestPaint_Load(object sender, EventArgs e)
{
}
//</Snippet35>
}
//<Snippet1>
// compile with: csc -target:library abstractshape.cs
public abstract class Shape
{
public Shape(string s)
{
// calling the set accessor of the Id property.
Id = s;
}
public string Id { get; set; }
// Area is a read-only property - only a get accessor is needed:
public abstract double Area
{
get;
}
public override string ToString()
{
return $"{Id} Area = {Area:F2}";
}
}
//</Snippet1>
//<Snippet2>
// compile with: csc -target:library -reference:abstractshape.dll shapes.cs
public class Square : Shape
{
private int _side;
public Square(int side, string id)
: base(id)
{
_side = side;
}
public override double Area
{
get
{
// Given the side, return the area of a square:
return _side * _side;
}
}
}
public class Circle : Shape
{
private int _radius;
public Circle(int radius, string id)
: base(id)
{
_radius = radius;
}
public override double Area
{
get
{
// Given the radius, return the area of a circle:
return _radius * _radius * Math.PI;
}
}
}
public class Rectangle : Shape
{
private int _width;
private int _height;
public Rectangle(int width, int height, string id)
: base(id)
{
_width = width;
_height = height;
}
public override double Area
{
get
{
// Given the width and height, return the area of a rectangle:
return _width * _height;
}
}
}
//</Snippet2>
//<Snippet3>
// compile with: csc -reference:abstractshape.dll;shapes.dll shapetest.cs
class TestClass
{
static void Main()
{
Shape[] shapes =
[
new Square(5, "Square #1"),
new Circle(3, "Circle #1"),
new Rectangle( 4, 5, "Rectangle #1")
];
Console.WriteLine("Shapes Collection");
foreach (Shape s in shapes)
{
Console.WriteLine(s);
}
}
}
/* Output:
Shapes Collection
Square #1 Area = 25.00
Circle #1 Area = 28.27
Rectangle #1 Area = 20.00
*/
//</Snippet3>
class CarStuff
{
//<Snippet4>
// Define the base class
class Car
{
public virtual void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
}
}
// Define the derived classes
class ConvertibleCar : Car
{
public new virtual void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("A roof that opens up.");
}
}
class Minivan : Car
{
public override void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("Carries seven people.");
}
}
//</Snippet4>
//<Snippet5>
public static void TestCars1()
{
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet5>
//<Snippet6>
public static void TestCars2()
{
Car[] cars = [new Car(), new ConvertibleCar(), new Minivan()];
}
//</Snippet6>
public static void TestCars3()
{
Car[] cars = new Car[3];
//<Snippet7>
foreach (Car vehicle in cars)
{
Console.WriteLine("Car object: " + vehicle.GetType());
vehicle.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet7>
}
}
//<Snippet8>
interface IDimensions
{
float GetLength();
float GetWidth();
}
class Box : IDimensions
{
float _lengthInches;
float _widthInches;
Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
// Explicit interface member implementation:
float IDimensions.GetLength()
{
return _lengthInches;
}
// Explicit interface member implementation:
float IDimensions.GetWidth()
{
return _widthInches;
}
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an interface instance dimensions:
IDimensions dimensions = box1;
// The following commented lines would produce compilation
// errors because they try to access an explicitly implemented
// interface member from a class instance:
//<Snippet45>
//Console.WriteLine($"Length: {box1.GetLength()}");
//Console.WriteLine($"Width: {box1.GetWidth()}");
//</Snippet45>
// Print out the dimensions of the box by calling the methods
// from an instance of the interface:
//<Snippet46>
Console.WriteLine($"Length: {dimensions.GetLength()}");
Console.WriteLine($"Width: {dimensions.GetWidth()}");
//</Snippet46>
}
}
/* Output:
Length: 30
Width: 20
*/
//</Snippet8>
namespace WrapBox
{
//<Snippet9>
// Declare the English units interface:
interface IEnglishDimensions
{
float Length();
float Width();
}
// Declare the metric units interface:
interface IMetricDimensions
{
float Length();
float Width();
}
// Declare the Box class that implements the two interfaces:
// IEnglishDimensions and IMetricDimensions:
class Box : IEnglishDimensions, IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float lengthInches, float widthInches)
{
_lengthInches = lengthInches;
_widthInches = widthInches;
}
// Explicitly implement the members of IEnglishDimensions:
float IEnglishDimensions.Length() => _lengthInches;
float IEnglishDimensions.Width() => _widthInches;
// Explicitly implement the members of IMetricDimensions:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an instance of the English units interface:
IEnglishDimensions eDimensions = box1;
// Declare an instance of the metric units interface:
IMetricDimensions mDimensions = box1;
// Print dimensions in English units:
Console.WriteLine($"Length(in): {eDimensions.Length()}");
Console.WriteLine($"Width (in): {eDimensions.Width()}");
// Print dimensions in metric units:
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
}
/* Output:
Length(in): 30
Width (in): 20
Length(cm): 76.2
Width (cm): 50.8
*/
//</Snippet9>
}
class Test
{
interface IMetricDimensions
{
float Length();
float Width();
}
class Box : IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
//<Snippet10>
// Normal implementation:
public float Length() => _lengthInches;
public float Width() => _widthInches;
// Explicit implementation:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
//</Snippet10>
//<Snippet11>
public static void Test()
{
Box box1 = new(30.0f, 20.0f);
IMetricDimensions mDimensions = box1;
Console.WriteLine($"Length(in): {box1.Length()}");
Console.WriteLine($"Width (in): {box1.Width()}");
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
//</Snippet11>
}
}
}
namespace WrapExample
{
// Inheritance (C# Programming Guide)
//<Snippet12>
public class A { }
public class B : A { }
//</Snippet12>
class Program
{
static void Main()
{
A a = new();
B b = new();
b = (B)a;
}
}
}
namespace WrapA
{
//<Snippet13>
public abstract class A
{
// Class members here.
}
//</Snippet13>
}
//<Snippet14>
public abstract class A
{
public abstract void DoWork(int i);
}
//</Snippet14>
namespace WrapD
{
//<Snippet15>
// compile with: -target:library
public class D
{
public virtual void DoWork(int i)
{
// Original implementation.
}
}
public abstract class E : D
{
public abstract override void DoWork(int i);
}
public class F : E
{
public override void DoWork(int i)
{
// New implementation.
}
}
//</Snippet15>
}
namespace WrapD2
{
//<Snippet16>
public sealed class D
{
// Class members here.
}
//</Snippet16>
}
public abstract class C
{
public virtual void DoWork() { }
}
//<Snippet17>
public class D : C
{
public sealed override void DoWork() { }
}
//</Snippet17>
namespace WrapClasses
{
//<Snippet22>
public class A
{
public virtual void DoWork() { }
}
public class B : A
{
public override void DoWork() { }
}
//</Snippet22>
//<Snippet23>
public class C : B
{
public override void DoWork() { }
}
//</Snippet23>
}
class TestGraphics
{
//<Snippet27>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
}
//</Snippet27>
//<Snippet28>
class YourDerivedGraphicsClass : GraphicsClass
{
public void DrawRectangle() { }
}
//</Snippet28>
}
class TestGraphics2
{
//<Snippet29>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//</Snippet29>
//<Snippet30>
class YourDerivedGraphicsClass : GraphicsClass
{
public override void DrawRectangle() { }
}
//</Snippet30>
}
class TestGraphics3
{
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//<Snippet31>
class YourDerivedGraphicsClass : GraphicsClass
{
public new void DrawRectangle() { }
}
//</Snippet31>
class X : YourDerivedGraphicsClass
{
public new void DrawRectangle()
{
//<Snippet44>
base.DrawRectangle();
//</Snippet44>
}
}
}
class TestSelection
{
public class Base
{
public virtual void DoWork(int param) { }
}
//<Snippet32>
public class Derived : Base
{
public override void DoWork(int param) { }
public void DoWork(double param) { }
}
//</Snippet32>
void Test()
{
//<Snippet33>
int val = 5;
Derived d = new();
d.DoWork(val); // Calls DoWork(double).
//</Snippet33>
//<Snippet34>
((Base)d).DoWork(val); // Calls DoWork(int) on Derived.
//</Snippet34>
}
}
class OverrideToString
{
//<Snippet36>
class Person
{
public string Name { get; set; }
public int Age { get; set; }
public override string ToString()
{
return "Person: " + Name + " " + Age;
}
}
//</Snippet36>
void test()
{
//<Snippet37>
int x = 42;
string strx = x.ToString();
Console.WriteLine(strx);
// Output:
// 42
//</Snippet37>
//<Snippet38>
Person person = new() { Name = "John", Age = 12 };
Console.WriteLine(person);
// Output:
// Person: John 12
//</Snippet38>
}
}
namespace ExplicitConversion
{
//<Snippet51>
class Test
{
public static void M()
{ Console.WriteLine("I'm just a Test"); }
}
class Program2
{
static void Main()
{
System.Collections.ArrayList list = [new Test(), new Test(), new Test()];
foreach (object o in list)
{
if (o is Test t)
{
Test.M();
}
}
}
}
/* Output:
I'm just a Test
I'm just a Test
I'm just a Test
*/
//</Snippet51>
}
namespace RainyDay
{
class JustInCase
{
static void Main()
{
//<Snippet52>
Console.WriteLine("Saving for a rainy day.");
//</Snippet52>
//<Snippet53>
Console.WriteLine("Saving for a rainy day.");
//</Snippet53>
//<Snippet54>
Console.WriteLine("Saving for a rainy day.");
//</Snippet54>
//<Snippet55>
Console.WriteLine("Saving for a rainy day.");
//</Snippet55>
//<Snippet56>
Console.WriteLine("Saving for a rainy day.");
//</Snippet56>
}
}
}
//<snippet70>
namespace OverrideAndNew
{
using System;
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
// The following two calls do what you would expect. They call
// the methods that are defined in BaseClass.
bc.Method1();
bc.Method2();
// Output:
// Base - Method1
// Base - Method2
// The following two calls do what you would expect. They call
// the methods that are defined in DerivedClass.
dc.Method1();
dc.Method2();
// Output:
// Derived - Method1
// Derived - Method2
// The following two calls produce different results for the
// method defined by using override (Method1) and the
// method defined by using new (Method2).
bcdc.Method1();
bcdc.Method2();
// Output:
// Derived - Method1
// Base - Method2
}
}
class BaseClass
{
//<snippet68>
public virtual void Method1()
{
Console.WriteLine("Base - Method1");
}
//</snippet68>
public virtual void Method2()
{
Console.WriteLine("Base - Method2");
}
}
class DerivedClass : BaseClass
{
//<snippet67>
public override void Method1()
{
Console.WriteLine("Derived - Method1");
}
//</snippet67>
//<snippet66>
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
//</snippet66>
}
}
//</snippet70>
//<snippet69>
// Output:
// Base - Method1
// Base - Method2
// Derived - Method1
// Derived - Method2
// Derived - Method1
// Base - Method2
//</snippet69>
namespace pieces
{
using System;
//<snippet61>
class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
}
class DerivedClass : BaseClass
{
public void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
//</snippet61>
//<snippet62>
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
bc.Method1();
dc.Method1();
dc.Method2();
bcdc.Method1();
}
// Output:
// Base - Method1
// Base - Method1
// Derived - Method2
// Base - Method1
}
//</snippet62>
class morePieces
{
public void callers()
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
//<snippet64>
bc.Method1();
bc.Method2();
dc.Method1();
dc.Method2();
bcdc.Method1();
bcdc.Method2();
//</snippet64>
//<snippet65>
// Output:
// Base - Method1
// Base - Method2
// Base - Method1
// Derived - Method2
// Base - Method1
// Base - Method2
//</snippet65>
}
public class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
//<snippet63>
public void Method2()
{
Console.WriteLine("Base - Method2");
}
//</snippet63>
}
class DerivedClass : BaseClass
{
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
}
}
//<snippet78>
namespace OverrideAndNew2
{
using System;
using System.Collections.Generic;
class Program
{
static void Main(string[] args)
{
// Declare objects of the derived classes and test which version
// of ShowDetails is run, base or derived.
TestCars1();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars2();
// Declare objects of the derived classes and call ShowDetails
// directly.
TestCars3();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars4();
}
//<snippet72>
public static void TestCars1()
{
Console.WriteLine("\nTestCars1");
Console.WriteLine("----------");
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
// Note the output from this test case. The ShowDetails method is
// declared with the new modifier.
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</snippet72>
//<snippet73>
// Output:
// TestCars1
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet73>
//<snippet74>
public static void TestCars2()
{
Console.WriteLine("\nTestCars2");
Console.WriteLine("----------");
var cars = new List<Car> { new(), new ConvertibleCar(),
new Minivan() };
foreach (Car car in cars)
{
car.DescribeCar();
Console.WriteLine("----------");
}
}
//</snippet74>
//<snippet75>
// Output:
// TestCars2
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet75>
//<snippet76>
public static void TestCars3()
{
Console.WriteLine("\nTestCars3");
Console.WriteLine("----------");
ConvertibleCar car2 = new();
Minivan car3 = new();
car2.ShowDetails();
car3.ShowDetails();
}
public static void TestCars4()
{
Console.WriteLine("\nTestCars4");
Console.WriteLine("----------");
Car car2 = new ConvertibleCar();
Car car3 = new Minivan();
car2.ShowDetails();
car3.ShowDetails();
}
//</snippet76>
//<snippet77>
// Output:
// TestCars3
// ----------
// A roof that opens up.
// Carries seven people.
// TestCars4
// ----------
// Standard transportation.
// Carries seven people.
//</snippet77>
}
//<snippet71>
// Define the base class, Car. The class defines two methods, DescribeCar
// and ShowDetails. DescribeCar calls ShowDetails, and each derived class
// also defines a ShowDetails method. The example tests which version of
// ShowDetails is selected, the base class method or the derived class method.
class Car
{
public void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
ShowDetails();
}
public virtual void ShowDetails()
{
Console.WriteLine("Standard transportation.");
}
}
// Define the derived classes.
// Class ConvertibleCar uses the new modifier to acknowledge that ShowDetails
// hides the base class method.
class ConvertibleCar : Car
{
public new void ShowDetails()
{
Console.WriteLine("A roof that opens up.");
}
}
// Class Minivan uses the override modifier to specify that ShowDetails
// extends the base class method.
class Minivan : Car
{
public override void ShowDetails()
{
Console.WriteLine("Carries seven people.");
}
}
//</snippet71>
}
//</snippet78>An abstract class cannot be instantiated. The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share. For example, a class library may define an abstract class that is used as a parameter to many of its functions, and require programmers using that library to provide their own implementation of the class by creating a derived class.
Abstract classes may also define abstract methods. This is accomplished by adding the keyword abstract before the return type of the method. For example:
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideInheritance/CS/Inheritance.cs14
using System;
using System.Drawing;
using System.Windows.Forms;
namespace Inheritance_ProgGuide
{
class TestPaint : Form
{
//<Snippet35>
protected override void OnPaint(PaintEventArgs e)
{
// Perform custom drawing
System.Drawing.Graphics gfx = e.Graphics;
gfx.DrawString("Hello, World!",
new System.Drawing.Font("Arial", 16),
new System.Drawing.SolidBrush(System.Drawing.Color.Blue),
new System.Drawing.Point(10, 10));
}
private void InitializeComponent()
{
SuspendLayout();
//
// TestPaint
//
ClientSize = new Size(284, 264);
Name = "TestPaint";
Load += new EventHandler(TestPaint_Load);
ResumeLayout(false);
}
private void TestPaint_Load(object sender, EventArgs e)
{
}
//</Snippet35>
}
//<Snippet1>
// compile with: csc -target:library abstractshape.cs
public abstract class Shape
{
public Shape(string s)
{
// calling the set accessor of the Id property.
Id = s;
}
public string Id { get; set; }
// Area is a read-only property - only a get accessor is needed:
public abstract double Area
{
get;
}
public override string ToString()
{
return $"{Id} Area = {Area:F2}";
}
}
//</Snippet1>
//<Snippet2>
// compile with: csc -target:library -reference:abstractshape.dll shapes.cs
public class Square : Shape
{
private int _side;
public Square(int side, string id)
: base(id)
{
_side = side;
}
public override double Area
{
get
{
// Given the side, return the area of a square:
return _side * _side;
}
}
}
public class Circle : Shape
{
private int _radius;
public Circle(int radius, string id)
: base(id)
{
_radius = radius;
}
public override double Area
{
get
{
// Given the radius, return the area of a circle:
return _radius * _radius * Math.PI;
}
}
}
public class Rectangle : Shape
{
private int _width;
private int _height;
public Rectangle(int width, int height, string id)
: base(id)
{
_width = width;
_height = height;
}
public override double Area
{
get
{
// Given the width and height, return the area of a rectangle:
return _width * _height;
}
}
}
//</Snippet2>
//<Snippet3>
// compile with: csc -reference:abstractshape.dll;shapes.dll shapetest.cs
class TestClass
{
static void Main()
{
Shape[] shapes =
[
new Square(5, "Square #1"),
new Circle(3, "Circle #1"),
new Rectangle( 4, 5, "Rectangle #1")
];
Console.WriteLine("Shapes Collection");
foreach (Shape s in shapes)
{
Console.WriteLine(s);
}
}
}
/* Output:
Shapes Collection
Square #1 Area = 25.00
Circle #1 Area = 28.27
Rectangle #1 Area = 20.00
*/
//</Snippet3>
class CarStuff
{
//<Snippet4>
// Define the base class
class Car
{
public virtual void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
}
}
// Define the derived classes
class ConvertibleCar : Car
{
public new virtual void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("A roof that opens up.");
}
}
class Minivan : Car
{
public override void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("Carries seven people.");
}
}
//</Snippet4>
//<Snippet5>
public static void TestCars1()
{
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet5>
//<Snippet6>
public static void TestCars2()
{
Car[] cars = [new Car(), new ConvertibleCar(), new Minivan()];
}
//</Snippet6>
public static void TestCars3()
{
Car[] cars = new Car[3];
//<Snippet7>
foreach (Car vehicle in cars)
{
Console.WriteLine("Car object: " + vehicle.GetType());
vehicle.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet7>
}
}
//<Snippet8>
interface IDimensions
{
float GetLength();
float GetWidth();
}
class Box : IDimensions
{
float _lengthInches;
float _widthInches;
Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
// Explicit interface member implementation:
float IDimensions.GetLength()
{
return _lengthInches;
}
// Explicit interface member implementation:
float IDimensions.GetWidth()
{
return _widthInches;
}
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an interface instance dimensions:
IDimensions dimensions = box1;
// The following commented lines would produce compilation
// errors because they try to access an explicitly implemented
// interface member from a class instance:
//<Snippet45>
//Console.WriteLine($"Length: {box1.GetLength()}");
//Console.WriteLine($"Width: {box1.GetWidth()}");
//</Snippet45>
// Print out the dimensions of the box by calling the methods
// from an instance of the interface:
//<Snippet46>
Console.WriteLine($"Length: {dimensions.GetLength()}");
Console.WriteLine($"Width: {dimensions.GetWidth()}");
//</Snippet46>
}
}
/* Output:
Length: 30
Width: 20
*/
//</Snippet8>
namespace WrapBox
{
//<Snippet9>
// Declare the English units interface:
interface IEnglishDimensions
{
float Length();
float Width();
}
// Declare the metric units interface:
interface IMetricDimensions
{
float Length();
float Width();
}
// Declare the Box class that implements the two interfaces:
// IEnglishDimensions and IMetricDimensions:
class Box : IEnglishDimensions, IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float lengthInches, float widthInches)
{
_lengthInches = lengthInches;
_widthInches = widthInches;
}
// Explicitly implement the members of IEnglishDimensions:
float IEnglishDimensions.Length() => _lengthInches;
float IEnglishDimensions.Width() => _widthInches;
// Explicitly implement the members of IMetricDimensions:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an instance of the English units interface:
IEnglishDimensions eDimensions = box1;
// Declare an instance of the metric units interface:
IMetricDimensions mDimensions = box1;
// Print dimensions in English units:
Console.WriteLine($"Length(in): {eDimensions.Length()}");
Console.WriteLine($"Width (in): {eDimensions.Width()}");
// Print dimensions in metric units:
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
}
/* Output:
Length(in): 30
Width (in): 20
Length(cm): 76.2
Width (cm): 50.8
*/
//</Snippet9>
}
class Test
{
interface IMetricDimensions
{
float Length();
float Width();
}
class Box : IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
//<Snippet10>
// Normal implementation:
public float Length() => _lengthInches;
public float Width() => _widthInches;
// Explicit implementation:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
//</Snippet10>
//<Snippet11>
public static void Test()
{
Box box1 = new(30.0f, 20.0f);
IMetricDimensions mDimensions = box1;
Console.WriteLine($"Length(in): {box1.Length()}");
Console.WriteLine($"Width (in): {box1.Width()}");
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
//</Snippet11>
}
}
}
namespace WrapExample
{
// Inheritance (C# Programming Guide)
//<Snippet12>
public class A { }
public class B : A { }
//</Snippet12>
class Program
{
static void Main()
{
A a = new();
B b = new();
b = (B)a;
}
}
}
namespace WrapA
{
//<Snippet13>
public abstract class A
{
// Class members here.
}
//</Snippet13>
}
//<Snippet14>
public abstract class A
{
public abstract void DoWork(int i);
}
//</Snippet14>
namespace WrapD
{
//<Snippet15>
// compile with: -target:library
public class D
{
public virtual void DoWork(int i)
{
// Original implementation.
}
}
public abstract class E : D
{
public abstract override void DoWork(int i);
}
public class F : E
{
public override void DoWork(int i)
{
// New implementation.
}
}
//</Snippet15>
}
namespace WrapD2
{
//<Snippet16>
public sealed class D
{
// Class members here.
}
//</Snippet16>
}
public abstract class C
{
public virtual void DoWork() { }
}
//<Snippet17>
public class D : C
{
public sealed override void DoWork() { }
}
//</Snippet17>
namespace WrapClasses
{
//<Snippet22>
public class A
{
public virtual void DoWork() { }
}
public class B : A
{
public override void DoWork() { }
}
//</Snippet22>
//<Snippet23>
public class C : B
{
public override void DoWork() { }
}
//</Snippet23>
}
class TestGraphics
{
//<Snippet27>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
}
//</Snippet27>
//<Snippet28>
class YourDerivedGraphicsClass : GraphicsClass
{
public void DrawRectangle() { }
}
//</Snippet28>
}
class TestGraphics2
{
//<Snippet29>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//</Snippet29>
//<Snippet30>
class YourDerivedGraphicsClass : GraphicsClass
{
public override void DrawRectangle() { }
}
//</Snippet30>
}
class TestGraphics3
{
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//<Snippet31>
class YourDerivedGraphicsClass : GraphicsClass
{
public new void DrawRectangle() { }
}
//</Snippet31>
class X : YourDerivedGraphicsClass
{
public new void DrawRectangle()
{
//<Snippet44>
base.DrawRectangle();
//</Snippet44>
}
}
}
class TestSelection
{
public class Base
{
public virtual void DoWork(int param) { }
}
//<Snippet32>
public class Derived : Base
{
public override void DoWork(int param) { }
public void DoWork(double param) { }
}
//</Snippet32>
void Test()
{
//<Snippet33>
int val = 5;
Derived d = new();
d.DoWork(val); // Calls DoWork(double).
//</Snippet33>
//<Snippet34>
((Base)d).DoWork(val); // Calls DoWork(int) on Derived.
//</Snippet34>
}
}
class OverrideToString
{
//<Snippet36>
class Person
{
public string Name { get; set; }
public int Age { get; set; }
public override string ToString()
{
return "Person: " + Name + " " + Age;
}
}
//</Snippet36>
void test()
{
//<Snippet37>
int x = 42;
string strx = x.ToString();
Console.WriteLine(strx);
// Output:
// 42
//</Snippet37>
//<Snippet38>
Person person = new() { Name = "John", Age = 12 };
Console.WriteLine(person);
// Output:
// Person: John 12
//</Snippet38>
}
}
namespace ExplicitConversion
{
//<Snippet51>
class Test
{
public static void M()
{ Console.WriteLine("I'm just a Test"); }
}
class Program2
{
static void Main()
{
System.Collections.ArrayList list = [new Test(), new Test(), new Test()];
foreach (object o in list)
{
if (o is Test t)
{
Test.M();
}
}
}
}
/* Output:
I'm just a Test
I'm just a Test
I'm just a Test
*/
//</Snippet51>
}
namespace RainyDay
{
class JustInCase
{
static void Main()
{
//<Snippet52>
Console.WriteLine("Saving for a rainy day.");
//</Snippet52>
//<Snippet53>
Console.WriteLine("Saving for a rainy day.");
//</Snippet53>
//<Snippet54>
Console.WriteLine("Saving for a rainy day.");
//</Snippet54>
//<Snippet55>
Console.WriteLine("Saving for a rainy day.");
//</Snippet55>
//<Snippet56>
Console.WriteLine("Saving for a rainy day.");
//</Snippet56>
}
}
}
//<snippet70>
namespace OverrideAndNew
{
using System;
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
// The following two calls do what you would expect. They call
// the methods that are defined in BaseClass.
bc.Method1();
bc.Method2();
// Output:
// Base - Method1
// Base - Method2
// The following two calls do what you would expect. They call
// the methods that are defined in DerivedClass.
dc.Method1();
dc.Method2();
// Output:
// Derived - Method1
// Derived - Method2
// The following two calls produce different results for the
// method defined by using override (Method1) and the
// method defined by using new (Method2).
bcdc.Method1();
bcdc.Method2();
// Output:
// Derived - Method1
// Base - Method2
}
}
class BaseClass
{
//<snippet68>
public virtual void Method1()
{
Console.WriteLine("Base - Method1");
}
//</snippet68>
public virtual void Method2()
{
Console.WriteLine("Base - Method2");
}
}
class DerivedClass : BaseClass
{
//<snippet67>
public override void Method1()
{
Console.WriteLine("Derived - Method1");
}
//</snippet67>
//<snippet66>
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
//</snippet66>
}
}
//</snippet70>
//<snippet69>
// Output:
// Base - Method1
// Base - Method2
// Derived - Method1
// Derived - Method2
// Derived - Method1
// Base - Method2
//</snippet69>
namespace pieces
{
using System;
//<snippet61>
class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
}
class DerivedClass : BaseClass
{
public void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
//</snippet61>
//<snippet62>
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
bc.Method1();
dc.Method1();
dc.Method2();
bcdc.Method1();
}
// Output:
// Base - Method1
// Base - Method1
// Derived - Method2
// Base - Method1
}
//</snippet62>
class morePieces
{
public void callers()
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
//<snippet64>
bc.Method1();
bc.Method2();
dc.Method1();
dc.Method2();
bcdc.Method1();
bcdc.Method2();
//</snippet64>
//<snippet65>
// Output:
// Base - Method1
// Base - Method2
// Base - Method1
// Derived - Method2
// Base - Method1
// Base - Method2
//</snippet65>
}
public class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
//<snippet63>
public void Method2()
{
Console.WriteLine("Base - Method2");
}
//</snippet63>
}
class DerivedClass : BaseClass
{
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
}
}
//<snippet78>
namespace OverrideAndNew2
{
using System;
using System.Collections.Generic;
class Program
{
static void Main(string[] args)
{
// Declare objects of the derived classes and test which version
// of ShowDetails is run, base or derived.
TestCars1();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars2();
// Declare objects of the derived classes and call ShowDetails
// directly.
TestCars3();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars4();
}
//<snippet72>
public static void TestCars1()
{
Console.WriteLine("\nTestCars1");
Console.WriteLine("----------");
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
// Note the output from this test case. The ShowDetails method is
// declared with the new modifier.
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</snippet72>
//<snippet73>
// Output:
// TestCars1
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet73>
//<snippet74>
public static void TestCars2()
{
Console.WriteLine("\nTestCars2");
Console.WriteLine("----------");
var cars = new List<Car> { new(), new ConvertibleCar(),
new Minivan() };
foreach (Car car in cars)
{
car.DescribeCar();
Console.WriteLine("----------");
}
}
//</snippet74>
//<snippet75>
// Output:
// TestCars2
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet75>
//<snippet76>
public static void TestCars3()
{
Console.WriteLine("\nTestCars3");
Console.WriteLine("----------");
ConvertibleCar car2 = new();
Minivan car3 = new();
car2.ShowDetails();
car3.ShowDetails();
}
public static void TestCars4()
{
Console.WriteLine("\nTestCars4");
Console.WriteLine("----------");
Car car2 = new ConvertibleCar();
Car car3 = new Minivan();
car2.ShowDetails();
car3.ShowDetails();
}
//</snippet76>
//<snippet77>
// Output:
// TestCars3
// ----------
// A roof that opens up.
// Carries seven people.
// TestCars4
// ----------
// Standard transportation.
// Carries seven people.
//</snippet77>
}
//<snippet71>
// Define the base class, Car. The class defines two methods, DescribeCar
// and ShowDetails. DescribeCar calls ShowDetails, and each derived class
// also defines a ShowDetails method. The example tests which version of
// ShowDetails is selected, the base class method or the derived class method.
class Car
{
public void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
ShowDetails();
}
public virtual void ShowDetails()
{
Console.WriteLine("Standard transportation.");
}
}
// Define the derived classes.
// Class ConvertibleCar uses the new modifier to acknowledge that ShowDetails
// hides the base class method.
class ConvertibleCar : Car
{
public new void ShowDetails()
{
Console.WriteLine("A roof that opens up.");
}
}
// Class Minivan uses the override modifier to specify that ShowDetails
// extends the base class method.
class Minivan : Car
{
public override void ShowDetails()
{
Console.WriteLine("Carries seven people.");
}
}
//</snippet71>
}
//</snippet78>Abstract methods have no implementation, so the method definition is followed by a semicolon instead of a normal method block. Derived classes of the abstract class must implement all abstract methods. When an abstract class inherits a virtual method from a base class, the abstract class can override the virtual method with an abstract method. For example:
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideInheritance/CS/Inheritance.cs15
using System;
using System.Drawing;
using System.Windows.Forms;
namespace Inheritance_ProgGuide
{
class TestPaint : Form
{
//<Snippet35>
protected override void OnPaint(PaintEventArgs e)
{
// Perform custom drawing
System.Drawing.Graphics gfx = e.Graphics;
gfx.DrawString("Hello, World!",
new System.Drawing.Font("Arial", 16),
new System.Drawing.SolidBrush(System.Drawing.Color.Blue),
new System.Drawing.Point(10, 10));
}
private void InitializeComponent()
{
SuspendLayout();
//
// TestPaint
//
ClientSize = new Size(284, 264);
Name = "TestPaint";
Load += new EventHandler(TestPaint_Load);
ResumeLayout(false);
}
private void TestPaint_Load(object sender, EventArgs e)
{
}
//</Snippet35>
}
//<Snippet1>
// compile with: csc -target:library abstractshape.cs
public abstract class Shape
{
public Shape(string s)
{
// calling the set accessor of the Id property.
Id = s;
}
public string Id { get; set; }
// Area is a read-only property - only a get accessor is needed:
public abstract double Area
{
get;
}
public override string ToString()
{
return $"{Id} Area = {Area:F2}";
}
}
//</Snippet1>
//<Snippet2>
// compile with: csc -target:library -reference:abstractshape.dll shapes.cs
public class Square : Shape
{
private int _side;
public Square(int side, string id)
: base(id)
{
_side = side;
}
public override double Area
{
get
{
// Given the side, return the area of a square:
return _side * _side;
}
}
}
public class Circle : Shape
{
private int _radius;
public Circle(int radius, string id)
: base(id)
{
_radius = radius;
}
public override double Area
{
get
{
// Given the radius, return the area of a circle:
return _radius * _radius * Math.PI;
}
}
}
public class Rectangle : Shape
{
private int _width;
private int _height;
public Rectangle(int width, int height, string id)
: base(id)
{
_width = width;
_height = height;
}
public override double Area
{
get
{
// Given the width and height, return the area of a rectangle:
return _width * _height;
}
}
}
//</Snippet2>
//<Snippet3>
// compile with: csc -reference:abstractshape.dll;shapes.dll shapetest.cs
class TestClass
{
static void Main()
{
Shape[] shapes =
[
new Square(5, "Square #1"),
new Circle(3, "Circle #1"),
new Rectangle( 4, 5, "Rectangle #1")
];
Console.WriteLine("Shapes Collection");
foreach (Shape s in shapes)
{
Console.WriteLine(s);
}
}
}
/* Output:
Shapes Collection
Square #1 Area = 25.00
Circle #1 Area = 28.27
Rectangle #1 Area = 20.00
*/
//</Snippet3>
class CarStuff
{
//<Snippet4>
// Define the base class
class Car
{
public virtual void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
}
}
// Define the derived classes
class ConvertibleCar : Car
{
public new virtual void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("A roof that opens up.");
}
}
class Minivan : Car
{
public override void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("Carries seven people.");
}
}
//</Snippet4>
//<Snippet5>
public static void TestCars1()
{
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet5>
//<Snippet6>
public static void TestCars2()
{
Car[] cars = [new Car(), new ConvertibleCar(), new Minivan()];
}
//</Snippet6>
public static void TestCars3()
{
Car[] cars = new Car[3];
//<Snippet7>
foreach (Car vehicle in cars)
{
Console.WriteLine("Car object: " + vehicle.GetType());
vehicle.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet7>
}
}
//<Snippet8>
interface IDimensions
{
float GetLength();
float GetWidth();
}
class Box : IDimensions
{
float _lengthInches;
float _widthInches;
Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
// Explicit interface member implementation:
float IDimensions.GetLength()
{
return _lengthInches;
}
// Explicit interface member implementation:
float IDimensions.GetWidth()
{
return _widthInches;
}
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an interface instance dimensions:
IDimensions dimensions = box1;
// The following commented lines would produce compilation
// errors because they try to access an explicitly implemented
// interface member from a class instance:
//<Snippet45>
//Console.WriteLine($"Length: {box1.GetLength()}");
//Console.WriteLine($"Width: {box1.GetWidth()}");
//</Snippet45>
// Print out the dimensions of the box by calling the methods
// from an instance of the interface:
//<Snippet46>
Console.WriteLine($"Length: {dimensions.GetLength()}");
Console.WriteLine($"Width: {dimensions.GetWidth()}");
//</Snippet46>
}
}
/* Output:
Length: 30
Width: 20
*/
//</Snippet8>
namespace WrapBox
{
//<Snippet9>
// Declare the English units interface:
interface IEnglishDimensions
{
float Length();
float Width();
}
// Declare the metric units interface:
interface IMetricDimensions
{
float Length();
float Width();
}
// Declare the Box class that implements the two interfaces:
// IEnglishDimensions and IMetricDimensions:
class Box : IEnglishDimensions, IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float lengthInches, float widthInches)
{
_lengthInches = lengthInches;
_widthInches = widthInches;
}
// Explicitly implement the members of IEnglishDimensions:
float IEnglishDimensions.Length() => _lengthInches;
float IEnglishDimensions.Width() => _widthInches;
// Explicitly implement the members of IMetricDimensions:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an instance of the English units interface:
IEnglishDimensions eDimensions = box1;
// Declare an instance of the metric units interface:
IMetricDimensions mDimensions = box1;
// Print dimensions in English units:
Console.WriteLine($"Length(in): {eDimensions.Length()}");
Console.WriteLine($"Width (in): {eDimensions.Width()}");
// Print dimensions in metric units:
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
}
/* Output:
Length(in): 30
Width (in): 20
Length(cm): 76.2
Width (cm): 50.8
*/
//</Snippet9>
}
class Test
{
interface IMetricDimensions
{
float Length();
float Width();
}
class Box : IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
//<Snippet10>
// Normal implementation:
public float Length() => _lengthInches;
public float Width() => _widthInches;
// Explicit implementation:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
//</Snippet10>
//<Snippet11>
public static void Test()
{
Box box1 = new(30.0f, 20.0f);
IMetricDimensions mDimensions = box1;
Console.WriteLine($"Length(in): {box1.Length()}");
Console.WriteLine($"Width (in): {box1.Width()}");
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
//</Snippet11>
}
}
}
namespace WrapExample
{
// Inheritance (C# Programming Guide)
//<Snippet12>
public class A { }
public class B : A { }
//</Snippet12>
class Program
{
static void Main()
{
A a = new();
B b = new();
b = (B)a;
}
}
}
namespace WrapA
{
//<Snippet13>
public abstract class A
{
// Class members here.
}
//</Snippet13>
}
//<Snippet14>
public abstract class A
{
public abstract void DoWork(int i);
}
//</Snippet14>
namespace WrapD
{
//<Snippet15>
// compile with: -target:library
public class D
{
public virtual void DoWork(int i)
{
// Original implementation.
}
}
public abstract class E : D
{
public abstract override void DoWork(int i);
}
public class F : E
{
public override void DoWork(int i)
{
// New implementation.
}
}
//</Snippet15>
}
namespace WrapD2
{
//<Snippet16>
public sealed class D
{
// Class members here.
}
//</Snippet16>
}
public abstract class C
{
public virtual void DoWork() { }
}
//<Snippet17>
public class D : C
{
public sealed override void DoWork() { }
}
//</Snippet17>
namespace WrapClasses
{
//<Snippet22>
public class A
{
public virtual void DoWork() { }
}
public class B : A
{
public override void DoWork() { }
}
//</Snippet22>
//<Snippet23>
public class C : B
{
public override void DoWork() { }
}
//</Snippet23>
}
class TestGraphics
{
//<Snippet27>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
}
//</Snippet27>
//<Snippet28>
class YourDerivedGraphicsClass : GraphicsClass
{
public void DrawRectangle() { }
}
//</Snippet28>
}
class TestGraphics2
{
//<Snippet29>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//</Snippet29>
//<Snippet30>
class YourDerivedGraphicsClass : GraphicsClass
{
public override void DrawRectangle() { }
}
//</Snippet30>
}
class TestGraphics3
{
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//<Snippet31>
class YourDerivedGraphicsClass : GraphicsClass
{
public new void DrawRectangle() { }
}
//</Snippet31>
class X : YourDerivedGraphicsClass
{
public new void DrawRectangle()
{
//<Snippet44>
base.DrawRectangle();
//</Snippet44>
}
}
}
class TestSelection
{
public class Base
{
public virtual void DoWork(int param) { }
}
//<Snippet32>
public class Derived : Base
{
public override void DoWork(int param) { }
public void DoWork(double param) { }
}
//</Snippet32>
void Test()
{
//<Snippet33>
int val = 5;
Derived d = new();
d.DoWork(val); // Calls DoWork(double).
//</Snippet33>
//<Snippet34>
((Base)d).DoWork(val); // Calls DoWork(int) on Derived.
//</Snippet34>
}
}
class OverrideToString
{
//<Snippet36>
class Person
{
public string Name { get; set; }
public int Age { get; set; }
public override string ToString()
{
return "Person: " + Name + " " + Age;
}
}
//</Snippet36>
void test()
{
//<Snippet37>
int x = 42;
string strx = x.ToString();
Console.WriteLine(strx);
// Output:
// 42
//</Snippet37>
//<Snippet38>
Person person = new() { Name = "John", Age = 12 };
Console.WriteLine(person);
// Output:
// Person: John 12
//</Snippet38>
}
}
namespace ExplicitConversion
{
//<Snippet51>
class Test
{
public static void M()
{ Console.WriteLine("I'm just a Test"); }
}
class Program2
{
static void Main()
{
System.Collections.ArrayList list = [new Test(), new Test(), new Test()];
foreach (object o in list)
{
if (o is Test t)
{
Test.M();
}
}
}
}
/* Output:
I'm just a Test
I'm just a Test
I'm just a Test
*/
//</Snippet51>
}
namespace RainyDay
{
class JustInCase
{
static void Main()
{
//<Snippet52>
Console.WriteLine("Saving for a rainy day.");
//</Snippet52>
//<Snippet53>
Console.WriteLine("Saving for a rainy day.");
//</Snippet53>
//<Snippet54>
Console.WriteLine("Saving for a rainy day.");
//</Snippet54>
//<Snippet55>
Console.WriteLine("Saving for a rainy day.");
//</Snippet55>
//<Snippet56>
Console.WriteLine("Saving for a rainy day.");
//</Snippet56>
}
}
}
//<snippet70>
namespace OverrideAndNew
{
using System;
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
// The following two calls do what you would expect. They call
// the methods that are defined in BaseClass.
bc.Method1();
bc.Method2();
// Output:
// Base - Method1
// Base - Method2
// The following two calls do what you would expect. They call
// the methods that are defined in DerivedClass.
dc.Method1();
dc.Method2();
// Output:
// Derived - Method1
// Derived - Method2
// The following two calls produce different results for the
// method defined by using override (Method1) and the
// method defined by using new (Method2).
bcdc.Method1();
bcdc.Method2();
// Output:
// Derived - Method1
// Base - Method2
}
}
class BaseClass
{
//<snippet68>
public virtual void Method1()
{
Console.WriteLine("Base - Method1");
}
//</snippet68>
public virtual void Method2()
{
Console.WriteLine("Base - Method2");
}
}
class DerivedClass : BaseClass
{
//<snippet67>
public override void Method1()
{
Console.WriteLine("Derived - Method1");
}
//</snippet67>
//<snippet66>
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
//</snippet66>
}
}
//</snippet70>
//<snippet69>
// Output:
// Base - Method1
// Base - Method2
// Derived - Method1
// Derived - Method2
// Derived - Method1
// Base - Method2
//</snippet69>
namespace pieces
{
using System;
//<snippet61>
class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
}
class DerivedClass : BaseClass
{
public void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
//</snippet61>
//<snippet62>
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
bc.Method1();
dc.Method1();
dc.Method2();
bcdc.Method1();
}
// Output:
// Base - Method1
// Base - Method1
// Derived - Method2
// Base - Method1
}
//</snippet62>
class morePieces
{
public void callers()
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
//<snippet64>
bc.Method1();
bc.Method2();
dc.Method1();
dc.Method2();
bcdc.Method1();
bcdc.Method2();
//</snippet64>
//<snippet65>
// Output:
// Base - Method1
// Base - Method2
// Base - Method1
// Derived - Method2
// Base - Method1
// Base - Method2
//</snippet65>
}
public class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
//<snippet63>
public void Method2()
{
Console.WriteLine("Base - Method2");
}
//</snippet63>
}
class DerivedClass : BaseClass
{
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
}
}
//<snippet78>
namespace OverrideAndNew2
{
using System;
using System.Collections.Generic;
class Program
{
static void Main(string[] args)
{
// Declare objects of the derived classes and test which version
// of ShowDetails is run, base or derived.
TestCars1();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars2();
// Declare objects of the derived classes and call ShowDetails
// directly.
TestCars3();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars4();
}
//<snippet72>
public static void TestCars1()
{
Console.WriteLine("\nTestCars1");
Console.WriteLine("----------");
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
// Note the output from this test case. The ShowDetails method is
// declared with the new modifier.
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</snippet72>
//<snippet73>
// Output:
// TestCars1
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet73>
//<snippet74>
public static void TestCars2()
{
Console.WriteLine("\nTestCars2");
Console.WriteLine("----------");
var cars = new List<Car> { new(), new ConvertibleCar(),
new Minivan() };
foreach (Car car in cars)
{
car.DescribeCar();
Console.WriteLine("----------");
}
}
//</snippet74>
//<snippet75>
// Output:
// TestCars2
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet75>
//<snippet76>
public static void TestCars3()
{
Console.WriteLine("\nTestCars3");
Console.WriteLine("----------");
ConvertibleCar car2 = new();
Minivan car3 = new();
car2.ShowDetails();
car3.ShowDetails();
}
public static void TestCars4()
{
Console.WriteLine("\nTestCars4");
Console.WriteLine("----------");
Car car2 = new ConvertibleCar();
Car car3 = new Minivan();
car2.ShowDetails();
car3.ShowDetails();
}
//</snippet76>
//<snippet77>
// Output:
// TestCars3
// ----------
// A roof that opens up.
// Carries seven people.
// TestCars4
// ----------
// Standard transportation.
// Carries seven people.
//</snippet77>
}
//<snippet71>
// Define the base class, Car. The class defines two methods, DescribeCar
// and ShowDetails. DescribeCar calls ShowDetails, and each derived class
// also defines a ShowDetails method. The example tests which version of
// ShowDetails is selected, the base class method or the derived class method.
class Car
{
public void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
ShowDetails();
}
public virtual void ShowDetails()
{
Console.WriteLine("Standard transportation.");
}
}
// Define the derived classes.
// Class ConvertibleCar uses the new modifier to acknowledge that ShowDetails
// hides the base class method.
class ConvertibleCar : Car
{
public new void ShowDetails()
{
Console.WriteLine("A roof that opens up.");
}
}
// Class Minivan uses the override modifier to specify that ShowDetails
// extends the base class method.
class Minivan : Car
{
public override void ShowDetails()
{
Console.WriteLine("Carries seven people.");
}
}
//</snippet71>
}
//</snippet78> If a virtual method is declared abstract, it is still virtual to any class inheriting from the abstract class. A class inheriting an abstract method cannot access the original implementation of the method—in the previous example, DoWork on class F cannot call DoWork on class D. In this way, an abstract class can force derived classes to provide new method implementations for virtual methods.
Sealed Classes and Class Members
Classes can be declared as sealed by putting the keyword sealed before the class definition. For example:
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideInheritance/CS/Inheritance.cs16
using System;
using System.Drawing;
using System.Windows.Forms;
namespace Inheritance_ProgGuide
{
class TestPaint : Form
{
//<Snippet35>
protected override void OnPaint(PaintEventArgs e)
{
// Perform custom drawing
System.Drawing.Graphics gfx = e.Graphics;
gfx.DrawString("Hello, World!",
new System.Drawing.Font("Arial", 16),
new System.Drawing.SolidBrush(System.Drawing.Color.Blue),
new System.Drawing.Point(10, 10));
}
private void InitializeComponent()
{
SuspendLayout();
//
// TestPaint
//
ClientSize = new Size(284, 264);
Name = "TestPaint";
Load += new EventHandler(TestPaint_Load);
ResumeLayout(false);
}
private void TestPaint_Load(object sender, EventArgs e)
{
}
//</Snippet35>
}
//<Snippet1>
// compile with: csc -target:library abstractshape.cs
public abstract class Shape
{
public Shape(string s)
{
// calling the set accessor of the Id property.
Id = s;
}
public string Id { get; set; }
// Area is a read-only property - only a get accessor is needed:
public abstract double Area
{
get;
}
public override string ToString()
{
return $"{Id} Area = {Area:F2}";
}
}
//</Snippet1>
//<Snippet2>
// compile with: csc -target:library -reference:abstractshape.dll shapes.cs
public class Square : Shape
{
private int _side;
public Square(int side, string id)
: base(id)
{
_side = side;
}
public override double Area
{
get
{
// Given the side, return the area of a square:
return _side * _side;
}
}
}
public class Circle : Shape
{
private int _radius;
public Circle(int radius, string id)
: base(id)
{
_radius = radius;
}
public override double Area
{
get
{
// Given the radius, return the area of a circle:
return _radius * _radius * Math.PI;
}
}
}
public class Rectangle : Shape
{
private int _width;
private int _height;
public Rectangle(int width, int height, string id)
: base(id)
{
_width = width;
_height = height;
}
public override double Area
{
get
{
// Given the width and height, return the area of a rectangle:
return _width * _height;
}
}
}
//</Snippet2>
//<Snippet3>
// compile with: csc -reference:abstractshape.dll;shapes.dll shapetest.cs
class TestClass
{
static void Main()
{
Shape[] shapes =
[
new Square(5, "Square #1"),
new Circle(3, "Circle #1"),
new Rectangle( 4, 5, "Rectangle #1")
];
Console.WriteLine("Shapes Collection");
foreach (Shape s in shapes)
{
Console.WriteLine(s);
}
}
}
/* Output:
Shapes Collection
Square #1 Area = 25.00
Circle #1 Area = 28.27
Rectangle #1 Area = 20.00
*/
//</Snippet3>
class CarStuff
{
//<Snippet4>
// Define the base class
class Car
{
public virtual void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
}
}
// Define the derived classes
class ConvertibleCar : Car
{
public new virtual void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("A roof that opens up.");
}
}
class Minivan : Car
{
public override void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("Carries seven people.");
}
}
//</Snippet4>
//<Snippet5>
public static void TestCars1()
{
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet5>
//<Snippet6>
public static void TestCars2()
{
Car[] cars = [new Car(), new ConvertibleCar(), new Minivan()];
}
//</Snippet6>
public static void TestCars3()
{
Car[] cars = new Car[3];
//<Snippet7>
foreach (Car vehicle in cars)
{
Console.WriteLine("Car object: " + vehicle.GetType());
vehicle.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet7>
}
}
//<Snippet8>
interface IDimensions
{
float GetLength();
float GetWidth();
}
class Box : IDimensions
{
float _lengthInches;
float _widthInches;
Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
// Explicit interface member implementation:
float IDimensions.GetLength()
{
return _lengthInches;
}
// Explicit interface member implementation:
float IDimensions.GetWidth()
{
return _widthInches;
}
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an interface instance dimensions:
IDimensions dimensions = box1;
// The following commented lines would produce compilation
// errors because they try to access an explicitly implemented
// interface member from a class instance:
//<Snippet45>
//Console.WriteLine($"Length: {box1.GetLength()}");
//Console.WriteLine($"Width: {box1.GetWidth()}");
//</Snippet45>
// Print out the dimensions of the box by calling the methods
// from an instance of the interface:
//<Snippet46>
Console.WriteLine($"Length: {dimensions.GetLength()}");
Console.WriteLine($"Width: {dimensions.GetWidth()}");
//</Snippet46>
}
}
/* Output:
Length: 30
Width: 20
*/
//</Snippet8>
namespace WrapBox
{
//<Snippet9>
// Declare the English units interface:
interface IEnglishDimensions
{
float Length();
float Width();
}
// Declare the metric units interface:
interface IMetricDimensions
{
float Length();
float Width();
}
// Declare the Box class that implements the two interfaces:
// IEnglishDimensions and IMetricDimensions:
class Box : IEnglishDimensions, IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float lengthInches, float widthInches)
{
_lengthInches = lengthInches;
_widthInches = widthInches;
}
// Explicitly implement the members of IEnglishDimensions:
float IEnglishDimensions.Length() => _lengthInches;
float IEnglishDimensions.Width() => _widthInches;
// Explicitly implement the members of IMetricDimensions:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an instance of the English units interface:
IEnglishDimensions eDimensions = box1;
// Declare an instance of the metric units interface:
IMetricDimensions mDimensions = box1;
// Print dimensions in English units:
Console.WriteLine($"Length(in): {eDimensions.Length()}");
Console.WriteLine($"Width (in): {eDimensions.Width()}");
// Print dimensions in metric units:
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
}
/* Output:
Length(in): 30
Width (in): 20
Length(cm): 76.2
Width (cm): 50.8
*/
//</Snippet9>
}
class Test
{
interface IMetricDimensions
{
float Length();
float Width();
}
class Box : IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
//<Snippet10>
// Normal implementation:
public float Length() => _lengthInches;
public float Width() => _widthInches;
// Explicit implementation:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
//</Snippet10>
//<Snippet11>
public static void Test()
{
Box box1 = new(30.0f, 20.0f);
IMetricDimensions mDimensions = box1;
Console.WriteLine($"Length(in): {box1.Length()}");
Console.WriteLine($"Width (in): {box1.Width()}");
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
//</Snippet11>
}
}
}
namespace WrapExample
{
// Inheritance (C# Programming Guide)
//<Snippet12>
public class A { }
public class B : A { }
//</Snippet12>
class Program
{
static void Main()
{
A a = new();
B b = new();
b = (B)a;
}
}
}
namespace WrapA
{
//<Snippet13>
public abstract class A
{
// Class members here.
}
//</Snippet13>
}
//<Snippet14>
public abstract class A
{
public abstract void DoWork(int i);
}
//</Snippet14>
namespace WrapD
{
//<Snippet15>
// compile with: -target:library
public class D
{
public virtual void DoWork(int i)
{
// Original implementation.
}
}
public abstract class E : D
{
public abstract override void DoWork(int i);
}
public class F : E
{
public override void DoWork(int i)
{
// New implementation.
}
}
//</Snippet15>
}
namespace WrapD2
{
//<Snippet16>
public sealed class D
{
// Class members here.
}
//</Snippet16>
}
public abstract class C
{
public virtual void DoWork() { }
}
//<Snippet17>
public class D : C
{
public sealed override void DoWork() { }
}
//</Snippet17>
namespace WrapClasses
{
//<Snippet22>
public class A
{
public virtual void DoWork() { }
}
public class B : A
{
public override void DoWork() { }
}
//</Snippet22>
//<Snippet23>
public class C : B
{
public override void DoWork() { }
}
//</Snippet23>
}
class TestGraphics
{
//<Snippet27>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
}
//</Snippet27>
//<Snippet28>
class YourDerivedGraphicsClass : GraphicsClass
{
public void DrawRectangle() { }
}
//</Snippet28>
}
class TestGraphics2
{
//<Snippet29>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//</Snippet29>
//<Snippet30>
class YourDerivedGraphicsClass : GraphicsClass
{
public override void DrawRectangle() { }
}
//</Snippet30>
}
class TestGraphics3
{
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//<Snippet31>
class YourDerivedGraphicsClass : GraphicsClass
{
public new void DrawRectangle() { }
}
//</Snippet31>
class X : YourDerivedGraphicsClass
{
public new void DrawRectangle()
{
//<Snippet44>
base.DrawRectangle();
//</Snippet44>
}
}
}
class TestSelection
{
public class Base
{
public virtual void DoWork(int param) { }
}
//<Snippet32>
public class Derived : Base
{
public override void DoWork(int param) { }
public void DoWork(double param) { }
}
//</Snippet32>
void Test()
{
//<Snippet33>
int val = 5;
Derived d = new();
d.DoWork(val); // Calls DoWork(double).
//</Snippet33>
//<Snippet34>
((Base)d).DoWork(val); // Calls DoWork(int) on Derived.
//</Snippet34>
}
}
class OverrideToString
{
//<Snippet36>
class Person
{
public string Name { get; set; }
public int Age { get; set; }
public override string ToString()
{
return "Person: " + Name + " " + Age;
}
}
//</Snippet36>
void test()
{
//<Snippet37>
int x = 42;
string strx = x.ToString();
Console.WriteLine(strx);
// Output:
// 42
//</Snippet37>
//<Snippet38>
Person person = new() { Name = "John", Age = 12 };
Console.WriteLine(person);
// Output:
// Person: John 12
//</Snippet38>
}
}
namespace ExplicitConversion
{
//<Snippet51>
class Test
{
public static void M()
{ Console.WriteLine("I'm just a Test"); }
}
class Program2
{
static void Main()
{
System.Collections.ArrayList list = [new Test(), new Test(), new Test()];
foreach (object o in list)
{
if (o is Test t)
{
Test.M();
}
}
}
}
/* Output:
I'm just a Test
I'm just a Test
I'm just a Test
*/
//</Snippet51>
}
namespace RainyDay
{
class JustInCase
{
static void Main()
{
//<Snippet52>
Console.WriteLine("Saving for a rainy day.");
//</Snippet52>
//<Snippet53>
Console.WriteLine("Saving for a rainy day.");
//</Snippet53>
//<Snippet54>
Console.WriteLine("Saving for a rainy day.");
//</Snippet54>
//<Snippet55>
Console.WriteLine("Saving for a rainy day.");
//</Snippet55>
//<Snippet56>
Console.WriteLine("Saving for a rainy day.");
//</Snippet56>
}
}
}
//<snippet70>
namespace OverrideAndNew
{
using System;
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
// The following two calls do what you would expect. They call
// the methods that are defined in BaseClass.
bc.Method1();
bc.Method2();
// Output:
// Base - Method1
// Base - Method2
// The following two calls do what you would expect. They call
// the methods that are defined in DerivedClass.
dc.Method1();
dc.Method2();
// Output:
// Derived - Method1
// Derived - Method2
// The following two calls produce different results for the
// method defined by using override (Method1) and the
// method defined by using new (Method2).
bcdc.Method1();
bcdc.Method2();
// Output:
// Derived - Method1
// Base - Method2
}
}
class BaseClass
{
//<snippet68>
public virtual void Method1()
{
Console.WriteLine("Base - Method1");
}
//</snippet68>
public virtual void Method2()
{
Console.WriteLine("Base - Method2");
}
}
class DerivedClass : BaseClass
{
//<snippet67>
public override void Method1()
{
Console.WriteLine("Derived - Method1");
}
//</snippet67>
//<snippet66>
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
//</snippet66>
}
}
//</snippet70>
//<snippet69>
// Output:
// Base - Method1
// Base - Method2
// Derived - Method1
// Derived - Method2
// Derived - Method1
// Base - Method2
//</snippet69>
namespace pieces
{
using System;
//<snippet61>
class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
}
class DerivedClass : BaseClass
{
public void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
//</snippet61>
//<snippet62>
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
bc.Method1();
dc.Method1();
dc.Method2();
bcdc.Method1();
}
// Output:
// Base - Method1
// Base - Method1
// Derived - Method2
// Base - Method1
}
//</snippet62>
class morePieces
{
public void callers()
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
//<snippet64>
bc.Method1();
bc.Method2();
dc.Method1();
dc.Method2();
bcdc.Method1();
bcdc.Method2();
//</snippet64>
//<snippet65>
// Output:
// Base - Method1
// Base - Method2
// Base - Method1
// Derived - Method2
// Base - Method1
// Base - Method2
//</snippet65>
}
public class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
//<snippet63>
public void Method2()
{
Console.WriteLine("Base - Method2");
}
//</snippet63>
}
class DerivedClass : BaseClass
{
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
}
}
//<snippet78>
namespace OverrideAndNew2
{
using System;
using System.Collections.Generic;
class Program
{
static void Main(string[] args)
{
// Declare objects of the derived classes and test which version
// of ShowDetails is run, base or derived.
TestCars1();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars2();
// Declare objects of the derived classes and call ShowDetails
// directly.
TestCars3();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars4();
}
//<snippet72>
public static void TestCars1()
{
Console.WriteLine("\nTestCars1");
Console.WriteLine("----------");
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
// Note the output from this test case. The ShowDetails method is
// declared with the new modifier.
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</snippet72>
//<snippet73>
// Output:
// TestCars1
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet73>
//<snippet74>
public static void TestCars2()
{
Console.WriteLine("\nTestCars2");
Console.WriteLine("----------");
var cars = new List<Car> { new(), new ConvertibleCar(),
new Minivan() };
foreach (Car car in cars)
{
car.DescribeCar();
Console.WriteLine("----------");
}
}
//</snippet74>
//<snippet75>
// Output:
// TestCars2
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet75>
//<snippet76>
public static void TestCars3()
{
Console.WriteLine("\nTestCars3");
Console.WriteLine("----------");
ConvertibleCar car2 = new();
Minivan car3 = new();
car2.ShowDetails();
car3.ShowDetails();
}
public static void TestCars4()
{
Console.WriteLine("\nTestCars4");
Console.WriteLine("----------");
Car car2 = new ConvertibleCar();
Car car3 = new Minivan();
car2.ShowDetails();
car3.ShowDetails();
}
//</snippet76>
//<snippet77>
// Output:
// TestCars3
// ----------
// A roof that opens up.
// Carries seven people.
// TestCars4
// ----------
// Standard transportation.
// Carries seven people.
//</snippet77>
}
//<snippet71>
// Define the base class, Car. The class defines two methods, DescribeCar
// and ShowDetails. DescribeCar calls ShowDetails, and each derived class
// also defines a ShowDetails method. The example tests which version of
// ShowDetails is selected, the base class method or the derived class method.
class Car
{
public void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
ShowDetails();
}
public virtual void ShowDetails()
{
Console.WriteLine("Standard transportation.");
}
}
// Define the derived classes.
// Class ConvertibleCar uses the new modifier to acknowledge that ShowDetails
// hides the base class method.
class ConvertibleCar : Car
{
public new void ShowDetails()
{
Console.WriteLine("A roof that opens up.");
}
}
// Class Minivan uses the override modifier to specify that ShowDetails
// extends the base class method.
class Minivan : Car
{
public override void ShowDetails()
{
Console.WriteLine("Carries seven people.");
}
}
//</snippet71>
}
//</snippet78>A sealed class cannot be used as a base class. For this reason, it cannot also be an abstract class. Sealed classes prevent derivation. Because they can never be used as a base class, some run-time optimizations can make calling sealed class members slightly faster.
A method, indexer, property, or event, on a derived class that is overriding a virtual member of the base class can declare that member as sealed. This negates the virtual aspect of the member for any further derived class. This is accomplished by putting the sealed keyword before the override keyword in the class member declaration. For example:
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideInheritance/CS/Inheritance.cs17
using System;
using System.Drawing;
using System.Windows.Forms;
namespace Inheritance_ProgGuide
{
class TestPaint : Form
{
//<Snippet35>
protected override void OnPaint(PaintEventArgs e)
{
// Perform custom drawing
System.Drawing.Graphics gfx = e.Graphics;
gfx.DrawString("Hello, World!",
new System.Drawing.Font("Arial", 16),
new System.Drawing.SolidBrush(System.Drawing.Color.Blue),
new System.Drawing.Point(10, 10));
}
private void InitializeComponent()
{
SuspendLayout();
//
// TestPaint
//
ClientSize = new Size(284, 264);
Name = "TestPaint";
Load += new EventHandler(TestPaint_Load);
ResumeLayout(false);
}
private void TestPaint_Load(object sender, EventArgs e)
{
}
//</Snippet35>
}
//<Snippet1>
// compile with: csc -target:library abstractshape.cs
public abstract class Shape
{
public Shape(string s)
{
// calling the set accessor of the Id property.
Id = s;
}
public string Id { get; set; }
// Area is a read-only property - only a get accessor is needed:
public abstract double Area
{
get;
}
public override string ToString()
{
return $"{Id} Area = {Area:F2}";
}
}
//</Snippet1>
//<Snippet2>
// compile with: csc -target:library -reference:abstractshape.dll shapes.cs
public class Square : Shape
{
private int _side;
public Square(int side, string id)
: base(id)
{
_side = side;
}
public override double Area
{
get
{
// Given the side, return the area of a square:
return _side * _side;
}
}
}
public class Circle : Shape
{
private int _radius;
public Circle(int radius, string id)
: base(id)
{
_radius = radius;
}
public override double Area
{
get
{
// Given the radius, return the area of a circle:
return _radius * _radius * Math.PI;
}
}
}
public class Rectangle : Shape
{
private int _width;
private int _height;
public Rectangle(int width, int height, string id)
: base(id)
{
_width = width;
_height = height;
}
public override double Area
{
get
{
// Given the width and height, return the area of a rectangle:
return _width * _height;
}
}
}
//</Snippet2>
//<Snippet3>
// compile with: csc -reference:abstractshape.dll;shapes.dll shapetest.cs
class TestClass
{
static void Main()
{
Shape[] shapes =
[
new Square(5, "Square #1"),
new Circle(3, "Circle #1"),
new Rectangle( 4, 5, "Rectangle #1")
];
Console.WriteLine("Shapes Collection");
foreach (Shape s in shapes)
{
Console.WriteLine(s);
}
}
}
/* Output:
Shapes Collection
Square #1 Area = 25.00
Circle #1 Area = 28.27
Rectangle #1 Area = 20.00
*/
//</Snippet3>
class CarStuff
{
//<Snippet4>
// Define the base class
class Car
{
public virtual void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
}
}
// Define the derived classes
class ConvertibleCar : Car
{
public new virtual void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("A roof that opens up.");
}
}
class Minivan : Car
{
public override void DescribeCar()
{
base.DescribeCar();
Console.WriteLine("Carries seven people.");
}
}
//</Snippet4>
//<Snippet5>
public static void TestCars1()
{
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet5>
//<Snippet6>
public static void TestCars2()
{
Car[] cars = [new Car(), new ConvertibleCar(), new Minivan()];
}
//</Snippet6>
public static void TestCars3()
{
Car[] cars = new Car[3];
//<Snippet7>
foreach (Car vehicle in cars)
{
Console.WriteLine("Car object: " + vehicle.GetType());
vehicle.DescribeCar();
Console.WriteLine("----------");
}
//</Snippet7>
}
}
//<Snippet8>
interface IDimensions
{
float GetLength();
float GetWidth();
}
class Box : IDimensions
{
float _lengthInches;
float _widthInches;
Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
// Explicit interface member implementation:
float IDimensions.GetLength()
{
return _lengthInches;
}
// Explicit interface member implementation:
float IDimensions.GetWidth()
{
return _widthInches;
}
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an interface instance dimensions:
IDimensions dimensions = box1;
// The following commented lines would produce compilation
// errors because they try to access an explicitly implemented
// interface member from a class instance:
//<Snippet45>
//Console.WriteLine($"Length: {box1.GetLength()}");
//Console.WriteLine($"Width: {box1.GetWidth()}");
//</Snippet45>
// Print out the dimensions of the box by calling the methods
// from an instance of the interface:
//<Snippet46>
Console.WriteLine($"Length: {dimensions.GetLength()}");
Console.WriteLine($"Width: {dimensions.GetWidth()}");
//</Snippet46>
}
}
/* Output:
Length: 30
Width: 20
*/
//</Snippet8>
namespace WrapBox
{
//<Snippet9>
// Declare the English units interface:
interface IEnglishDimensions
{
float Length();
float Width();
}
// Declare the metric units interface:
interface IMetricDimensions
{
float Length();
float Width();
}
// Declare the Box class that implements the two interfaces:
// IEnglishDimensions and IMetricDimensions:
class Box : IEnglishDimensions, IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float lengthInches, float widthInches)
{
_lengthInches = lengthInches;
_widthInches = widthInches;
}
// Explicitly implement the members of IEnglishDimensions:
float IEnglishDimensions.Length() => _lengthInches;
float IEnglishDimensions.Width() => _widthInches;
// Explicitly implement the members of IMetricDimensions:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
static void Main()
{
// Declare a class instance box1:
Box box1 = new(30.0f, 20.0f);
// Declare an instance of the English units interface:
IEnglishDimensions eDimensions = box1;
// Declare an instance of the metric units interface:
IMetricDimensions mDimensions = box1;
// Print dimensions in English units:
Console.WriteLine($"Length(in): {eDimensions.Length()}");
Console.WriteLine($"Width (in): {eDimensions.Width()}");
// Print dimensions in metric units:
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
}
/* Output:
Length(in): 30
Width (in): 20
Length(cm): 76.2
Width (cm): 50.8
*/
//</Snippet9>
}
class Test
{
interface IMetricDimensions
{
float Length();
float Width();
}
class Box : IMetricDimensions
{
float _lengthInches;
float _widthInches;
public Box(float length, float width)
{
_lengthInches = length;
_widthInches = width;
}
//<Snippet10>
// Normal implementation:
public float Length() => _lengthInches;
public float Width() => _widthInches;
// Explicit implementation:
float IMetricDimensions.Length() => _lengthInches * 2.54f;
float IMetricDimensions.Width() => _widthInches * 2.54f;
//</Snippet10>
//<Snippet11>
public static void Test()
{
Box box1 = new(30.0f, 20.0f);
IMetricDimensions mDimensions = box1;
Console.WriteLine($"Length(in): {box1.Length()}");
Console.WriteLine($"Width (in): {box1.Width()}");
Console.WriteLine($"Length(cm): {mDimensions.Length()}");
Console.WriteLine($"Width (cm): {mDimensions.Width()}");
}
//</Snippet11>
}
}
}
namespace WrapExample
{
// Inheritance (C# Programming Guide)
//<Snippet12>
public class A { }
public class B : A { }
//</Snippet12>
class Program
{
static void Main()
{
A a = new();
B b = new();
b = (B)a;
}
}
}
namespace WrapA
{
//<Snippet13>
public abstract class A
{
// Class members here.
}
//</Snippet13>
}
//<Snippet14>
public abstract class A
{
public abstract void DoWork(int i);
}
//</Snippet14>
namespace WrapD
{
//<Snippet15>
// compile with: -target:library
public class D
{
public virtual void DoWork(int i)
{
// Original implementation.
}
}
public abstract class E : D
{
public abstract override void DoWork(int i);
}
public class F : E
{
public override void DoWork(int i)
{
// New implementation.
}
}
//</Snippet15>
}
namespace WrapD2
{
//<Snippet16>
public sealed class D
{
// Class members here.
}
//</Snippet16>
}
public abstract class C
{
public virtual void DoWork() { }
}
//<Snippet17>
public class D : C
{
public sealed override void DoWork() { }
}
//</Snippet17>
namespace WrapClasses
{
//<Snippet22>
public class A
{
public virtual void DoWork() { }
}
public class B : A
{
public override void DoWork() { }
}
//</Snippet22>
//<Snippet23>
public class C : B
{
public override void DoWork() { }
}
//</Snippet23>
}
class TestGraphics
{
//<Snippet27>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
}
//</Snippet27>
//<Snippet28>
class YourDerivedGraphicsClass : GraphicsClass
{
public void DrawRectangle() { }
}
//</Snippet28>
}
class TestGraphics2
{
//<Snippet29>
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//</Snippet29>
//<Snippet30>
class YourDerivedGraphicsClass : GraphicsClass
{
public override void DrawRectangle() { }
}
//</Snippet30>
}
class TestGraphics3
{
class GraphicsClass
{
public virtual void DrawLine() { }
public virtual void DrawPoint() { }
public virtual void DrawRectangle() { }
}
//<Snippet31>
class YourDerivedGraphicsClass : GraphicsClass
{
public new void DrawRectangle() { }
}
//</Snippet31>
class X : YourDerivedGraphicsClass
{
public new void DrawRectangle()
{
//<Snippet44>
base.DrawRectangle();
//</Snippet44>
}
}
}
class TestSelection
{
public class Base
{
public virtual void DoWork(int param) { }
}
//<Snippet32>
public class Derived : Base
{
public override void DoWork(int param) { }
public void DoWork(double param) { }
}
//</Snippet32>
void Test()
{
//<Snippet33>
int val = 5;
Derived d = new();
d.DoWork(val); // Calls DoWork(double).
//</Snippet33>
//<Snippet34>
((Base)d).DoWork(val); // Calls DoWork(int) on Derived.
//</Snippet34>
}
}
class OverrideToString
{
//<Snippet36>
class Person
{
public string Name { get; set; }
public int Age { get; set; }
public override string ToString()
{
return "Person: " + Name + " " + Age;
}
}
//</Snippet36>
void test()
{
//<Snippet37>
int x = 42;
string strx = x.ToString();
Console.WriteLine(strx);
// Output:
// 42
//</Snippet37>
//<Snippet38>
Person person = new() { Name = "John", Age = 12 };
Console.WriteLine(person);
// Output:
// Person: John 12
//</Snippet38>
}
}
namespace ExplicitConversion
{
//<Snippet51>
class Test
{
public static void M()
{ Console.WriteLine("I'm just a Test"); }
}
class Program2
{
static void Main()
{
System.Collections.ArrayList list = [new Test(), new Test(), new Test()];
foreach (object o in list)
{
if (o is Test t)
{
Test.M();
}
}
}
}
/* Output:
I'm just a Test
I'm just a Test
I'm just a Test
*/
//</Snippet51>
}
namespace RainyDay
{
class JustInCase
{
static void Main()
{
//<Snippet52>
Console.WriteLine("Saving for a rainy day.");
//</Snippet52>
//<Snippet53>
Console.WriteLine("Saving for a rainy day.");
//</Snippet53>
//<Snippet54>
Console.WriteLine("Saving for a rainy day.");
//</Snippet54>
//<Snippet55>
Console.WriteLine("Saving for a rainy day.");
//</Snippet55>
//<Snippet56>
Console.WriteLine("Saving for a rainy day.");
//</Snippet56>
}
}
}
//<snippet70>
namespace OverrideAndNew
{
using System;
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
// The following two calls do what you would expect. They call
// the methods that are defined in BaseClass.
bc.Method1();
bc.Method2();
// Output:
// Base - Method1
// Base - Method2
// The following two calls do what you would expect. They call
// the methods that are defined in DerivedClass.
dc.Method1();
dc.Method2();
// Output:
// Derived - Method1
// Derived - Method2
// The following two calls produce different results for the
// method defined by using override (Method1) and the
// method defined by using new (Method2).
bcdc.Method1();
bcdc.Method2();
// Output:
// Derived - Method1
// Base - Method2
}
}
class BaseClass
{
//<snippet68>
public virtual void Method1()
{
Console.WriteLine("Base - Method1");
}
//</snippet68>
public virtual void Method2()
{
Console.WriteLine("Base - Method2");
}
}
class DerivedClass : BaseClass
{
//<snippet67>
public override void Method1()
{
Console.WriteLine("Derived - Method1");
}
//</snippet67>
//<snippet66>
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
//</snippet66>
}
}
//</snippet70>
//<snippet69>
// Output:
// Base - Method1
// Base - Method2
// Derived - Method1
// Derived - Method2
// Derived - Method1
// Base - Method2
//</snippet69>
namespace pieces
{
using System;
//<snippet61>
class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
}
class DerivedClass : BaseClass
{
public void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
//</snippet61>
//<snippet62>
class Program
{
static void Main(string[] args)
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
bc.Method1();
dc.Method1();
dc.Method2();
bcdc.Method1();
}
// Output:
// Base - Method1
// Base - Method1
// Derived - Method2
// Base - Method1
}
//</snippet62>
class morePieces
{
public void callers()
{
BaseClass bc = new();
DerivedClass dc = new();
BaseClass bcdc = new DerivedClass();
//<snippet64>
bc.Method1();
bc.Method2();
dc.Method1();
dc.Method2();
bcdc.Method1();
bcdc.Method2();
//</snippet64>
//<snippet65>
// Output:
// Base - Method1
// Base - Method2
// Base - Method1
// Derived - Method2
// Base - Method1
// Base - Method2
//</snippet65>
}
public class BaseClass
{
public void Method1()
{
Console.WriteLine("Base - Method1");
}
//<snippet63>
public void Method2()
{
Console.WriteLine("Base - Method2");
}
//</snippet63>
}
class DerivedClass : BaseClass
{
public new void Method2()
{
Console.WriteLine("Derived - Method2");
}
}
}
}
//<snippet78>
namespace OverrideAndNew2
{
using System;
using System.Collections.Generic;
class Program
{
static void Main(string[] args)
{
// Declare objects of the derived classes and test which version
// of ShowDetails is run, base or derived.
TestCars1();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars2();
// Declare objects of the derived classes and call ShowDetails
// directly.
TestCars3();
// Declare objects of the base class, instantiated with the
// derived classes, and repeat the tests.
TestCars4();
}
//<snippet72>
public static void TestCars1()
{
Console.WriteLine("\nTestCars1");
Console.WriteLine("----------");
Car car1 = new();
car1.DescribeCar();
Console.WriteLine("----------");
// Note the output from this test case. The ShowDetails method is
// declared with the new modifier.
ConvertibleCar car2 = new();
car2.DescribeCar();
Console.WriteLine("----------");
Minivan car3 = new();
car3.DescribeCar();
Console.WriteLine("----------");
}
//</snippet72>
//<snippet73>
// Output:
// TestCars1
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet73>
//<snippet74>
public static void TestCars2()
{
Console.WriteLine("\nTestCars2");
Console.WriteLine("----------");
var cars = new List<Car> { new(), new ConvertibleCar(),
new Minivan() };
foreach (Car car in cars)
{
car.DescribeCar();
Console.WriteLine("----------");
}
}
//</snippet74>
//<snippet75>
// Output:
// TestCars2
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Standard transportation.
// ----------
// Four wheels and an engine.
// Carries seven people.
// ----------
//</snippet75>
//<snippet76>
public static void TestCars3()
{
Console.WriteLine("\nTestCars3");
Console.WriteLine("----------");
ConvertibleCar car2 = new();
Minivan car3 = new();
car2.ShowDetails();
car3.ShowDetails();
}
public static void TestCars4()
{
Console.WriteLine("\nTestCars4");
Console.WriteLine("----------");
Car car2 = new ConvertibleCar();
Car car3 = new Minivan();
car2.ShowDetails();
car3.ShowDetails();
}
//</snippet76>
//<snippet77>
// Output:
// TestCars3
// ----------
// A roof that opens up.
// Carries seven people.
// TestCars4
// ----------
// Standard transportation.
// Carries seven people.
//</snippet77>
}
//<snippet71>
// Define the base class, Car. The class defines two methods, DescribeCar
// and ShowDetails. DescribeCar calls ShowDetails, and each derived class
// also defines a ShowDetails method. The example tests which version of
// ShowDetails is selected, the base class method or the derived class method.
class Car
{
public void DescribeCar()
{
Console.WriteLine("Four wheels and an engine.");
ShowDetails();
}
public virtual void ShowDetails()
{
Console.WriteLine("Standard transportation.");
}
}
// Define the derived classes.
// Class ConvertibleCar uses the new modifier to acknowledge that ShowDetails
// hides the base class method.
class ConvertibleCar : Car
{
public new void ShowDetails()
{
Console.WriteLine("A roof that opens up.");
}
}
// Class Minivan uses the override modifier to specify that ShowDetails
// extends the base class method.
class Minivan : Car
{
public override void ShowDetails()
{
Console.WriteLine("Carries seven people.");
}
}
//</snippet71>
}
//</snippet78>