In a generic type or method definition, a type parameter is a placeholder for a specific type that a client specifies when they create an instance of the generic type. A generic class, such as GenericList<T> listed in Introduction to Generics, cannot be used as-is because it is not really a type; it is more like a blueprint for a type. To use GenericList<T>, client code must declare and instantiate a constructed type by specifying a type argument inside the angle brackets. The type argument for this particular class can be any type recognized by the compiler. Any number of constructed type instances can be created, each one using a different type argument, as follows:
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideGenerics/CS/Generics.cs7
using System.Collections.Generic;
namespace CsCsrefProgrammingGenerics
{
public class GenericList<T>
{
// The nested class is also generic on T.
private class Node
{
// T used in non-generic constructor.
public Node(T t)
{
data = t;
}
public Node? Next { get; set; }
// T as private member data type.
private T data;
// T as return type of property.
public T Data
{
get { return data; }
set { data = value; }
}
}
private Node? head;
// constructor
public GenericList()
{
head = null;
}
// T as method parameter type:
public void AddHead(T t)
{
Node n = new Node(t);
n.Next = head;
head = n;
}
public IEnumerator<T> GetEnumerator()
{
Node? current = head;
while (current != null)
{
yield return current.Data;
current = current.Next;
}
}
}
class TestGenericList2
{
private class ExampleClass { }
private struct ExampleStruct { }
static void Main()
{
//<Snippet7>
GenericList<float> list1 = new GenericList<float>();
GenericList<ExampleClass> list2 = new GenericList<ExampleClass>();
GenericList<ExampleStruct> list3 = new GenericList<ExampleStruct>();
//</Snippet7>
}
}
//---------------------------------------------------------------------------
public class WrapBenefits
{
public static void Test1()
{
//<Snippet4>
// The .NET Framework 1.1 way to create a list:
System.Collections.ArrayList list1 = new System.Collections.ArrayList();
list1.Add(3);
list1.Add(105);
System.Collections.ArrayList list2 = new System.Collections.ArrayList();
list2.Add("It is raining in Redmond.");
list2.Add("It is snowing in the mountains.");
//</Snippet4>
}
public static void Test2()
{
//<Snippet5>
System.Collections.ArrayList list = new System.Collections.ArrayList();
// Add an integer to the list.
list.Add(3);
// Add a string to the list. This will compile, but may cause an error later.
list.Add("It is raining in Redmond.");
int t = 0;
// This causes an InvalidCastException to be returned.
foreach (int x in list)
{
t += x;
}
//</Snippet5>
}
public static void Test3()
{
//<Snippet6>
// The .NET Framework 2.0 way to create a list
List<int> list1 = new List<int>();
// No boxing, no casting:
list1.Add(3);
// Compile-time error:
// list1.Add("It is raining in Redmond.");
//</Snippet6>
}
}
//---------------------------------------------------------------------------
public class WrapConstraints
{
//<Snippet11>
public class Employee
{
private string name;
private int id;
public Employee(string s, int i)
{
name = s;
id = i;
}
public string Name
{
get { return name; }
set { name = value; }
}
public int ID
{
get { return id; }
set { id = value; }
}
}
public class GenericList<T> where T : Employee
{
private class Node
{
private Node? next;
private T data;
public Node(T t)
{
next = null;
data = t;
}
public Node? Next
{
get { return next; }
set { next = value; }
}
public T Data
{
get { return data; }
set { data = value; }
}
}
private Node? head;
public GenericList() //constructor
{
head = null;
}
public void AddHead(T t)
{
Node n = new Node(t);
n.Next = head;
head = n;
}
public IEnumerator<T> GetEnumerator()
{
Node? current = head;
while (current != null)
{
yield return current.Data;
current = current.Next;
}
}
public T? FindFirstOccurrence(string s)
{
Node? current = head;
T? t = null;
while (current != null)
{
// The constraint enables access to the Name property.
if (current.Data.Name == s)
{
t = current.Data;
break;
}
else
{
current = current.Next;
}
}
return t;
}
}
//</Snippet11>
interface IEmployee { }
//<Snippet12>
class EmployeeList<T> where T : Employee, IEmployee, System.IComparable<T>, new()
{
// ...
}
//</Snippet12>
//<Snippet13>
public static void OpTest<T>(T s, T t) where T : class
{
System.Console.WriteLine(s == t);
}
static void Main()
{
string s1 = "target";
System.Text.StringBuilder sb = new System.Text.StringBuilder("target");
string s2 = sb.ToString();
OpTest<string>(s1, s2);
}
//</Snippet13>
//<Snippet14>
class List<T>
{
void Add<U>(List<U> items) where U : T {/*...*/}
}
//</Snippet14>
//<Snippet15>
//Type parameter V is used as a type constraint.
public class SampleClass<T, U, V> where T : V { }
//</Snippet15>
}
//---------------------------------------------------------------------------
namespace WrapClasses
{
//<Snippet16>
class BaseNode { }
class BaseNodeGeneric<T> { }
// concrete type
class NodeConcrete<T> : BaseNode { }
//closed constructed type
class NodeClosed<T> : BaseNodeGeneric<int> { }
//open constructed type
class NodeOpen<T> : BaseNodeGeneric<T> { }
//</Snippet16>
//<Snippet17>
//No error
class Node1 : BaseNodeGeneric<int> { }
//Generates an error
//class Node2 : BaseNodeGeneric<T> {}
//Generates an error
//class Node3 : T {}
//</Snippet17>
//<Snippet18>
class BaseNodeMultiple<T, U> { }
//No error
class Node4<T> : BaseNodeMultiple<T, int> { }
//No error
class Node5<T, U> : BaseNodeMultiple<T, U> { }
//Generates an error
//class Node6<T> : BaseNodeMultiple<T, U> {}
//</Snippet18>
//<Snippet19>
class NodeItem<T> where T : System.IComparable<T>, new() { }
class SpecialNodeItem<T> : NodeItem<T> where T : System.IComparable<T>, new() { }
//</Snippet19>
//<Snippet20>
class SuperKeyType<K, V, U>
where U : System.IComparable<U>
where V : new()
{ }
//</Snippet20>
class Test
{
//<Snippet21>
void Swap<T>(List<T> list1, List<T> list2)
{
//code to swap items
}
void Swap(List<int> list1, List<int> list2)
{
//code to swap items
}
//</Snippet21>
}
}
//---------------------------------------------------------------------------
namespace WrapMethods
{
public class Test
{
//<Snippet22>
static void Swap<T>(ref T lhs, ref T rhs)
{
T temp;
temp = lhs;
lhs = rhs;
rhs = temp;
}
//</Snippet22>
//<Snippet23>
public static void TestSwap()
{
int a = 1;
int b = 2;
Swap<int>(ref a, ref b);
System.Console.WriteLine(a + " " + b);
}
//</Snippet23>
public static void TestSwapInfer()
{
int a = 1;
int b = 2;
//<Snippet24>
Swap(ref a, ref b);
//</Snippet24>
System.Console.WriteLine(a + " " + b);
}
}
//<Snippet25>
class SampleClass<T>
{
void Swap(ref T lhs, ref T rhs) { }
}
//</Snippet25>
class Test2
{
//<Snippet27>
void SwapIfGreater<T>(ref T lhs, ref T rhs) where T : System.IComparable<T>
{
T temp;
if (lhs.CompareTo(rhs) > 0)
{
temp = lhs;
lhs = rhs;
rhs = temp;
}
}
//</Snippet27>
//<Snippet28>
void DoWork() { }
void DoWork<T>() { }
void DoWork<T, U>() { }
//</Snippet28>
}
class Test3
{
//<Snippet29>
T[] Swap<T>(T a, T b)
{
return [b, a];
}
//</Snippet29>
}
}
//---------------------------------------------------------------------------
namespace WrapArrays
{
//<Snippet35>
class Program
{
static void Main()
{
int[] arr = [0, 1, 2, 3, 4];
List<int> list = new List<int>();
for (int x = 5; x < 10; x++)
{
list.Add(x);
}
ProcessItems<int>(arr);
ProcessItems<int>(list);
}
static void ProcessItems<T>(IList<T> coll)
{
// IsReadOnly returns True for the array and False for the List.
System.Console.WriteLine
("IsReadOnly returns {0} for this collection.",
coll.IsReadOnly);
// The following statement causes a run-time exception for the
// array, but not for the List.
//coll.RemoveAt(4);
foreach (T item in coll)
{
System.Console.Write(item?.ToString() + " ");
}
System.Console.WriteLine();
}
}
//</Snippet35>
}
//---------------------------------------------------------------------------
class WrapDelegates
{
//<Snippet36>
public delegate void Del<T>(T item);
public static void Notify(int i) { }
Del<int> m1 = new Del<int>(Notify);
//</Snippet36>
//<Snippet37>
Del<int> m2 = Notify;
//</Snippet37>
//<Snippet38>
class Stack<T>
{
public delegate void StackDelegate(T[] items);
}
//</Snippet38>
//<Snippet39>
private static void DoWork(float[] items) { }
public static void TestStack()
{
Stack<float> s = new Stack<float>();
Stack<float>.StackDelegate d = DoWork;
}
//</Snippet39>
}
//---------------------------------------------------------------------------
class WrapDelegates2
{
//<Snippet40>
delegate void StackEventHandler<T, U>(T sender, U eventArgs);
class Stack<T>
{
public class StackEventArgs : System.EventArgs { }
public event StackEventHandler<Stack<T>, StackEventArgs>? StackEvent;
protected virtual void OnStackChanged(StackEventArgs a)
{
if (StackEvent is not null)
StackEvent(this, a);
}
}
class SampleClass
{
public void HandleStackChange<T>(Stack<T> stack, Stack<T>.StackEventArgs args) { }
}
public static void Test()
{
Stack<double> s = new Stack<double>();
SampleClass o = new SampleClass();
s.StackEvent += o.HandleStackChange;
}
//</Snippet40>
}
//---------------------------------------------------------------------------
class WrapRuntime
{
//<Snippet43>
Stack<int> stackOne = new Stack<int>();
Stack<int> stackTwo = new Stack<int>();
//</Snippet43>
//<Snippet47>
class Customer { }
class Order { }
//</Snippet47>
public static void Test()
{
//<Snippet44>
Stack<Customer> customers;
//</Snippet44>
//<Snippet45>
Stack<Order> orders = new Stack<Order>();
//</Snippet45>
//<Snippet46>
customers = new Stack<Customer>();
//</Snippet46>
}
}
}In each of these instances of GenericList<T>, every occurrence of T in the class is substituted at run time with the type argument. By means of this substitution, we have created three separate type-safe and efficient objects using a single class definition. For more information on how this substitution is performed by the CLR, see Generics in the Runtime.
You can learn the naming conventions for generic type parameters in the article on naming conventions.