Local variables can be declared without giving an explicit type. The var keyword instructs the compiler to infer the type of the variable from the expression on the right side of the initialization statement. The inferred type may be a built-in type, an anonymous type, a user-defined type, or a type defined in the .NET class library. For more information about how to initialize arrays with var, see Implicitly Typed Arrays.
The following examples show various ways in which local variables can be declared with var:
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideLINQ/CS/csRef30LangFeatures_2.cs43
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
namespace Immutable
{
//how to implement an immutable class
//<snippet30>
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// constructor to initialize its properties.
class Contact
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Public constructor.
public Contact(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
}
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// static method and private constructor to initialize its properties.
public class Contact2
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Private constructor.
private Contact2(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
// Public factory method.
public static Contact2 CreateContact(string name, string address)
{
return new Contact2(name, address);
}
}
public class Program
{
static void Main()
{
// Some simple data sources.
string[] names = {"Terry Adams","Fadi Fakhouri", "Hanying Feng",
"Cesar Garcia", "Debra Garcia"};
string[] addresses = {"123 Main St.", "345 Cypress Ave.", "678 1st Ave",
"12 108th St.", "89 E. 42nd St."};
// Simple query to demonstrate object creation in select clause.
// Create Contact objects by using a constructor.
var query1 = from i in Enumerable.Range(0, 5)
select new Contact(names[i], addresses[i]);
// List elements cannot be modified by client code.
var list = query1.ToList();
foreach (var contact in list)
{
Console.WriteLine("{0}, {1}", contact.Name, contact.Address);
}
// Create Contact2 objects by using a static factory method.
var query2 = from i in Enumerable.Range(0, 5)
select Contact2.CreateContact(names[i], addresses[i]);
// Console output is identical to query1.
var list2 = query2.ToList();
// List elements cannot be modified by client code.
// CS0272:
// list2[0].Name = "Eugene Zabokritski";
// Keep the console open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
/* Output:
Terry Adams, 123 Main St.
Fadi Fakhouri, 345 Cypress Ave.
Hanying Feng, 678 1st Ave
Cesar Garcia, 12 108th St.
Debra Garcia, 89 E. 42nd St.
*/
//</snippet30>
}
namespace csrefLINQExamples
{
//Return Subsets of Element Properties
class AnonymousTypes : StudentClass
{
static void Main(string[] args)
{
QueryByScore();
}
//<snippet31>
private static void QueryByScore()
{
// Create the query. var is required because
// the query produces a sequence of anonymous types.
var queryHighScores =
from student in students
where student.ExamScores[0] > 95
select new { student.FirstName, student.LastName };
// Execute the query.
foreach (var obj in queryHighScores)
{
// The anonymous type's properties were not named. Therefore
// they have the same names as the Student properties.
Console.WriteLine(obj.FirstName + ", " + obj.LastName);
}
}
/* Output:
Adams, Terry
Fakhouri, Fadi
Garcia, Cesar
Omelchenko, Svetlana
Zabokritski, Eugene
*/
//</snippet31>
}
// How to use implicitly typed locals
class ImplicitTyping : StudentClass
{
static void Main(string[] args)
{
QueryNames('S');
}
//<snippet32>
private static void QueryNames(char firstLetter)
{
// Create the query. Use of var is required because
// the query produces a sequence of anonymous types:
// System.Collections.Generic.IEnumerable<????>.
var studentQuery =
from student in students
where student.FirstName[0] == firstLetter
select new { student.FirstName, student.LastName };
// Execute the query and display the results.
foreach (var anonType in studentQuery)
{
Console.WriteLine("First = {0}, Last = {1}", anonType.FirstName, anonType.LastName);
}
}
//</snippet32>
static void QueryMethod(List<Student> students)
{
//how to use implicitly typed locals #2
//<snippet33>
// Variable queryId could be declared by using
// System.Collections.Generic.IEnumerable<string>
// instead of var.
var queryId =
from student in students
where student.Id > 111
select student.LastName;
// Variable str could be declared by using var instead of string.
foreach (string str in queryId)
{
Console.WriteLine($"Last name: {str}");
}
//</snippet33>
}
}
//how to init a dictionary with coll initializer
//<snippet34>
class StudentName
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
}
class CollInit
{
Dictionary<int, StudentName> students = new Dictionary<int, StudentName>()
{
{ 111, new StudentName { FirstName = "Sachin", LastName="Karnik", Id = 211 } },
{ 112, new StudentName { FirstName = "Dina", LastName="Salimzianova", Id = 317 } },
{ 113, new StudentName { FirstName = "Andy", LastName="Ruth", Id = 198 } }
};
}
//</snippet34>
class ObjInit
{
//How to: Initialize Objects without Calling a Constructor (C# Programming Guide)
//<snippet35>
public class Program
{
public static void Main()
{
// Declare a StudentName by using the constructor that has two parameters.
StudentName student1 = new StudentName("Craig", "Playstead");
// Make the same declaration by using an object initializer and sending
// arguments for the first and last names. The default constructor is
// invoked in processing this declaration, not the constructor that has
// two parameters.
StudentName student2 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
};
// Declare a StudentName by using an object initializer and sending
// an argument for only the Id property. No corresponding constructor is
// necessary. Only the default constructor is used to process object
// initializers.
StudentName student3 = new StudentName
{
Id = 183
};
// Declare a StudentName by using an object initializer and sending
// arguments for all three properties. No corresponding constructor is
// defined in the class.
StudentName student4 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
Id = 116
};
System.Console.WriteLine(student1.ToString());
System.Console.WriteLine(student2.ToString());
System.Console.WriteLine(student3.ToString());
System.Console.WriteLine(student4.ToString());
}
// Output:
// Craig 0
// Craig 0
// 183
// Craig 116
}
public class StudentName
{
// The default constructor has no parameters. The default constructor
// is invoked in the processing of object initializers.
// You can test this by changing the access modifier from public to
// private. The declarations in Main that use object initializers will
// fail.
public StudentName() { }
// The following constructor has parameters for two of the three
// properties.
public StudentName(string first, string last)
{
FirstName = first;
LastName = last;
}
// Properties.
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
public override string ToString()
{
return FirstName + " " + Id;
}
}
//</snippet35>
//How to: Initialize Objects without Calling a Constructor #2 (C# Programming Guide)
//<snippet36>
List<StudentName> students = new List<StudentName>()
{
new StudentName { FirstName = "Craig", LastName = "Playstead", Id = 116 },
new StudentName { FirstName = "Shu", LastName="Ito", Id = 112 },
new StudentName { FirstName = "Gretchen", LastName="Rivas", Id = 113 },
new StudentName { FirstName = "Rajesh", LastName="Rotti", Id = 114 }
};
//</snippet36>
}
//Object and collection initializers
//<snippet46>
// The following code consolidates examples from the topic.
class ObjInitializers
{
//<snippet39>
class Cat
{
// Automatically implemented properties.
public int Age { get; set; }
public string Name { get; set; }
}
//</snippet39>
static void Main()
{
//<snippet45>
Cat cat = new Cat { Age = 10, Name = "Fluffy" };
//</snippet45>
//<snippet41>
List<Cat> cats = new List<Cat>
{
new Cat(){ Name = "Sylvester", Age=8 },
new Cat(){ Name = "Whiskers", Age=2 },
new Cat(){ Name = "Sasha", Age=14 }
};
//</snippet41>
//<snippet42>
List<Cat> moreCats = new List<Cat>
{
new Cat(){ Name = "Furrytail", Age=5 },
new Cat(){ Name = "Peaches", Age=4 },
null
};
//</snippet42>
// Display results.
System.Console.WriteLine(cat.Name);
foreach (Cat c in cats)
System.Console.WriteLine(c.Name);
foreach (Cat c in moreCats)
if (c != null)
System.Console.WriteLine(c.Name);
else
System.Console.WriteLine("List element has null value.");
}
// Output:
//Fluffy
//Sylvester
//Whiskers
//Sasha
//Furrytail
//Peaches
//List element has null value.
}
//</snippet46>
class ObjCollInitializers
{
class Product
{
public string ProductName { get; set; }
public decimal UnitPrice { get; set; }
}
void MethodB()
{
List<Product> products = new List<Product>();
//<snippet40>
var productInfos =
from p in products
select new { p.ProductName, p.UnitPrice };
//</snippet40>
}
}
//Implicitly Typed Local Variables Conceptual Topic
class ImplicitlyTypedLocals
{
// compilation hack
class Customer
{
public string City { get; set; }
}
static List<Customer> customers = new List<Customer>();
static void Main()
{
//<snippet43>
// i is compiled as an int
var i = 5;
// s is compiled as a string
var s = "Hello";
// a is compiled as int[]
var a = new[] { 0, 1, 2 };
// expr is compiled as IEnumerable<Customer>
// or perhaps IQueryable<Customer>
var expr =
from c in customers
where c.City == "London"
select c;
// anon is compiled as an anonymous type
var anon = new { Name = "Terry", Age = 34 };
// list is compiled as List<int>
var list = new List<int>();
//</snippet43>
Console.WriteLine(s + i.ToString());
}
}
//<snippet44>
class ImplicitlyTypedLocals2
{
static void Main()
{
string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };
// If a query produces a sequence of anonymous types,
// then use var in the foreach statement to access the properties.
var upperLowerWords =
from w in words
select new { Upper = w.ToUpper(), Lower = w.ToLower() };
// Execute the query
foreach (var ul in upperLowerWords)
{
Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);
}
}
}
/* Outputs:
Uppercase: APPLE, Lowercase: apple
Uppercase: BLUEBERRY, Lowercase: blueberry
Uppercase: CHERRY, Lowercase: cherry
*/
//</snippet44>
// no snippets from this class yet.
//<snippet64>
class CustomJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product { Name="Tea", CategoryId = 001 },
new Product { Name="Mustard", CategoryId = 002 },
new Product { Name="Pickles", CategoryId = 002 },
new Product { Name="Carrots", CategoryId = 003 },
new Product { Name="Bok Choy", CategoryId = 003 },
new Product { Name="Peaches", CategoryId = 005 },
new Product { Name="Melons", CategoryId = 005 },
new Product { Name="Ice Cream", CategoryId = 007 },
new Product { Name="Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
CustomJoins app = new CustomJoins();
app.CrossJoin();
app.NonEquijoin();
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void CrossJoin()
{
var crossJoinQuery =
from c in categories
from p in products
select new { c.Id, p.Name };
Console.WriteLine("Cross Join Query:");
foreach (var v in crossJoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.Id, v.Name);
}
}
void NonEquijoin()
{
var nonEquijoinQuery =
from p in products
let catIds = from c in categories
select c.Id
where catIds.Contains(p.CategoryId) == true
select new { Product = p.Name, CategoryId = p.CategoryId };
Console.WriteLine("Non-equijoin query:");
foreach (var v in nonEquijoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.CategoryId, v.Product);
}
}
}
/* Output:
Cross Join Query:
1 Tea
1 Mustard
1 Pickles
1 Carrots
1 Bok Choy
1 Peaches
1 Melons
1 Ice Cream
1 Mackerel
2 Tea
2 Mustard
2 Pickles
2 Carrots
2 Bok Choy
2 Peaches
2 Melons
2 Ice Cream
2 Mackerel
3 Tea
3 Mustard
3 Pickles
3 Carrots
3 Bok Choy
3 Peaches
3 Melons
3 Ice Cream
3 Mackerel
Non-equijoin query:
1 Tea
2 Mustard
2 Pickles
3 Carrots
3 Bok Choy
Press any key to exit.
*/
//</snippet64>
//<snippet 66>
class Test2
{
// Func<int, bool> f;
bool M(Func<double, bool> func, int i)
{
int local = i;
Console.WriteLine(func(local));
Thread.Sleep(5300);
return func(local);
}
static void Main()
{
Test2 app = new Test2();
bool b = app.M(x => x % 2 == 0, DateTime.Now.Second);
//bool b2 = app.M(x => x +
Console.WriteLine(b.ToString());
Console.ReadKey();
}
}
//</snippet 66>
//reserving numbers 67-79 for anonymous methods
//<snippet80>
class MQ
{
// QueryMethod1 returns a query as its value.
IEnumerable<string> QueryMethod1(ref int[] ints)
{
var intsToStrings = from i in ints
where i > 4
select i.ToString();
return intsToStrings;
}
// QueryMethod2 returns a query as the value of parameter returnQ.
void QueryMethod2(ref int[] ints, out IEnumerable<string> returnQ)
{
var intsToStrings = from i in ints
where i < 4
select i.ToString();
returnQ = intsToStrings;
}
static void Main()
{
MQ app = new MQ();
int[] nums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// QueryMethod1 returns a query as the value of the method.
var myQuery1 = app.QueryMethod1(ref nums);
// Query myQuery1 is executed in the following foreach loop.
Console.WriteLine("Results of executing myQuery1:");
// Rest the mouse pointer over myQuery1 to see its type.
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// You also can execute the query returned from QueryMethod1
// directly, without using myQuery1.
Console.WriteLine("\nResults of executing myQuery1 directly:");
// Rest the mouse pointer over the call to QueryMethod1 to see its
// return type.
foreach (string s in app.QueryMethod1(ref nums))
{
Console.WriteLine(s);
}
IEnumerable<string> myQuery2;
// QueryMethod2 returns a query as the value of its out parameter.
app.QueryMethod2(ref nums, out myQuery2);
// Execute the returned query.
Console.WriteLine("\nResults of executing myQuery2:");
foreach (string s in myQuery2)
{
Console.WriteLine(s);
}
// You can modify a query by using query composition. A saved query
// is nested inside a new query definition that revises the results
// of the first query.
myQuery1 = from item in myQuery1
orderby item descending
select item;
// Execute the modified query.
Console.WriteLine("\nResults of executing modified myQuery1:");
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet80>
//<snippet81>
class HowToOrderJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name="Condiments", Id = 002 },
new Category() { Name="Vegetables", Id = 003 },
new Category() { Name="Grains", Id = 004 },
new Category() { Name="Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
HowToOrderJoins app = new HowToOrderJoins();
app.OrderJoin1();
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void OrderJoin1()
{
var groupJoinQuery2 =
from category in categories
join prod in products on category.Id equals prod.CategoryId into prodGroup
orderby category.Name
select new
{
Category = category.Name,
Products = from prod2 in prodGroup
orderby prod2.Name
select prod2
};
foreach (var productGroup in groupJoinQuery2)
{
Console.WriteLine(productGroup.Category);
foreach (var prodItem in productGroup.Products)
{
Console.WriteLine(" {0,-10} {1}", prodItem.Name, prodItem.CategoryId);
}
}
}
/* Output:
Beverages
Cola 1
Tea 1
Condiments
Mustard 2
Pickles 2
Fruit
Melons 5
Peaches 5
Grains
Vegetables
Bok Choy 3
Carrots 3
*/
}
//</snippet81>
class NullValuesInQueries
{
#region Data
class Product
{
public string Name { get; set; }
public int? CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int? Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
new Category() { Name = "Grains", Id = 004 },
new Category() { Name = "Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
NullValuesInQueries app = new NullValuesInQueries();
Northwind db = new Northwind();
app.TestMethod(db);
}
void NullHandlingQuery()
{
//<snippet82>
var query1 =
from c in categories
where c != null
join p in products on c.Id equals
(p == null ? null : p.CategoryId)
select new { Category = c.Name, Name = p.Name };
//</snippet82>
}
#region compilation only
class Order
{
public int? OrderId = 1;
public int? EmployeeId = 1;
}
class Employee
{
public int EmployeeId = 1;
public string FirstName = string.Empty;
}
class Northwind
{
public Order[] Orders = null;
public Employee[] Employees = null;
}
#endregion
//<snippet83>
void TestMethod(Northwind db)
{
var query =
from o in db.Orders
join e in db.Employees
on o.EmployeeId equals (int?)e.EmployeeId
select new { o.OrderId, e.FirstName };
}
//</snippet83>
}
//<snippet 84 >
class DynamicPreds2
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category(){ Name = "Beverages", Id = 001 },
new Category(){ Name = "Condiments", Id = 002 },
new Category(){ Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
new Product{ Name = "Ice Cream", CategoryId = 007 },
new Product{ Name = "Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
DynamicPreds2 app = new DynamicPreds2();
var q = from p in app.products
select p;
// Identity select
Console.WriteLine("First iteration");
//foreach(var v in query)
// Console.WriteLine(v.Name);
// Mod 1
var q2 = from item in q
where item.Name[0] == 'M'
select item;
Console.WriteLine("2nd iteration");
foreach (var v in q2)
Console.WriteLine(v.Name);
//var q3 = from item in
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet 84>
}It is important to understand that the var keyword does not mean "variant" and does not indicate that the variable is loosely typed, or late-bound. It just means that the compiler determines and assigns the most appropriate type.
The var keyword may be used in the following contexts:
On local variables (variables declared at method scope) as shown in the previous example.
In a for initialization statement.
for (var x = 1; x < 10; x++)In a foreach initialization statement.
foreach (var item in list) {...}In a using statement.
using (var file = new StreamReader("C:\\myfile.txt")) {...}
For more information, see How to use implicitly typed local variables and arrays in a query expression.
var and anonymous types
In many cases the use of var is optional and is just a syntactic convenience. However, when a variable is initialized with an anonymous type you must declare the variable as var if you need to access the properties of the object at a later point. This is a common scenario in LINQ query expressions. For more information, see Anonymous Types.
From the perspective of your source code, an anonymous type has no name. Therefore, if a query variable has been initialized with var, then the only way to access the properties in the returned sequence of objects is to use var as the type of the iteration variable in the foreach statement.
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideLINQ/CS/csRef30LangFeatures_2.cs44
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
namespace Immutable
{
//how to implement an immutable class
//<snippet30>
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// constructor to initialize its properties.
class Contact
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Public constructor.
public Contact(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
}
// This class is immutable. After an object is created,
// it cannot be modified from outside the class. It uses a
// static method and private constructor to initialize its properties.
public class Contact2
{
// Read-only properties.
public string Name { get; private set; }
public string Address { get; private set; }
// Private constructor.
private Contact2(string contactName, string contactAddress)
{
Name = contactName;
Address = contactAddress;
}
// Public factory method.
public static Contact2 CreateContact(string name, string address)
{
return new Contact2(name, address);
}
}
public class Program
{
static void Main()
{
// Some simple data sources.
string[] names = {"Terry Adams","Fadi Fakhouri", "Hanying Feng",
"Cesar Garcia", "Debra Garcia"};
string[] addresses = {"123 Main St.", "345 Cypress Ave.", "678 1st Ave",
"12 108th St.", "89 E. 42nd St."};
// Simple query to demonstrate object creation in select clause.
// Create Contact objects by using a constructor.
var query1 = from i in Enumerable.Range(0, 5)
select new Contact(names[i], addresses[i]);
// List elements cannot be modified by client code.
var list = query1.ToList();
foreach (var contact in list)
{
Console.WriteLine("{0}, {1}", contact.Name, contact.Address);
}
// Create Contact2 objects by using a static factory method.
var query2 = from i in Enumerable.Range(0, 5)
select Contact2.CreateContact(names[i], addresses[i]);
// Console output is identical to query1.
var list2 = query2.ToList();
// List elements cannot be modified by client code.
// CS0272:
// list2[0].Name = "Eugene Zabokritski";
// Keep the console open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
/* Output:
Terry Adams, 123 Main St.
Fadi Fakhouri, 345 Cypress Ave.
Hanying Feng, 678 1st Ave
Cesar Garcia, 12 108th St.
Debra Garcia, 89 E. 42nd St.
*/
//</snippet30>
}
namespace csrefLINQExamples
{
//Return Subsets of Element Properties
class AnonymousTypes : StudentClass
{
static void Main(string[] args)
{
QueryByScore();
}
//<snippet31>
private static void QueryByScore()
{
// Create the query. var is required because
// the query produces a sequence of anonymous types.
var queryHighScores =
from student in students
where student.ExamScores[0] > 95
select new { student.FirstName, student.LastName };
// Execute the query.
foreach (var obj in queryHighScores)
{
// The anonymous type's properties were not named. Therefore
// they have the same names as the Student properties.
Console.WriteLine(obj.FirstName + ", " + obj.LastName);
}
}
/* Output:
Adams, Terry
Fakhouri, Fadi
Garcia, Cesar
Omelchenko, Svetlana
Zabokritski, Eugene
*/
//</snippet31>
}
// How to use implicitly typed locals
class ImplicitTyping : StudentClass
{
static void Main(string[] args)
{
QueryNames('S');
}
//<snippet32>
private static void QueryNames(char firstLetter)
{
// Create the query. Use of var is required because
// the query produces a sequence of anonymous types:
// System.Collections.Generic.IEnumerable<????>.
var studentQuery =
from student in students
where student.FirstName[0] == firstLetter
select new { student.FirstName, student.LastName };
// Execute the query and display the results.
foreach (var anonType in studentQuery)
{
Console.WriteLine("First = {0}, Last = {1}", anonType.FirstName, anonType.LastName);
}
}
//</snippet32>
static void QueryMethod(List<Student> students)
{
//how to use implicitly typed locals #2
//<snippet33>
// Variable queryId could be declared by using
// System.Collections.Generic.IEnumerable<string>
// instead of var.
var queryId =
from student in students
where student.Id > 111
select student.LastName;
// Variable str could be declared by using var instead of string.
foreach (string str in queryId)
{
Console.WriteLine($"Last name: {str}");
}
//</snippet33>
}
}
//how to init a dictionary with coll initializer
//<snippet34>
class StudentName
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
}
class CollInit
{
Dictionary<int, StudentName> students = new Dictionary<int, StudentName>()
{
{ 111, new StudentName { FirstName = "Sachin", LastName="Karnik", Id = 211 } },
{ 112, new StudentName { FirstName = "Dina", LastName="Salimzianova", Id = 317 } },
{ 113, new StudentName { FirstName = "Andy", LastName="Ruth", Id = 198 } }
};
}
//</snippet34>
class ObjInit
{
//How to: Initialize Objects without Calling a Constructor (C# Programming Guide)
//<snippet35>
public class Program
{
public static void Main()
{
// Declare a StudentName by using the constructor that has two parameters.
StudentName student1 = new StudentName("Craig", "Playstead");
// Make the same declaration by using an object initializer and sending
// arguments for the first and last names. The default constructor is
// invoked in processing this declaration, not the constructor that has
// two parameters.
StudentName student2 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
};
// Declare a StudentName by using an object initializer and sending
// an argument for only the Id property. No corresponding constructor is
// necessary. Only the default constructor is used to process object
// initializers.
StudentName student3 = new StudentName
{
Id = 183
};
// Declare a StudentName by using an object initializer and sending
// arguments for all three properties. No corresponding constructor is
// defined in the class.
StudentName student4 = new StudentName
{
FirstName = "Craig",
LastName = "Playstead",
Id = 116
};
System.Console.WriteLine(student1.ToString());
System.Console.WriteLine(student2.ToString());
System.Console.WriteLine(student3.ToString());
System.Console.WriteLine(student4.ToString());
}
// Output:
// Craig 0
// Craig 0
// 183
// Craig 116
}
public class StudentName
{
// The default constructor has no parameters. The default constructor
// is invoked in the processing of object initializers.
// You can test this by changing the access modifier from public to
// private. The declarations in Main that use object initializers will
// fail.
public StudentName() { }
// The following constructor has parameters for two of the three
// properties.
public StudentName(string first, string last)
{
FirstName = first;
LastName = last;
}
// Properties.
public string FirstName { get; set; }
public string LastName { get; set; }
public int Id { get; set; }
public override string ToString()
{
return FirstName + " " + Id;
}
}
//</snippet35>
//How to: Initialize Objects without Calling a Constructor #2 (C# Programming Guide)
//<snippet36>
List<StudentName> students = new List<StudentName>()
{
new StudentName { FirstName = "Craig", LastName = "Playstead", Id = 116 },
new StudentName { FirstName = "Shu", LastName="Ito", Id = 112 },
new StudentName { FirstName = "Gretchen", LastName="Rivas", Id = 113 },
new StudentName { FirstName = "Rajesh", LastName="Rotti", Id = 114 }
};
//</snippet36>
}
//Object and collection initializers
//<snippet46>
// The following code consolidates examples from the topic.
class ObjInitializers
{
//<snippet39>
class Cat
{
// Automatically implemented properties.
public int Age { get; set; }
public string Name { get; set; }
}
//</snippet39>
static void Main()
{
//<snippet45>
Cat cat = new Cat { Age = 10, Name = "Fluffy" };
//</snippet45>
//<snippet41>
List<Cat> cats = new List<Cat>
{
new Cat(){ Name = "Sylvester", Age=8 },
new Cat(){ Name = "Whiskers", Age=2 },
new Cat(){ Name = "Sasha", Age=14 }
};
//</snippet41>
//<snippet42>
List<Cat> moreCats = new List<Cat>
{
new Cat(){ Name = "Furrytail", Age=5 },
new Cat(){ Name = "Peaches", Age=4 },
null
};
//</snippet42>
// Display results.
System.Console.WriteLine(cat.Name);
foreach (Cat c in cats)
System.Console.WriteLine(c.Name);
foreach (Cat c in moreCats)
if (c != null)
System.Console.WriteLine(c.Name);
else
System.Console.WriteLine("List element has null value.");
}
// Output:
//Fluffy
//Sylvester
//Whiskers
//Sasha
//Furrytail
//Peaches
//List element has null value.
}
//</snippet46>
class ObjCollInitializers
{
class Product
{
public string ProductName { get; set; }
public decimal UnitPrice { get; set; }
}
void MethodB()
{
List<Product> products = new List<Product>();
//<snippet40>
var productInfos =
from p in products
select new { p.ProductName, p.UnitPrice };
//</snippet40>
}
}
//Implicitly Typed Local Variables Conceptual Topic
class ImplicitlyTypedLocals
{
// compilation hack
class Customer
{
public string City { get; set; }
}
static List<Customer> customers = new List<Customer>();
static void Main()
{
//<snippet43>
// i is compiled as an int
var i = 5;
// s is compiled as a string
var s = "Hello";
// a is compiled as int[]
var a = new[] { 0, 1, 2 };
// expr is compiled as IEnumerable<Customer>
// or perhaps IQueryable<Customer>
var expr =
from c in customers
where c.City == "London"
select c;
// anon is compiled as an anonymous type
var anon = new { Name = "Terry", Age = 34 };
// list is compiled as List<int>
var list = new List<int>();
//</snippet43>
Console.WriteLine(s + i.ToString());
}
}
//<snippet44>
class ImplicitlyTypedLocals2
{
static void Main()
{
string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };
// If a query produces a sequence of anonymous types,
// then use var in the foreach statement to access the properties.
var upperLowerWords =
from w in words
select new { Upper = w.ToUpper(), Lower = w.ToLower() };
// Execute the query
foreach (var ul in upperLowerWords)
{
Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);
}
}
}
/* Outputs:
Uppercase: APPLE, Lowercase: apple
Uppercase: BLUEBERRY, Lowercase: blueberry
Uppercase: CHERRY, Lowercase: cherry
*/
//</snippet44>
// no snippets from this class yet.
//<snippet64>
class CustomJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product { Name="Tea", CategoryId = 001 },
new Product { Name="Mustard", CategoryId = 002 },
new Product { Name="Pickles", CategoryId = 002 },
new Product { Name="Carrots", CategoryId = 003 },
new Product { Name="Bok Choy", CategoryId = 003 },
new Product { Name="Peaches", CategoryId = 005 },
new Product { Name="Melons", CategoryId = 005 },
new Product { Name="Ice Cream", CategoryId = 007 },
new Product { Name="Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
CustomJoins app = new CustomJoins();
app.CrossJoin();
app.NonEquijoin();
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void CrossJoin()
{
var crossJoinQuery =
from c in categories
from p in products
select new { c.Id, p.Name };
Console.WriteLine("Cross Join Query:");
foreach (var v in crossJoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.Id, v.Name);
}
}
void NonEquijoin()
{
var nonEquijoinQuery =
from p in products
let catIds = from c in categories
select c.Id
where catIds.Contains(p.CategoryId) == true
select new { Product = p.Name, CategoryId = p.CategoryId };
Console.WriteLine("Non-equijoin query:");
foreach (var v in nonEquijoinQuery)
{
Console.WriteLine("{0,-5}{1}", v.CategoryId, v.Product);
}
}
}
/* Output:
Cross Join Query:
1 Tea
1 Mustard
1 Pickles
1 Carrots
1 Bok Choy
1 Peaches
1 Melons
1 Ice Cream
1 Mackerel
2 Tea
2 Mustard
2 Pickles
2 Carrots
2 Bok Choy
2 Peaches
2 Melons
2 Ice Cream
2 Mackerel
3 Tea
3 Mustard
3 Pickles
3 Carrots
3 Bok Choy
3 Peaches
3 Melons
3 Ice Cream
3 Mackerel
Non-equijoin query:
1 Tea
2 Mustard
2 Pickles
3 Carrots
3 Bok Choy
Press any key to exit.
*/
//</snippet64>
//<snippet 66>
class Test2
{
// Func<int, bool> f;
bool M(Func<double, bool> func, int i)
{
int local = i;
Console.WriteLine(func(local));
Thread.Sleep(5300);
return func(local);
}
static void Main()
{
Test2 app = new Test2();
bool b = app.M(x => x % 2 == 0, DateTime.Now.Second);
//bool b2 = app.M(x => x +
Console.WriteLine(b.ToString());
Console.ReadKey();
}
}
//</snippet 66>
//reserving numbers 67-79 for anonymous methods
//<snippet80>
class MQ
{
// QueryMethod1 returns a query as its value.
IEnumerable<string> QueryMethod1(ref int[] ints)
{
var intsToStrings = from i in ints
where i > 4
select i.ToString();
return intsToStrings;
}
// QueryMethod2 returns a query as the value of parameter returnQ.
void QueryMethod2(ref int[] ints, out IEnumerable<string> returnQ)
{
var intsToStrings = from i in ints
where i < 4
select i.ToString();
returnQ = intsToStrings;
}
static void Main()
{
MQ app = new MQ();
int[] nums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// QueryMethod1 returns a query as the value of the method.
var myQuery1 = app.QueryMethod1(ref nums);
// Query myQuery1 is executed in the following foreach loop.
Console.WriteLine("Results of executing myQuery1:");
// Rest the mouse pointer over myQuery1 to see its type.
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// You also can execute the query returned from QueryMethod1
// directly, without using myQuery1.
Console.WriteLine("\nResults of executing myQuery1 directly:");
// Rest the mouse pointer over the call to QueryMethod1 to see its
// return type.
foreach (string s in app.QueryMethod1(ref nums))
{
Console.WriteLine(s);
}
IEnumerable<string> myQuery2;
// QueryMethod2 returns a query as the value of its out parameter.
app.QueryMethod2(ref nums, out myQuery2);
// Execute the returned query.
Console.WriteLine("\nResults of executing myQuery2:");
foreach (string s in myQuery2)
{
Console.WriteLine(s);
}
// You can modify a query by using query composition. A saved query
// is nested inside a new query definition that revises the results
// of the first query.
myQuery1 = from item in myQuery1
orderby item descending
select item;
// Execute the modified query.
Console.WriteLine("\nResults of executing modified myQuery1:");
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet80>
//<snippet81>
class HowToOrderJoins
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name="Condiments", Id = 002 },
new Category() { Name="Vegetables", Id = 003 },
new Category() { Name="Grains", Id = 004 },
new Category() { Name="Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
HowToOrderJoins app = new HowToOrderJoins();
app.OrderJoin1();
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
void OrderJoin1()
{
var groupJoinQuery2 =
from category in categories
join prod in products on category.Id equals prod.CategoryId into prodGroup
orderby category.Name
select new
{
Category = category.Name,
Products = from prod2 in prodGroup
orderby prod2.Name
select prod2
};
foreach (var productGroup in groupJoinQuery2)
{
Console.WriteLine(productGroup.Category);
foreach (var prodItem in productGroup.Products)
{
Console.WriteLine(" {0,-10} {1}", prodItem.Name, prodItem.CategoryId);
}
}
}
/* Output:
Beverages
Cola 1
Tea 1
Condiments
Mustard 2
Pickles 2
Fruit
Melons 5
Peaches 5
Grains
Vegetables
Bok Choy 3
Carrots 3
*/
}
//</snippet81>
class NullValuesInQueries
{
#region Data
class Product
{
public string Name { get; set; }
public int? CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int? Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category() { Name = "Beverages", Id = 001 },
new Category() { Name = "Condiments", Id = 002 },
new Category() { Name = "Vegetables", Id = 003 },
new Category() { Name = "Grains", Id = 004 },
new Category() { Name = "Fruit", Id = 005 }
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Cola", CategoryId = 001 },
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
};
#endregion
static void Main()
{
NullValuesInQueries app = new NullValuesInQueries();
Northwind db = new Northwind();
app.TestMethod(db);
}
void NullHandlingQuery()
{
//<snippet82>
var query1 =
from c in categories
where c != null
join p in products on c.Id equals
(p == null ? null : p.CategoryId)
select new { Category = c.Name, Name = p.Name };
//</snippet82>
}
#region compilation only
class Order
{
public int? OrderId = 1;
public int? EmployeeId = 1;
}
class Employee
{
public int EmployeeId = 1;
public string FirstName = string.Empty;
}
class Northwind
{
public Order[] Orders = null;
public Employee[] Employees = null;
}
#endregion
//<snippet83>
void TestMethod(Northwind db)
{
var query =
from o in db.Orders
join e in db.Employees
on o.EmployeeId equals (int?)e.EmployeeId
select new { o.OrderId, e.FirstName };
}
//</snippet83>
}
//<snippet 84 >
class DynamicPreds2
{
#region Data
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Category
{
public string Name { get; set; }
public int Id { get; set; }
}
// Specify the first data source.
List<Category> categories = new List<Category>()
{
new Category(){ Name = "Beverages", Id = 001 },
new Category(){ Name = "Condiments", Id = 002 },
new Category(){ Name = "Vegetables", Id = 003 },
};
// Specify the second data source.
List<Product> products = new List<Product>()
{
new Product{ Name = "Tea", CategoryId = 001 },
new Product{ Name = "Mustard", CategoryId = 002 },
new Product{ Name = "Pickles", CategoryId = 002 },
new Product{ Name = "Carrots", CategoryId = 003 },
new Product{ Name = "Bok Choy", CategoryId = 003 },
new Product{ Name = "Peaches", CategoryId = 005 },
new Product{ Name = "Melons", CategoryId = 005 },
new Product{ Name = "Ice Cream", CategoryId = 007 },
new Product{ Name = "Mackerel", CategoryId = 012 },
};
#endregion
static void Main()
{
DynamicPreds2 app = new DynamicPreds2();
var q = from p in app.products
select p;
// Identity select
Console.WriteLine("First iteration");
//foreach(var v in query)
// Console.WriteLine(v.Name);
// Mod 1
var q2 = from item in q
where item.Name[0] == 'M'
select item;
Console.WriteLine("2nd iteration");
foreach (var v in q2)
Console.WriteLine(v.Name);
//var q3 = from item in
// Keep console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
//</snippet 84>
}Remarks
The following restrictions apply to implicitly-typed variable declarations:
varcan only be used when a local variable is declared and initialized in the same statement; the variable cannot be initialized to null, or to a method group or an anonymous function.varcannot be used on fields at class scope.Variables declared by using
varcannot be used in the initialization expression. In other words, this expression is legal:int i = (i = 20);but this expression produces a compile-time error:var i = (i = 20);Multiple implicitly-typed variables cannot be initialized in the same statement.
If a type named
varis in scope, then thevarkeyword will resolve to that type name and will not be treated as part of an implicitly typed local variable declaration.
Implicit typing with the var keyword can only be applied to variables at local method scope. Implicit typing is not available for class fields as the C# compiler would encounter a logical paradox as it processed the code: the compiler needs to know the type of the field, but it cannot determine the type until the assignment expression is analyzed, and the expression cannot be evaluated without knowing the type. Consider the following code:
private var bookTitles;bookTitles is a class field given the type var. As the field has no expression to evaluate, it is impossible for the compiler to infer what type bookTitles is supposed to be. In addition, adding an expression to the field (like you would for a local variable) is also insufficient:
private var bookTitles = new List<string>();When the compiler encounters fields during code compilation, it records each field's type before processing any expressions associated with it. The compiler encounters the same paradox trying to parse bookTitles: it needs to know the type of the field, but the compiler would normally determine var's type by analyzing the expression, which isn't possible without knowing the type beforehand.
You may find that var can also be useful with query expressions in which the exact constructed type of the query variable is difficult to determine. This can occur with grouping and ordering operations.
The var keyword can also be useful when the specific type of the variable is tedious to type on the keyboard, or is obvious, or does not add to the readability of the code. One example where var is helpful in this manner is with nested generic types such as those used with group operations. In the following query, the type of the query variable is IEnumerable<IGrouping<string, Student>>. As long as you and others who must maintain your code understand this, there is no problem with using implicit typing for convenience and brevity.
var studentQuery =
from student in students
group student by student.Last;The use of var helps simplify your code, but its use should be restricted to cases where it is required, or when it makes your code easier to read. For more information about when to use var properly, see the Implicitly typed local variables section on the C# Coding Guidelines article.