Implicitly Typed Local Variables

Local variables can be given an inferred "type" of var instead of 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 Framework class library. For more information about how to initialize arrays with var, see Implicitly Typed Arrays (C# Programming Guide).

The following examples show various ways in which local variables can be declared with var:

C#

// 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
// or perhaps IQueryable
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                             
var list = new List<int>();

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 (C# Programming Guide).

var and Anonymous Types

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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 (C# Programming Guide).

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.

C#

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        
 */