C# Implicit Typed Local Variable - var

C# 3.0 introduced the implicit typed local variable "var". Var can only be defined in a method as a local variable. The compiler will infer its type based on the value to the right of the "=" operator.

Example: Explicitly Typed Variable
int i = 100;// explicitly typed 
var j = 100; // implicityly type

The following example shows how var can have a different type based on its value:

Example: Implicit Typed Variable
static void Main(string[] args)
{
    var i = 10;
    Console.WriteLine("Type of i is {0}",i.GetType().ToString());

    var str = "Hello World!!";
    Console.WriteLine("Type of str is {0}", str.GetType().ToString());

    var d = 100.50d;
    Console.WriteLine("Type of d is {0}", d.GetType().ToString());

    var b = true;
    Console.WriteLine("Type of b is {0}", b.GetType().ToString());
}
Output:
Type of i is System.Int32
Type of str is System.String
Type of d is System.Double
Type of b is System.Boolean

Implicitly-typed variables must initialized at the time of declaration, otherwise C# compiler would give an error: Implicitly-typed variables must be initialized.

var i = 100; // Valid
var j; // Compile-time error: Implicitly-typed variables must be initialized

Multiple declerations of var variables in a single statement is not allowed.

var i = 100, j = 200, k = 300; // Compile-time error
var i = 100; var j = 200; var k = 300; // Valid
// the followings are also valid
var i = 100; 
var j = 200; 
var k = 300; 

var cannot be used for function parameters.

void Display(var param) //Compile-time error
{
    Console.Write(param);
}

var variables can be used in the following different contexts:

  • Local variable in a function
  • For loop
  • Foreach loop
  • Using statement
  • As an anonymous type
  • In a LINQ query expression
Points to Remember :
  1. var can only be declared and initialized in a single statement. Following is not valid:
    var i; i = 10;
  2. var cannot be used as a field type at the class level.
  3. var cannot be used in an expression like var i += 10;
  4. Multiple vars cannot be declared and initialized in a single statement. For example, var i=10, j=20; is invalid.
Further Reading