C# Implicitly-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.

int i = 100;// explicitly typed 
var j = 100; // implicitly typed

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

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());
}

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.