Sunday, September 28, 2014

var - Implicitly Typed Local Variables

 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.

E.g. ways in which local variables can be declared with var
var a = 1; // a is compiled as int
var b = "Narendra"; // b is compiled as string
var c = new List<int>(); // c is compiled as List<int>
var d = new { Name = "Naren", Age = 27 }; // d is compiled as an anonymous type

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.

E.g. var myVariable = 1; // myVariable is compiled as int
If I try to assign it differnt value than int as  :
myVariable = "some string"; then it will throw error as " Error: Cannot implicitly convert type 'string' to 'int' "               

Following restrictions apply to implicitly-typed variable declarations:
·         var can 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.

NOTE: The compile-time type value of var variable cannot be null but the runtime value can be null.
E.g. If we try var a = null; It throws error " Error: Cannot assign <null> to an implicitly-typed local variable."
But following is valid statement
           string s=null;
            var a = s;            
·         var cannot be used on fields at class scope.
·         Multiple implicitly-typed variables cannot be initialized in the same statement.
            E.g. var param1 = 1, param2 = 2; --> This is not allowed. This will throw compilation error "Error: Implicitly-typed local variables cannot have multiple declarators"
Following is valid declaration.
  var param1 = 1;
 var param2 = 2;

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