Friday, October 22, 2010

Implicitly typed local variables

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In an implicitly typed local variable declaration, the type of the local variable being declared is inferred from the expression used to initialize the variable. When a local variable declaration specifies var as the type and no type named var is in scope, the declaration is an implicitly typed local variable declaration. For example:
var i = 5;
var s = "Hello";
var d = 1.0;
var numbers = new int[] {1, 2, 3};
var orders = new Dictionary();
The implicitly typed local variable declarations above are precisely equivalent to the following explicitly typed declarations:
int i = 5;
string s = "Hello";
double d = 1.0;
int[] numbers = new int[] {1, 2, 3};
Dictionary orders = new Dictionary();
A local variable declarator in an implicitly typed local variable declaration is subject to the following restrictions:

  • The declarator must include an initializer.
  • The initializer must be an expression. The initializer cannot be an object or collection initializer by itself, but it can be a new expression that includes an object or collection initializer.
  • The compile-time type of the initializer expression cannot be the null type.
  • If the local variable declaration includes multiple declarators, the initializers must all have the same compile-time type.


The following are examples of incorrect implicitly typed local variable declarations:
var x; // Error, no initializer to infer type from
var y = {1, 2, 3}; // Error, collection initializer not permitted
var z = null; // Error, null type not permitted