when we use var keyword for variable declaration?

hi,

Var is an implicit type.Implicit means indirect way of defining variable types.
note that var define data type statically (compile time) not run time.

So you can use it anywhere.

Please refer to know more about var.
http://msdn.microsoft.com/en-us/library/bb383973.aspx\

http://www.dotnetfunda.com/articles/article1567-what-is-the-use-of-var-keyword-in-csharp.aspx

hope this will help you.