Pros & Cons of var datatype in .net 3.5

Answer 1

These are the arguements for it:

It induces better naming for local variables. When you read local variable declaration with explicit type, you have more information at that moment and something like "IUnitTestElement current" makes sense. However, when this local variable is used later, you read "current" which takes some time to figure out the meaning. Using "var currentElement" makes it easier to read at any place.

It induces better API. When you let compiler deduce type from method return type or property type, you have to have good types in the first place. When you don't have explicit type in the initialization expression, you have to have best names for members.

It induces variable initialization. It is generally a good practice to initialize variable in the declaration, and compiler needs initializer to infer type for local variable declared with "var" keyword.

It removes code noise. There are a lot of cases, when implicitly typed local will reduce amount of text developer needs to read, or rather skip. Declaring local variable from new object expression or cast expression requires specifying type twice, if we don't use "var". With generics it can lead to a lot of otherwise redundant code. Another example would be iteration variable in foreach over Dictionary.

It doesn't require using directive. With var, you don't have explicit reference to type, as compiler infers type for you, so you don't need to import namespace when you need a temporary variable.

From http://resharper.blogspot.com/2008/03/varification-using-implicitly-typed.html

The cons are potentially less readable code. For instance the line int myInt = 0; is still more straightforward for most than var myInt = 0; but this is mainly due to the syntax we're all been looking at for years.

Answer 2

var is not a data type, it is simply "syntactic sugar" for "let-the-compiler-infer-at-compile-time-what-actual-type-to-use".

So, you just need to understand the following type inferences:

var x = 4; //int
var y = 4.0; //double
var z = 4M; //decimal
var w = (string)null; //string