object and var difference in C#

What is the difference between object 开发者_开发百科and var?

• var - Not specifying the type explicitly. Letting compiler figure out what that type is.
  • Type is fixed at design time and cannot refer to object of other type.
  • As Pauli noted in a comment, you get intelliSense.
  • Must be initialized. var i; won't compile.
  • Cannot be used as return type of a method.
  • Must be a local variable. Not a field or property.
  • Works great with Anonymous Types. You get intelliSense.
• object - System.Object.
  
  • Can be used to refer any type at runtime.
  • Here you don't get intelliSense.

Example:

var i = 0; // i is of type `System.Int32`.  Same as "int i = 0;"
i = "Some String"; // Compile time error.

object o = 0;  
o = "Some String"; // Works

• object will be determined in runtime, but var determined in compile time.

for example:

var i = 2;
object j = 2;

and you look at it in ildasm:

  IL_0000:  nop
  IL_0001:  ldc.i4.2
  IL_0002:  stloc.0
  IL_0003:  ldc.i4.2
  IL_0004:  box        [mscorlib]System.Int32
  IL_0009:  stloc.1

You can see object item should be boxed and var item no need to boxing.

MSDN for object and var

• Also you can do:

     object i;
     i = 2;
  

  but you can't do:

     var i;
     i = 2;
  

  you will get compile error.

• Object is type which all things in .Net inherited from it, so you can do object x = y for any type of y because of inheritance, but var is a keyword for implicit type definition, for example var i = 2 means int i = 2.