Will using 'var' affect performance?

Question

There's no extra IL code for the var keyword: the resulting IL should be identical for non-anonymous types. If the compiler can't create that IL because it can't figure out what type you intended to use, you'll get a compiler error.

The only trick is that var will infer an exact type where you may have chosen an Interface or parent type if you were to set the type manually.

As Joel says, the compiler works out at compile-time what type var should be, effectively it's just a trick the compiler performs to save keystrokes, so for example

var s = "hi";

gets replaced by

string s = "hi";

by the compiler before any IL is generated. The Generated IL will be exactly the same as if you'd typed string.

As nobody has mentioned reflector yet...

If you compile the following C# code:

static void Main(string[] args)
{
    var x = "hello";
    string y = "hello again!";
    Console.WriteLine(x);
    Console.WriteLine(y);
}

Then use reflector on it, you get:

// Methods
private static void Main(string[] args)
{
    string x = "hello";
    string y = "hello again!";
    Console.WriteLine(x);
    Console.WriteLine(y);
}

So the answer is clearly no runtime performance hit!

For the following method:

   private static void StringVsVarILOutput()
    {
        var string1 = new String(new char[9]);

        string string2 = new String(new char[9]);
    }

The IL Output is this:

        {
          .method private hidebysig static void  StringVsVarILOutput() cil managed
          // Code size       28 (0x1c)
          .maxstack  2
          .locals init ([0] string string1,
                   [1] string string2)
          IL_0000:  nop
          IL_0001:  ldc.i4.s   9
          IL_0003:  newarr     [mscorlib]System.Char
          IL_0008:  newobj     instance void [mscorlib]System.String::.ctor(char[])
          IL_000d:  stloc.0
          IL_000e:  ldc.i4.s   9
          IL_0010:  newarr     [mscorlib]System.Char
          IL_0015:  newobj     instance void [mscorlib]System.String::.ctor(char[])
          IL_001a:  stloc.1
          IL_001b:  ret
        } // end of method Program::StringVsVarILOutput