C# extention methods

Very nice feature.
According to definition: "Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type"

Example:

namespace ExtensionMethods
{
 public static class MyExtensions
 {
     public static int WordCount(this String str)
     {
         return str.Split(new char[] { ' ', '.', '?' }, StringSplitOptions.RemoveEmptyEntries).Length;
     }
 }
}

And use it:

string s = "Hello Extension Methods";
int i = s.WordCount();

Note the following:

1. Extensions should be defined in a static class
2. Extensions should be defined in a static methods
3. First parameter should be "this {class} {var}"