Question

I am wondering if I can use extension method or other techniques to extend static class like
System.Net.Mime.MediaTypeNames.Image, it has fewer type than I need.

Answer 1

No, extension methods can only be used to add instance methods, not static methods (or even properties). Extension methods are really just syntactic sugar around static methods. For instance, when you use an extension method such as Count():

var list = GetList();
var size = list.Count();

This is actually compiled to:

var list = GetList();
var size = Enumerable.Count(list);

You can’t add additional static methods to an existing class using extension methods.