Static Method of a Static Class vs. Static Method of a Non-Static Class ( C# )

Declaring a static class documents your intent for that class to be a collection of static functionality, and anyone adding instance members will get a compilation error.

A non-static class with static members usually indicates that the class is designed to be instantiated at some point. Static methods of these classes usually do one of two things:

  1. Provide a factory method for creating an instance of that type;
  2. Provide helper functionality that does not require an instance of the type;

Also, as mentioned already, extension methods can only be declared on a static class.