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.

I assume you were asked for the differences?

A static method on a static class can be used to define an extension method. A static method on a non-static class cannot.

In terms of performance and memory usage; precisely nothing. Having a static class means you know there are no instances, but back in 1.1 having a private constructor sufficed. Use a static class if it simply makes no sense to have an instance! (utility classes etc)