Extension Methods vs. Regular Methods - Best Practice Ideas

Question

I'm having a bit of a difficult time determining when to implement a method as an extension method and when to implement a method as a stand-alone method. What are some best practices people follow in determining this?

Answer 1

Use an extension method if any of the following conditions are true:

• You need a method on a type and you don't own the source.
• You need a method on a type, you do own the source, and the type is an interface.
• You need a method on a type, you do own the source, but adding the method creates undesired coupling.*

Otherwise, you should use a real method on the actual type itself.

I don't think it makes a whole lot of sense to create an extension method for a class or struct that you own the source for - why confuse readers with an extension method when a regular method will suffice?

Suggested reading: Framework Design Guidelines: Extension Methods

* Imagine that you wanted to add convenience methods to a type but don't want to create dependencies to assemblies or types that shouldn't be part of the API. You could use extension methods to manage this.