Partial Class vs Extension Method

Question

I dont have much experience of using these 2 ways to extend a class or create extension methods against a class. By looking others work, I have a question here.

I saw people using a partial class to extend an entity class in a project. Meanwhile, in the same project, there is another folder containing a lot extension methods to the entity class.

Is it right to do so? I mean these 2 ways both work well. Could you give me some real idea of how to pick one or the other when I want extend a class?

Answer 1

Some of differences that will determine whether you want to use a Partial Class or an Extension Method are

Partial Class

• Only works against classes in the same project/assembly
• Target class has to be marked as partial
• Has access to the Target class' fields and protected members
• Target must be a class implementation

Extension Method

• Can be applied against classes in other assembles
• Must be static, has access to only the Target classes public members
• Target of extension can be a concrete type, or an abstract type or interface

Answer 2

Partial classes should be used in code generation scenarios.

Since the generated file might get overwritten at any time, one uses partial classes to write into the non-generated file.

Additionally, partials will only work if they are part of the same assembly - they cannot cross assembly boundaries.

If these are not your constraints, you can and should use extension methods - of course, after considering other possibilities such as inheritance and composition for suitability.