How do extension methods work under-the-hood?

Question

A contractor where I work is using extension methods to implement CRUD on well-known internal classes that we own. I say it is better to use normal inheritance over extension methods for the following reasons.

• Using extension methods obfuscates, hides & confuses the source of the CRUD methods.
• I assume extension methods make heavy use of reflection (which is slower).

His logic is, "It's compiled, so it's fast." Maybe I'm wrong...but just because it is compiled doesn't mean it doesn't use reflection, nor does it mean it is faster than normal inheritance.

So my questions are:

1. How do extension methods work under-the-hood?
2. Is it better to use inheritance or extension methods on WELL-KNOWN classes that you OWN?

Answer 1

How do extension methods work under-the-hood?

They're just static methods; the compiler rewrites calls like myObject.MyExtensionMethod() to MyExtensionClass.MyExtensionMethod(myObject).

Is it better to use inheretance or extension methods on WELL-KNOWN classes that you OWN?

There's not single answer to this question, it all depends on the context. But usually extension methods are most useful in those cases:

• you don't own the code for the extended type
• the method targets an interface and will be the same for all implementations of this interface (e.g. IEnumerable<T> and Linq extension methods)

Answer 2

I assume extension methods make heavy use of reflection (which is slower).

No. Extension methods are resolved at compile-time, no reflection required.
That negates your performance concerns.

Is it better to use inheretance or extension methods ?

I would say neither. Use a Repository (DAL). An entity should be persistence-agnostic (so: no inheritance from a base that does CRUD) and not pretend to be involved where it's not (no extensions).

You are right that "Using extension methods obfuscates & confuses the source of the CRUD methods" but inheritance is not the solution.