If an extension method has the same signature as a method in the sealed class, what is the call precedence ?

Question

I was reading about extension methods in C# 3.0. The text I'm reading implies that an extension method with the same signature as a method in the class being extended would be second in order of execution - that is, the method in the sealed class gets called. If this is the case, how can you extend the sealed class ?

Answer 1

Indeed, the actual method takes precedence over the extension method. And just to make it clear - "order of execution" suggests both might be called; only the original method will be invoked. Perhaps pick another name / signature; you can't use extension methods to monkey-patch, if that is your intent.

If there is some base-class / interface (that the type implements) that doesn't have this method, you could perhaps cast it to there...?

Answer 2

Use another method signature. Extension methods imply that you are extending the sealed class with new functionality and not overriding the ones already implemented.

Extension methods have "hide-by-name" semantics with instance members on a type. This means that any accessible instance member on a type will always shadow any extension methods with the same name, even if the extension method is a better fit. As a result, if an instance member is ever added to a type with the same name as an extension method, then the extension method can be rendered uncallable.

For more details, take a look at this post: Extension Methods Best Practices (Extension Methods Part 6)