问题
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 ?
回答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...?
回答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)
来源：https://stackoverflow.com/questions/2303885/if-an-extension-method-has-the-same-signature-as-a-method-in-the-sealed-class-w