How to override an existing extension method

UPDATE: This question was the subject of my blog in December of 2013. Thanks for the great question!

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You can do this, in a sense. But I should start by talking briefly about the basic design principle of overload resolution in C#. All overload resolution is, of course, about taking a set of methods with the same name and choosing from that set the unique best member to call.

There are many factors involved in determining which is the "best" method; different languages use a different "mixture" of factors to figure this out. C# in particular heavily weights "closeness" of a given method to the call site. If given the choice between an applicable method in a base class or a new applicable method in a derived class, C# takes the one in the derived class because it is closer, even if the one in the base class is in every other way a better match.

And so we run down the list. Derived classes are closer than base classes. Inner classes are closer than outer classes. Methods in the class hierarchy are closer than extension methods.

And now we come to your question. The closeness of an extension method depends on (1) how many namespaces "out" did we have to go? and (2) did we find the extension method via using or was it right there in the namespace? Therefore you can influence overload resolution by changing in what namespace your static extension class appears, to put it in a closer namespace to the call site. Or, you can change your using declarations, to put the using of the namespace that contains the desired static class closer than the other.

For example, if you have

namespace FrobCo.Blorble
{
  using BazCo.TheirExtensionNamespace;
  using FrobCo.MyExtensionNamespace;
  ... some extension method call
}

then it is ambiguous which is closer. If you want to prioritize yours over theirs, you could choose to do this:

namespace FrobCo
{
  using BazCo.TheirExtensionNamespace;
  namespace Blorble
  {
    using FrobCo.MyExtensionNamespace;
    ... some extension method call
  }

And now when overload resolution goes to resolve the extension method call, classes in Blorple get first go, then classes in FrobCo.MyExtensionNamespace, then classes in FrobCo, and then classes in BazCo.TheirExtensionNamespace.

Is that clear?

Extension methods cannot be overridden since they are not instance methods and they are not virtual.

The compiler will complain if you import both extension method classes via namespace as it will not know which method to call:

The call is ambiguous between the following methods or properties: ...

The only way around this is to call your extension method using normal static method syntax. So instead of this:

a.Foo();

you would have to do this:

YourExtensionMethodClass.Foo(a);