Extensions methods and forward compatibilty of source code

Question

I would like solve the problem (now hypothetical but propably real in future) of using extension methods and maginification of class interface in future development.

Example:

/* the code written in 17. March 2010 */
public class MySpecialList : IList<MySpecialClass> {
    // ... implementation
}
// ... somewhere elsewhere ...
MySpecialList list = GetMySpecialList(); // returns list of special classes
var reversedList = list.Reverse().ToList(); // .Reverse() is extension method
/* now the "list" is unchanged and "reveresedList" has same items in reversed order */

/* --- in future the interface of MySpecialList will be changed because of reason XYZ*/

/* the code written in some future */
public class MySpecialList : IList<MySpecialClass> {
    // ... implementation
    public MySpecialList Reverse() {
        // reverse order of items in this collection
        return this;
    }
}
// ... somewhere elsewhere ...
MySpecialList list = GetMySpecialList(); // returns list of special classes
var reversedList = list.Reverse().ToList(); // .Reverse() was extension method but now is instance method and do something else !
/* now the "list" is reversed order of items and "reveresedList" has same items lake in "list" */

My question is: Is there some way how to prevent this case (I didn't find them)? If is now way how to prevent it, is there some way how to find possible issues like this? If is now way how to find possible issues, should I forbid usage of extension methods?

Thanks.

EDIT:

Yours answer was usefull. Can I found where in code are used extension methods? And/or can I found where in code are used instance methods but exists extension method with same signature?

Answer 1

It looks like what you are describing is the following situation

1. In V1 of your product MySpecialList has no Reverse method so all calls to Reverse bind to an extension method of the same name
2. In V2 of your product MySpecialList gains a Reverse method and now all previous bindings to the extension method bind to the instance method instead.

If you want to call Reverse in the instance / extension method form there is no way to prevent this as it's the designed behavior. Instance methods will always be preferred over extension methods if they are at least as good as the extension method version.

The only way to 100% prevent this is to call extension methods as static methods. For example

ExtensionMethods.Reverse(list);

This problem of binding to new methods with new version of the product is not just limited to extension methods (although the problem is likely a bit worse). There are many things you can do to a type to alter the way method binding would be affected such as implementing a new interface, inheriting or adding a new conversion

Answer 2

And this is why we write unit tests.

First, write extension methods. Name them precisely. So that one day, IF an extension method is implemented as a real method on the class with the same name, there is a good chance, it does the same thing as your extension method, and nothing breaks.

Secondly, with unit tests, you'll quickly see what broke, and track down that it broke because an extension method is no longer being called, since the class now has a it's own method with that name. Given that, you can choose to rename your method, call your extension method as a static method, OR rewrite your code to properly use the new method.