Extension methods versus inheritance

Question

Are there rules of thumb that help determine which to use in what case? Should I prefer one over the other most times?

Thanks!

Answer 1

Whenever possible, use inheritance instead of extension methods.

edit

I prefer to keep this short and simple, but I will of course answer follow-up questions.

In the cases where inheritance is possible, which is to say classes that are not sealed, it is almost always a better option than extension methods. In fact, this is what the best practices document that womp referenced says. It has headings such as "Be wary of extension methods", "Think twice before extending types you don't own", and "Prefer interface extensions over class extensions". In other words, it just says what my one-liner did, with greater detail.

The article does give detailed reasons, but the bottom line is that this is how extension methods were designed to be used. They were added to the language late in the game as a bit of syntactic sugar to allow MS to wedge in LINQ without having to go back and reinvent the wheel. This is the canonical example of what they are good for. Another good example is adding utility methods, such as:

public static string FormatWith(this string format, params object[] args)
{ return string.Format(CultureInfo.InvariantCulture, format, args); }

Note that, in this case, extension methods were the only way to accomplish this additional feature, since strings are sealed.

As for composition over inheritance, while this is a truism, I fail to see the relevance here. Whether we're using extension methods or inheritance, the goal is to change the interface to allow another method. How this method is implemented, whether by composition, generics or some other technique, is orthogonal.

Answer 2

Well... you can't always use inheritance. String, for example, is a sealed class. It's in those cases where an extension method really shines.

In general, extension methods are best for little utilities that you might otherwise put into a static class, but that operate against an instance of a particular type. Strings are a great example -- almost everyone has their own little string extension methods to do little operations on a string.

Another great place for extension methods is against enumerations. I almost always include a HasFlag extension method against any [Flags] enumerations I create.

Answer 3

Extension methods break good OO design. To say they should be used on sealed classes that you do not have access to the code base is ridiculous. Classes that are sealed and you do not have access to are probably sealed for a reason (performance, thread safety) and to tag functionality blindly to these classes is down right dangerous. There is always a way of implementing the decorator pattern in a pure OO way and to not do it that way makes the code harder to read, maintain and refactor. As a rule of thumb, if a feature of a language smells bad then it should be avoided. I'm sure you could find one example where extension methods are useful however the truth is that the feature will be abused by those developers with minimal OO training.