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

It really depends on the problem you need to solve, in most situations class inheritance and interfaces make naturally more sense than extension methods and thus should be preferred.

On the other hand, Extensions allow you to create useful methods applying not just to one class - which would otherwise be much more cumbersome to do with inheritance, if not almost impossible to achieve.

Last but not least, Extensions allow you to extend .NET Framework's builtin classes as well as 3rd party classes, even if you don't own or have no access to the sourcecode.

Here are some examples where extension methods are used for good reasons:

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LinqPad uses extension methods, for example the .Dump() method with which you can dump (print) the contents of every kind of object to the output window.

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The .NET framework itself uses extension methods in many places, for example in Linq:

public static TSource FirstOrDefault(this 
                        System.Collections.Generic.IEnumerable source)

which returns the first element or default of any enumerable collection of any object type.

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An example, where extension methods are better than inheritance is the following: Say you want to create a method which is able to create a clone (copy) of any existing object. With Extensions (and generics, plus reflection) you can do it this way.