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

MSDN

In the page on extension methods in the C# programming guide it says:

General Guidelines

In general, we recommend that you implement extension methods sparingly and only when you have to. Whenever possible, client code that must extend an existing type should do so by creating a new type derived from the existing type. For more information, see Inheritance (C# Programming Guide).

When using an extension method to extend a type whose source code you cannot change, you run the risk that a change in the implementation of the type will cause your extension method to break.

Answer 2

I would stick to inheritance except in the cases that extension methods were primarily designed for - extending sealed classes or creating scope-specific extensions. Also remember that they are static, so if these are methods and behaviours that you would need to override in other classes, you can't really use extensions.

Extension methods do have one really great feature that is an inherent benefit of their implementation. Since they are static methods, you can call them on a null object.

For instance:

string a = null;
return a.IfNullOrEmpty("Default Value");

Implementations like this are great, though they are technically just syntactical sugar. IMHO, anything that keeps your code cleaner and more readable is great.

Though I don't like that they aren't really discoverable. If I copy that code from one class to another, i would then have to search for the namespace in which it was defined.

Answer 3

Extension methods are useful, but they are harder to discover through the IDE than regular methods, since they are not attached to the original class and there are no clues as to where the code for them might reside. There are some best practice suggestions as to where to put them and how to name them, but these are only guidelines and there is no guarantee that someone will follow them.

Usually you would use extension methods if you are only adding functionality to a well known, well used class or interface such as the .Net base classes, that you don't have access to the code for. Extension methods also have the constraint in that you not only have to have the original assembly, you have to have the assembly with the extension methods in it, which must be understood by consumers of your code.

Using inheritance will allow you to add, remove or override functionality, and ensure that it is always present with the class when you build it.

Answer 4

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<TSource>(this 
                        System.Collections.Generic.IEnumerable<TSource> 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.

Answer 5

They are very different, for example LINQ standard query operators are great example of extension methods that should be difficult to implement with inheritance, but if you have access to class and can change source it will be better to use inheritance,
EDIT
and here is some rules that I find here C# 3.0 Features: Extension Methods

• Extension methods cannot be used to override existing methods
• An extension method with the same name and signature as an instance method will not be called
• The concept of extension methods cannot be applied to fields, properties or events
• Use extension methods sparingly....overuse can be a bad thing!

Answer 6

Extension methods should be used when you want to provide an implementation across a variety of types that should share the same behavior, but would otherwise be disimilar. That's why you see extension methods being used on interfaces a lot, because it's a very powerful tool to ensure that any given implementation of an interface will have the same implementation of a given behavior.

For example, the Skip and Take extension methods.