Question

When I first learned about extension methods I read this:

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.

However, I have numerious times seen a very liberal use of extension methods in various production code bases.

Granted, my experience is not representative of the majority but I would like to know if there’s a shift in the guidelines, an alternate design philosophy, or if I just happened to see enough code that ignored the guidlines to make me think so?

NOTE: I am not trying to spark a debate (which will promptly lead to this question closing) – I’ve honestly been wondering about this for a while and feel my best chance at getting an answer is here on SO.

Answer 1

Extending class is good approach, but often it is not available for code that uses third party libraries (which is common case for production code unlike in education/sample code). So derive new class if it makes sense, but feel free to use extension methods when they make code more readable.

There are multiple reasons why there are a lot of extension methods:

• often you can’t extend a class to add methods that would make your product code to be more readable. I.e. value types like String or some base classes in hierarchies like Stream.
• extension methods is valuable way to add methods to interfaces without polluting the interface. LINQ is good example how it produces more readable code.
• some frameworks recommend to use extension methods in particular cases. I.e. for MVC it is recommended to add extensions to HtmlHelper.