Extension Methods vs Static Utility Class

c# .net design-patterns extension-methods
AJM picture AJM · Jan 10, 2011 · Viewed 14.1k times · Source

I'm looking for some pros and cons for using extension methods over static utility classes in a C# app.

For instance, a plus in the extension methods column is the convinience of calling by the class name rather than something like "StringUtils". But a con would be that it can blur the lines between what is in the framework and what isn't.

Answer

Jon Skeet picture Jon Skeet · Jan 10, 2011

I would say that a pro is that it blurs the lines between what is in the framework and what isn't: you can use your own code as naturally as framework code, operating on framework types.

Extension methods shouldn't be used arbitrarily, of course - it's not like all static methods should become extension methods.

I try to think of it as whether the method is logically operating "on" its first parameter. Would it make sense as an instance method, if you were able to include it as an instance method?

A "con" which you may not be aware of: if the extended type later adds an instance method of the same name which is applicable with the same parameters, your calling code will transparently start calling that instance method next time you recompile. For example, Stream gained CopyTo in .NET 4... I'd previously written a CopyTo extension method, which then wouldn't be called. There's no warning that this is occurring, so you have to be on your guard.

One caveat: extension methods haven't been around for long enough for best practices to really become established. You should weigh up all opinions (even - or perhaps especially - my own ones) carefully.

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