What is so great about extension methods? [duplicate]

This question already has answers here: Closed 13 years ago.

Possible Duplicate:

What Advantages of Extension Methods have you found?

All right, first of all, I realize this sounds controversial, but I don't mean to be confrontational. I am asking a serious question out of genuine curiosity (or maybe puzzlement is a better word).

Why were extension methods ever introduced to .NET? What benefit do they provide, aside from making things look nice (and by "nice" I mean "deceptively like instance methods")?

To me, any code that uses an extension method like this:

Thing initial = GetThing();
Thing manipulated = initial.SomeExtensionMethod();

is misleading, because it implies that SomeExtensionMethod is an instance member of Thing开发者_StackOverflow中文版, which misleads developers into believing (at least as a gut feeling... you may deny it but I've definitely observed this) that (1) SomeExtensionMethod is probably implemented efficiently, and (2) since SomeExtensionMethod actually looks like it's part of the Thing class, surely it will remain valid if Thing is revised at some point in the future (as long as the author of Thing knows what he/she's doing).

But the fact is that extension methods don't have access to protected members or any of the internal workings of the class they're extending, so they're just as prone to breakage as any other static methods.

We all know that the above could easily be:

Thing initial = GetThing();
Thing manipulated = SomeNonExtensionMethod(initial);

To me, this seems a lot more, for lack of a better word, honest.

What am I missing? Why do extension methods exist?

---

Extension methods were needed to make Linq work in the clean way that it does, with method chaining. If you have to use the "long" form, it causes the function calls and the parameters to become separated from each other, making the code very hard to read. Compare:

IEnumerable<int> r = list.Where(x => x > 10).Take(5)

versus

// What does the 5 do here?
IEnumerable<int> r = Enumerable.Take(Enumerable.Where(list, x => x > 10), 5);

Like anything, they can be abused, but extension methods are really useful when used properly.

---

I think that the main upside is discoverability. Type initial and a dot, and there you have all the stuff that you can do with it. It's a lot harder to find static methods tucked away in some class somewhere else.

---

First of all, in the Thing manipulated = SomeNonExtensionMethod(initial); case, SomeNonExtensionMethod is based on exactly the same assumptions like in the Thing manipulated = initial.SomeExtensionMethod(); case. Thing can change, SomeExtensionMethod can break. That's life for us programmers.

Second, when I see Thing manipulated = initial.SomeExtensionMethod();, it doesn't tell me exactly where SomeExtensionMethod() is implemented. Thing could inherit it from TheThing, which inherits it from TheOriginalThing. So the "misleading" argument leads to nowhere. I bet the IDE takes care of leading you to the right source, doesn't it?

What's so great? It makes code more consistent. If it works on a string, it looks like if it was a member of string. It's ugly to have several MyThing.doThis() methods and several static ThingUtil.doSomethingElse(Mything thing) methods in another class.

---

SO you can extend someone else's class. not yours... that's the advantage. (and you can say.. oh I wish they implement this / that.... you just do it yourself..)

---

they are great for automatically mixing in functionality based on Interfaces that a class inherits without that class having to explicitly re implement it.

Linq makes use of this a lot.

Great way to decorate classes with extra functionality. Most effective when applied to an Interface rather than a specific class. Still a good way to extend Framework classes though.

---

It's just convenient syntactic sugar so that you can call a method with the same syntax regardless of whether it's actually part of the class. If party A releases a lib, and party B releases stuff that uses that lib, it's easier to just call everything with class.method(args) than to have to remember what gets called with method(class, args) vs. class.method(args).