Why C# is not allowing non-member functions like C++

Question

See this blog posting:

http://blogs.msdn.com/ericlippert/archive/2009/06/22/why-doesn-t-c-implement-top-level-methods.aspx

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I am asked "why doesn't C# implement feature X?" all the time. The answer is always the same: because no one ever designed, specified, implemented, tested, documented and shipped that feature. All six of those things are necessary to make a feature happen. All of them cost huge amounts of time, effort and money. Features are not cheap, and we try very hard to make sure that we are only shipping those features which give the best possible benefits to our users given our constrained time, effort and money budgets.

I understand that such a general answer probably does not address the specific question.

In this particular case, the clear user benefit was in the past not large enough to justify the complications to the language which would ensue. By stricting how different language entities nest inside each other we (1) restrict legal programs to be in a common, easily understood style, and (2) make it possible to define "identifier lookup" rules which are comprehensible, specifiable, implementable, testable and documentable.

By restricting method bodies to always be inside a struct or class, we make it easier to reason about the meaning of an unqualified identifier used in an invocation context; such a thing is always an invocable member of the current type (or a base type).

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and this follow-up posting:

http://blogs.msdn.com/ericlippert/archive/2009/06/24/it-already-is-a-scripting-language.aspx

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Like all design decisions, when we're faced with a number of competing, compelling, valuable and noncompossible ideas, we've got to find a workable compromise. We don't do that except by considering all the possibilites, which is what we're doing in this case.

(emphasis from original text)

C# doesn't allow it because Java didn't allow it.

I can think of several reasons why the designers of Java probably didn't allow it

• Java was designed to be simple. They attempted to make a language without random shortcuts, so that you generally have just one simple way to do everything, even if other approaches would have been cleaner or more concise. They wanted to minimize the learning curve, and learning "a class may contain methods" is simpler than "a class may contain methods, and functions may exist outside classes".
• Superficially, it looks less object-oriented. (Anything that isn't part of an object obviously can't be object-oriented? Can it? of course, C++ says yes, but C++ wasn't involved in this decision)

As I already said in comments, I think this is a good question, and there are plenty of cases where non-member functions would've been preferable. (this part is mostly a response to all the other answers saying "you don't need it")

In C++, where non-member functions are allowed, they are often preferred, for several reasons:

• It aids encapsulation. The fewer methods have access to the private members of a class, the easier that class will be to refactor or maintain. Encapsulation is an important part of OOP.
• Code can be reused much easier when it is not part of a class. For example, the C++ standard library defines std::find or std::sort` as non-member functions, so that they can be reused on any type of sequences, whether it is arrays, sets, linked lists or (for std::find, at least) streams. Code reuse is also an important part of OOP.
• It gives us better decoupling. The find function doesn't need to know about the LinkedList class in order to be able to work on it. If it had been defined as a member function, it would be a member of the LinkedList class, basically merging the two concepts into one big blob.
• Extensibility. If you accept that the interface of a class is not just "all its public members", but also "all non-member functions that operate on the class", then it becomes possible to extend the interface of a class without having to edit or even recompile the class itself.

The ability to have non-member functions may have originated with C (where you had no other choice), but in modern C++, it is a vital feature in its own right, not just for backward-comparibility purposes, but because of the simpler, cleaner and more reusable code it allows.

In fact, C# seems to have realized much the same things, much later. Why do you think extension methods were added? They are an attempt at achieving the above, while preserving the simple Java-like syntax. Lambdas are also interesting examples, as they too are essentially small functions defined freely, not as members of any particular class. So yes, the concept of non-member functions is useful, and C#'s designers have realized the same thing. They've just tried to sneak the concept in through the back door.

http://www.ddj.com/cpp/184401197 and http://www.gotw.ca/publications/mill02.htm are two articles written by C++ experts on the subject.

Non member functions are a good thing because they improve encapsulation and reduce coupling between types. Most modern programming languages such as Haskell and F# support free functions.

What's the benefit of not putting each method in a named class? Why would a non-member function "pollute" the class's interface? If you don't want it as part of the public API of a class, either don't make it public or don't put it in that class. You can always create a different class.

I can't remember ever wanting to write a method floating around with no appropriate scope - other than anonymous functions, of course (which aren't really the same).

In short, I can't see any benefit in non-member functions, but I can see benefits in terms of consistency, naming and documentation in putting all methods in an appropriately named class.