为什么 C# 不允许像 C++ 这样的非成员函数

Question

C# will not allow to write non-member functions and every method should be part of a class. I was thinking this as a restriction in all CLI languages. But I was wrong and I found that C++/CLI supports non-member functions. When it is compiled, compiler will make the method as member of some unnamed class.

Here is what C++/CLI standard says,

[Note: Non-member functions are treated by the CLI as members of some unnamed class; however, in C++/CLI source code, such functions cannot be qualified explicitly with that class name. end note]

The encoding of non-member functions in metadata is unspecified. [Note: This does not cause interop problems because such functions cannot have public visibility. end note]

So my question is why don't C# implement something like this? Or do you think there should not be non-member functions and every method should belong to some class?

My opinion is to have non-member function support and it helps to avoid polluting class's interface.

Any thoughts..?

Answer 1

See this blog posting:

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

(...)

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).

(...)

and this follow-up posting:

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

(...)

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)

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

The CLS (common language specification) says that you shouldn't have non-member functions in a library that conforms to the CLS. It's like an extra set of restrictions in addition to the basic restrictions of the CLI (common language interface).

It is possible that a future version of C# will add the ability to write a using directive that allows the static members of a class to be accessed without the class name qualification:

using System.Linq.Enumerable; // Enumerable is a static class

...

IEnumerable<int> range = Range(1, 10); // finds Enumerable.Range

Then there will be no need to change the CLS and existing libraries.

These blog posts demonstrate a library for functional programming in C#, and they use a class name that is just one letter long, to try and cut down the noise caused by the requirement to qualify static method calls. Examples like that would be made a little nicer if using directives could target classes.

Answer 6

• Having all code lie within classes allows for a more powerful set of reflection capabilities.
• It allows the use of static intializers, which can initialize the data needed by static methods within a class.
• It avoids name clashes between methods by explicitly enclosing them within a unit that cannot be added to by another compilation unit.

Answer 7

Since Java, most programmers have easily accepted that any method is a member of a class. I doesn't make any considerable obstacles and make the concept of method more narrow, which make a language easier.

However, indeed, class infers object, and object infers state, so the concept of class containing only static methods looks a little absurd.

Answer 8

I think you really need to clarify what you would want to create non-member static methods to achieve.

For instance, some of the things you might want them for could be handled with Extension Methods

Another typical use (of a class which only contains static methods) is in a library. In this case, there is little harm in creating a class in an assembly which is entirely composed of static methods. It keeps them together, avoids naming collisions. After all, there are static methods in Math which serve the same purpose.

Also, you should not necessarily compare C++'s object model with C#. C++ is largely (but not perfectly) compatible with C, which didn't have a class system at all - so C++ had to support this programming idiom out of the C legacy, not for any particular design imperative.

Answer 9

Bear something in mind: C++ is a much more complicated language than C#. And although they may be similiar syntactically, they are very different beasts semantically. You wouldn't think it would be terribly difficult to make a change like this, but I could see how it could be. ANTLR has a good wiki page called What makes a language problem hard? that's good to consult for questions like this. In this case:

Context sensitive lexer? You can't decide what vocabulay symbol to match unless you know what kind of sentence you are parsing.

Now instead of just worrying about functions defined in classes, we have to worry about functions defined outside classes. Conceptually, there isn't much difference. But in terms of lexing and parsing the code, now you have the added problem of having to say "if a function is outside a class, it belongs to this unnamed class. However, if it is inside the class, then it belongs to that class."

Also, if the compiler comes across a method like this:

public void Foo()
{
    Bar();
}

...it now has to answer the question "is Bar located within this class or is it a global class?"

Forward or external references? Ie, multiple passes needed? Pascal has a "forward" reference to handle intra-file procedure references, but references to procedures in other files via the USES clauses etc... require special handling.

This is another thing that causes problems. Remember that C# doesn't require forward declarations. The compiler will make one pass just to determine what classes are named and what functions those classes contain. Now you have to worry about finding classes and functions where functions can be either inside or outside of a class. This is something a C++ parser doesn't have to worry about as it parses everything in order.

Now don't get me wrong, it could probably be done in C#, and I would probably use such a feature. But is it really worth all the trouble of overcoming these obstacles when you could just type a class name in front of a static method?

Answer 10

While it's true you need a class (eg a static class called FreeFunctions ) to hold such functions, you're free to place using static FreeFunctions; at the top of any file that needs the functions from it, without having to litter your code with FreeFunctions. qualifiers. I'm not sure if there's actually a case where this is demonstrably inferior to not requiring the function definitions to be contained in a class.

Answer 11

Look, other programming languages have a hard time to define the internal nature of a function instance from the compiler's point of view. In Pascal and C, the instances are basically defined as something that can be processed as pointer only . Especially, since reading/writing to executable code positions is what 7 out of 9 computer science professors are dead set against. As member of a class, no one does need to care how to treat its manifestation because this manifestation's type is derived from a class property. It is possible to create something that is exactly processed like a global function: a lambda function, assigned to a variable:

 Func<int,int> myFunc = delegate(int var1) { Console.WriteLine("{0}",var1*2); return var1*3; };

. And it can simply be called like a global function by its variable name. If so, the difference would be implementing a new object type on the lowest level with same behavior as another one. That is considered bad practice by experienced programmers, and was perhaps scrapped because of this.

Answer 12

Free functions are very useful if you combine them with duck typing. The whole C++ STL is based on it. Hence I am sure that C# will introduce free functions when they manage to add true generics.

Like economics, language design is also about psychology. If you create appetite for true generics via free functions in C# and not deliver, then you would kill C#. Then all C# developers would move to C++ and nobody wants that to happen, not the C# community and most certainly not those invested in C++.

Answer 13

Csharp does not have non-member function because it has copied or inspired by java's philosophy that only OOPs is the solution for all the problems and it will only allow things to be solved using OO way.

Non-member functions are very important feature if we really want to do generic programming. They are more reusable compared to putting them in a class.

CSharp has to come up with ExtensionMethods due to absence of non-member functions.

As now programming languages are moving towards functional programming paradigm and it seems to be the better way to approach and solve the problem and is the future. CSharp should rethink about it.

Answer 14

If you think about it, the counter argument would be "why doesn't C++ support extension methods" and the answer lies within their design goals.

C++ gives you namespace functions that you can call on any object which would help you "extending" that object in some way.

Most of the times I used namespace functions in C++ were to create functions that accept objects and do functionality that I don't want to put inside a class member function.

In C# you can create an extension method that does the job for you (most of the time). For what's left of the cases you will miss that feature.

Take the following code for example:

template<typename T>
unsigned int findCount(vector<T>& vec, T data){
    unsigned int count = 0;
    for(auto val : vec)
        if(val == data) ++count;
    return count;
}

Now if I needed that functionality for some purpose in my class, I can just add it to the class's namespace and use it without "polluting" my class with that function.

In C# you can achieve the same goal with extension method:

static class Extensions {
    public static uint FindCount<T>(this List<T> list, T data) {
        uint counts = 0;
        foreach (var item in list)
            if (item.Equals(data)) ++counts;
        return counts;
    }
}

And both of them work the same way which is cool. Many will argue about C++ missing extension methods, many will argue about C# missing namespace functions.

I think the best way to put it is by not comparing these languages to some detail because they are different languages with different implementations.

As you quoted in your question, C++/CLI does not "really" support namespace member functions since it adds an unnamed class to use them which is by no way close to C++ implementation, maybe close in the way it looks, but really different.

In the end, functionality is always something to look forward to, and as much as I want namespace member functions in C# is as much as I want extension methods in C++. Which isn't much anyway.