Why Doesn't C# Allow Static Methods to Impleme

问题:

Why was C# designed this way?

As I understand it, an interface only describes behaviour, and serves the purpose of describing a contractual obligation for classes implementing the interface that certain behaviour is implemented.

If classes wish to implement that behavour in a shared method, why shouldn't they?

Here is an example of what I have in mind:

// These items will be displayed in a list on the screen.
public interface IListItem {
  string ScreenName();
  ...
}

public class Animal: IListItem {
    // All animals will be called "Animal".
    public static string ScreenName() {
        return "Animal";
    }
....
}

public class Person: IListItem {

    private string name;

    // All persons will be called by their individual names.
    public string ScreenName() {
        return name;
    }

    ....

 }

回答1:

Assuming you are asking why you can't do this:

public interface IFoo {
    void Bar();
}

public class Foo: IFoo {
    public static void Bar() {}
}

This doesn't make sense to me, semantically. Methods specified on an interface should be there to specify the contract for interacting with an object. Static methods do not allow you to interact with an object - if you find yourself in the position where your implementation could be made static, you may need to ask yourself if that method really belongs in the interface.

---

To implement your example, I would give Animal a const property, which would still allow it to be accessed from a static context, and return that value in the implementation.

public class Animal: IListItem {
    /* Can be tough to come up with a different, yet meaningful name!
     * A different casing convention, like Java has, would help here.
     */
    public const string AnimalScreenName = "Animal";
    public string ScreenName(){ return AnimalScreenName; }
}

For a more complicated situation, you could always declare another static method and delegate to that. In trying come up with an example, I couldn't think of any reason you would do something non-trivial in both a static and instance context, so I'll spare you a FooBar blob, and take it as an indication that it might not be a good idea.

回答2:

My (simplified) technical reason is that static methods are not in the vtable, and the call site is chosen at compile time. It's the same reason you can't have override or virtual static members. For more details, you'd need a CS grad or compiler wonk - of which I'm neither.

For the political reason, I'll quote Eric Lippert (who is a compiler wonk, and holds a Bachelor of Mathematics, Computer science and Applied Mathematics from University of Waterloo (source: LinkedIn):

...the core design principle of static methods, the principle that gives them their name...[is]...it can always be determined exactly, at compile time, what method will be called. That is, the method can be resolved solely by static analysis of the code.

Note that Lippert does leave room for a so-called type method:

That is, a method associated with a type (like a static), which does not take a non-nullable “this” argument (unlike an instance or virtual), but one where the method called would depend on the constructed type of T (unlike a static, which must be determinable at compile time).

but is yet to be convinced of its usefulness.

回答3:

Most answers here seem to miss the whole point. Polymorphism can be used not only between instances, but also between types. This is often needed, when we use generics.

Suppose we have type parameter in generic method and we need to do some operation with it. We dont want to instantinate, because we are unaware of the constructors.

For example:

Repository GetRepository<T>()
{
  //need to call T.IsQueryable, but can't!!!
  //need to call T.RowCount
  //need to call T.DoSomeStaticMath(int param)
}

...
var r = GetRepository<Customer>()

Unfortunately, I can come up only with "ugly" alternatives:

• Use reflection Ugly and beats the idea of interfaces and polymorphism.

• Create completely separate factory class

  This might greatly increase the complexity of the code. For example, if we are trying to model domain objects, each object would need another repository class.

• Instantiate and then call the desired interface method

  This can be hard to implement even if we control the source for the classes, used as generic parameters. The reason is that, for example we might need the instances to be only in well-known, "connected to DB" state.

Example:

public class Customer 
{
  //create new customer
  public Customer(Transaction t) { ... }

  //open existing customer
  public Customer(Transaction t, int id) { ... }

  void SomeOtherMethod() 
  { 
    //do work...
  }
}

in order to use instantination for solving the static interface problem we need to do the following thing:

public class Customer: IDoSomeStaticMath
{
  //create new customer
  public Customer(Transaction t) { ... }

  //open existing customer
  public Customer(Transaction t, int id) { ... }

  //dummy instance
  public Customer() { IsDummy = true; }

  int DoSomeStaticMath(int a) { }

  void SomeOtherMethod() 
  { 
    if(!IsDummy) 
    {
      //do work...
    }
  }
}

This is obviously ugly and also unnecessary complicates the code for all other methods. Obviously, not an elegant solution either!

回答4:

I know it's an old question, but it's interesting. The example isn't the best. I think it would be much clearer if you showed a usage case:

string DoSomething<T>() where T:ISomeFunction
{
  if (T.someFunction())
    ...
}

Merely being able to have static methods implement an interface would not achieve what you want; what would be needed would be to have static members as part of an interface. I can certainly imagine many usage cases for that, especially when it comes to being able to create things. Two approaches I could offer which might be helpful:

1. Create a static generic class whose type parameter will be the type you'd be passing to DoSomething above. Each variation of this class will have one or more static members holding stuff related to that type. This information could supplied either by having each class of interest call a "register information" routine, or by using Reflection to get the information when the class variation's static constructor is run. I believe the latter approach is used by things like Comparer<T>.Default().
2. For each class T of interest, define a class or struct which implements IGetWhateverClassInfo<T> and satisfies a "new" constraint. The class won't actually contain any fields, but will have a static property which returns a static field with the type information. Pass the type of that class or struct to the generic routine in question, which will be able to create an instance and use it to get information about the other class. If you use a class for this purpose, you should probably define a static generic class as indicated above, to avoid having to construct a new descriptor-object instance each time. If you use a struct, instantiation cost should be nil, but every different struct type would require a different expansion of the DoSomething routine.

None of these approaches is really appealing. On the other hand, I would expect that if the mechanisms existed in CLR to provide this sort of functionality cleanly, .net would allow one to specify parameterized "new" constraints (since knowing if a class has a constructor with a particular signature would seem to be comparable in difficulty to knowing if it has a static method with a particular signature).

回答5:

Short-sightedness, I'd guess.

When originally designed, interfaces were intended only to be used with instances of class

IMyInterface val = GetObjectImplementingIMyInterface();
val.SomeThingDefinedinInterface();

It was only with the introduction of interfaces as constraints for generics did adding a static method to an interface have a practical use.

(responding to comment:) I believe changing it now would require a change to the CLR, which would lead to incompatibilities with existing assemblies.

回答6:

Interfaces specify behavior of an object.

Static methods do not specify a behavior of an object, but behavior that affects an object in some way.

回答7:

To the extent that interfaces represent "contracts", it seems quiet reasonable for static classes to implement interfaces.

The above arguments all seem to miss this point about contracts.

回答8:

Because the purpose of an interface is to allow polymorphism, being able to pass an instance of any number of defined classes that have all been defined to implement the defined interface... guaranteeing that within your polymorphic call, the code will be able to find the method you are calling. it makes no sense to allow a static method to implement the interface,

How would you call it??

---

public interface MyInterface { void MyMethod(); }
public class MyClass: MyInterface
{
    public static void MyMethod() { //Do Something; }
}

 // inside of some other class ...  
 // How would you call the method on the interface ???
    MyClass.MyMethod();  // this calls the method normally 
                         // not through the interface...

    // This next fails you can't cast a classname to a different type... 
    // Only instances can be Cast to a different type...
    MyInterface myItf = MyClass as MyInterface;  

回答9:

Regarding static methods used in non-generic contexts I agree that it doesn't make much sense to allow them in interfaces, since you wouldn't be able to call them if you had a reference to the interface anyway. However there is a fundamental hole in the language design created by using interfaces NOT in a polymorphic context, but in a generic one. In this case the interface is not an interface at all but rather a constraint. Because C# has no concept of a constraint outside of an interface it is missing substantial functionality. Case in point:

T SumElements<T>(T initVal, T[] values)
{
    foreach (var v in values)
    {
        initVal += v;
    }
}

Here there is no polymorphism, the generic uses the actual type of the object and calls the += operator, but this fails since it can't say for sure that that operator exists. The simple solution is to specify it in the constraint; the simple solution is impossible because operators are static and static methods can't be in an interface and (here is the problem) constraints are represented as interfaces.

What C# needs is a real constraint type, all interfaces would also be constraints, but not all constraints would be interfaces then you could do this:

constraint CHasPlusEquals
{
    static CHasPlusEquals operator + (CHasPlusEquals a, CHasPlusEquals b);
}

T SumElements<T>(T initVal, T[] values) where T : CHasPlusEquals
{
    foreach (var v in values)
    {
        initVal += v;
    }
}

There has been lots of talk already about making an IArithmetic for all numeric types to implement, but there is concern about efficiency, since a constraint is not a polymorphic construct, making a CArithmetic constraint would solve that problem.

回答10:

Because interfaces are in inheritance structure, and static methods don't inherit well.

回答11:

What you seem to want would allow for a static method to be called via both the Type or any instance of that type. This would at very least result in ambiguity which is not a desirable trait.

There would be endless debates about whether it mattered, which is best practice and whether there are performance issues doing it one way or another. By simply not supporting it C# saves us having to worry about it.

Its also likely that a compilier that conformed to this desire would lose some optimisations that may come with a more strict separation between instance and static methods.

回答12:

You can think of the static methods and non-static methods of a class as being different interfaces. When called, static methods resolve to the singleton static class object, and non-static methods resolve to the instance of the class you deal with. So, if you use static and non-static methods in an interface, you'd effectively be declaring two interfaces when really we want interfaces to be used to access one cohesive thing.

回答13:

To give an example where I am missing either static implementation of interface methods or what Mark Brackett introduced as the "so-called type method":

When reading from a database storage, we have a generic DataTable class that handles reading from a table of any structure. All table specific information is put in one class per table that also holds data for one row from the DB and which must implement an IDataRow interface. Included in the IDataRow is a description of the structure of the table to read from the database. The DataTable must ask for the datastructure from the IDataRow before reading from the DB. Currently this looks like:

interface IDataRow {
  string GetDataSTructre();  // How to read data from the DB
  void Read(IDBDataRow);     // How to populate this datarow from DB data
}

public class DataTable<T> : List<T> where T : IDataRow {

  public string GetDataStructure()
    // Desired: Static or Type method:
    // return (T.GetDataStructure());
    // Required: Instantiate a new class:
    return (new T().GetDataStructure());
  }

}

The GetDataStructure is only required once for each table to read, the overhead for instantiating one more instance is minimal. However, it would be nice in this case here.

回答14:

FYI: You could get a similar behavior to what you want by creating extension methods for the interface. The extension method would be a shared, non overridable static behavior. However, unfortunately, this static method would not be part of the contract.

回答15:

Interfaces are abstract sets of defined available functionality.

Whether or not a method in that interface behaves as static or not is an implementation detail that should be hidden behind the interface. It would be wrong to define an interface method as static because you would be unnecessarily forcing the method to be implemented in a certain way.

If methods were defined as static, the class implementing the interface wouldn't be as encapsulated as it could be. Encapsulation is a good thing to strive for in object oriented design (I won't go into why, you can read that here: http://en.wikipedia.org/wiki/Object-oriented). For this reason, static methods aren't permitted in interfaces.

回答16:

Static classes should be able to do this so they can be used generically. I had to instead implement a Singleton to achieve the desired results.

I had a bunch of Static Business Layer classes that implemented CRUD methods like "Create", "Read", "Update", "Delete" for each entity type like "User", "Team", ect.. Then I created a base control that had an abstract property for the Business Layer class that implemented the CRUD methods. This allowed me to automate the "Create", "Read", "Update", "Delete" operations from the base class. I had to use a Singleton because of the Static limitation.

回答17:

Most people seem to forget that in OOP Classes are objects too, and so they have messages, which for some reason c# calls "static method". The fact that differences exist between instance objects and class objects only shows flaws or shortcomings in the language. Optimist about c# though...

回答18:

OK here is an example of needing a 'type method'. I am creating one of a set of classes based on some source XML. So I have a

  static public bool IsHandled(XElement xml)

function which is called in turn on each class.

The function should be static as otherwise we waste time creating inappropriate objects. As @Ian Boyde points out it could be done in a factory class, but this just adds complexity.

It would be nice to add it to the interface to force class implementors to implement it. This would not cause significant overhead - it is only a compile/link time check and does not affect the vtable.

However, it would also be a fairly minor improvement. As the method is static, I as the caller, must call it explicitly and so get an immediate compile error if it is not implemented. Allowing it to be specified on the interface would mean this error comes marginally earlier in the development cycle, but this is trivial compared to other broken-interface issues.

So it is a minor potential feature which on balance is probably best left out.

回答19:

The fact that a static class is implemented in C# by Microsoft creating a special instance of a class with the static elements is just an oddity of how static functionality is achieved. It is isn't a theoretical point.

An interface SHOULD be a descriptor of the class interface - or how it is interacted with, and that should include interactions that are static. The general definition of interface (from Meriam-Webster): the place or area at which different things meet and communicate with or affect each other. When you omit static components of a class or static classes entirely, we are ignoring large sections of how these bad boys interact.

Here is a very clear example of where being able to use interfaces with static classes would be quite useful:

public interface ICrudModel<T, Tk>
{
    Boolean Create(T obj);
    T Retrieve(Tk key);
    Boolean Update(T obj);
    Boolean Delete(T obj);
}

Currently, I write the static classes that contain these methods without any kind of checking to make sure that I haven't forgotten anything. Is like the bad old days of programming before OOP.

回答20:

C# and the CLR should support static methods in interfaces as Java does. The static modifier is part of a contract definition and does have meaning, specifically that the behavior and return value do not vary base on instance although it may still vary from call to call.

That said, I recommend that when you want to use a static method in an interface and cannot, use an annotation instead. You will get the functionality you are looking for.

回答21:

I think the short answer is "because it is of zero usefulness". To call an interface method, you need an instance of the type. From instance methods you can call any static methods you want to.

回答22:

I think the question is getting at the fact that C# needs another keyword, for precisely this sort of situation. You want a method whose return value depends only on the type on which it is called. You can't call it "static" if said type is unknown. But once the type becomes known, it will become static. "Unresolved static" is the idea -- it's not static yet, but once we know the receiving type, it will be. This is a perfectly good concept, which is why programmers keep asking for it. But it didn't quite fit into the way the designers thought about the language.

Since it's not available, I have taken to using non-static methods in the way shown below. Not exactly ideal, but I can't see any approach that makes more sense, at least not for me.

public interface IZeroWrapper<TNumber> {
  TNumber Zero {get;}
}

public class DoubleWrapper: IZeroWrapper<double> {
  public double Zero { get { return 0; } }
}

回答23:

As per Object oriented concept Interface implemented by classes and have contract to access these implemented function(or methods) using object.

So if you want to access Interface Contract methods you have to create object. It is always must that is not allowed in case of Static methods. Static classes ,method and variables never require objects and load in memory without creating object of that area(or class) or you can say do not require Object Creation.

回答24:

Conceptually there is no reason why an interface could not define a contract that includes static methods.

For the current C# language implementation, the restriction is due to the allowance of inheritance of a base class and interfaces. If "class SomeBaseClass" implements "interface ISomeInterface" and "class SomeDerivedClass : SomeBaseClass, ISomeInterface" also implements the interface, a static method to implement an interface method would fail compile because a static method cannot have same signature as an instance method (which would be present in base class to implement the interface).

A static class is functionally identical to a singleton and serves the same purpose as a singleton with cleaner syntax. Since a singleton can implement an interface, interface implementations by statics are conceptually valid.

So it simply boils down to the limitation of C# name conflict for instance and static methods of the same name across inheritance. There is no reason why C# could not be "upgraded" to support static method contracts (interfaces).

回答25:

When a class implements an interface,it is creating instance for the interface members. While a static type doesnt have an instance,there is no point in having static signatures in an interface.

回答26:

don't do that, try abstract classes instead

public abstract class ExampleBase
{
    /// <summary>
    /// Do it
    /// </summary>
    public virtual abstract static void DoIt();
}