Why Doesn't C# Allow Static Methods to Impleme

Question

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;
    }

    ....

 }

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

don't do that, try abstract classes instead

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

Answer 5

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.

Answer 6

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;