Virtual Static Interface Methods in C# - Introduction

For quite some time now I wanted to blog about a feature I missed in C# for a long time, what I used to call static interfaces or static class interfaces.

In a class definition you can always add static properties and methods. In my opinion, these static elements really are elements of  a separate class which acts as a “StaticClass” for the original class. For example, a framework might need to operate on static methods defined by such “StaticClasses” depending on the type of a parameter passed to the framework.

Pseudo Code:

static interface IStaticClass
{
    void DoSomething();
}
class Foo : IStaticClass
{
    public static virtual void DoSomething()
    {
        Console.WriteLine("hello from Foo");
    }
}
class Bar : IStaticClass
{
    public static virtual void DoSomething()
    {
        Console.WriteLine("hello from Bar");
    }
}
class FooEx : Foo
{
    public static override void DoSomething()
    {
        Console.WriteLine("hello from FooEx");
    }
}
void Main()
{
    Bar bar = new Bar();
    Foo foo = new FooEx();

    // magic happens here:
    (bar.GetType() as IStaticClass).DoSomething();
    foo.class.DoSomething();
}

Of course I know that this is not possible in C#, and my personal impression is now that such a solution (or the desire to have such a solution) would clearly be an indication of bad design.

One reason for this indication is that there is a fundamental difference between virtual methods and static methods:

Static methods are statically identified by Namespace + Class name + Method name + Parameter types.

Virtual methods are dynamically identified by Method name + Parameter types of the Virtual Method Table of the Class definition (Namespace + Class name), optionally implementing an Interface definition (Namespace + Interface name), of an instantiated object.

I am not alone, though. A quick search on StackOverflow to research for these blogs came up with a quite a lot of questions:

• Workaround or alternative to no static methods on an interface

• C# static function in interface

• interface declaration with a static method

• static method cannot implement interface method, why?

• Why can’t I declare C# methods virtual and static?

• Static Methods in an Interface/Abstract Class

• c#: Inherited/interface static member?

• Why can’t you have static methods in an Interface?

The next two blogs illustrate a solution using techniques already available in C#.

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