C# Interfaces and static methods

[dans]

Hi,
I have recently started writing in C# on Linux using mono and come from a C++ background.

I would like to create interfaces that expose methods for objects constructed using the singleton and factory method patterns. Both of these typically use a static method to return an object that has been constructed.

Is there any way of doing this using C# interfaces? or do I need to use an abstract base class?

My implementation looks something like this:

interface IDisplay
{
    // some property accesses and methods
}
 
public class CDisplay: IDisplay
{
    static CDisplay s_instance = null;
    public static IDisplay instance {
        get {
            if (null == s_instance)
                s_instance = new CDisplay();
            return s_instance;
        }
    }
 
    // implement constructor and interface
}
 

The problem with this implementation is that I need to expose CDisplay to my calling code when I think that it should be hidden as private or internal.

Is there a way of adding static methods to interfaces?

thanks

dan

[r035198x]

Hi,
I have recently started writing in C# on Linux using mono and come from a C++ background.

I would like to create interfaces that expose methods for objects constructed using the singleton and factory method patterns. Both of these typically use a static method to return an object that has been constructed.

Is there any way of doing this using C# interfaces? or do I need to use an abstract base class?

My implementation looks something like this:

interface IDisplay
{
    // some property accesses and methods
}
 
public class CDisplay: IDisplay
{
    static CDisplay s_instance = null;
    public static IDisplay instance {
        get {
            if (null == s_instance)
                s_instance = new CDisplay();
            return s_instance;
        }
    }
 
    // implement constructor and interface
}
 

The problem with this implementation is that I need to expose CDisplay to my calling code when I think that it should be hidden as private or internal.

Is there a way of adding static methods to interfaces?

thanks

dan

Interfaces cannot have static members.
Your problem is probably because your interface is not public but you are trying to return an object of that interface type in a public context.

[dans]

Interfaces cannot have static members.
Your problem is probably because your interface is not public but you are trying to return an object of that interface type in a public context.

The interface is public.

[Plater]

Could you do something like this:

public static class ContainerClass
{
   private interface iMyInterface
   {
      void somefunc();
   }
   private class MyClass : iMyInterface
   {
      //do stuff
      //like implement the interface's stuff
   }
 
   public static MyClass CreateAMyClass()
   {
      return new MyClass();
   }
}
 

Then your class and interface are "hidden" (sort of) but you can still get a reference to one of them?

Or, replace "public static MyClass CreateAMyClass()" with:

public static MyClass TheOnlyValidMyClass = new MyClass();
 

Then reference with:

ContainerClass.TheOnlyValidMyClass 
 

?

[r035198x]

The interface is public.

No it is not. Look at the code you posted above. There is no public modifier for the interface.


Here is a lazy implementation which has the interface declared as public. You'll notice that created is only printed once.

using System;
public interface IDisplay
{
// some property accesses and methods
}
 
public class CDisplay: IDisplay {
    static IDisplay s_instance = null;
    public static IDisplay instance {
        get {
            if (null == s_instance)
                s_instance = new CDisplay();
 
            return s_instance;
        }
    }
    private CDisplay() {
        Console.WriteLine("created");
    } 
    public static void Main(String[] args ) {
        IDisplay test = CDisplay.instance;
        IDisplay test2 = CDisplay.instance;
    }
 
// implement constructor and interface
}

[dans]

No it is not. Look at the code you posted above. There is no public modifier for the interface.


Here is a lazy implementation which has the interface declared as public. You'll notice that created is only printed once.

Sorry, I should have said that the interface is public in my code. Is there a way of editing previous posts?

Your implementation is pretty much what I have come up with, except that CDisplay does not implement Main.

What I'm really trying to avoid is exposing any of CDisplay to the caller as it is being implemented as a separate assembly and only the interface should be exposed. I could have something like:

public interface IDisplay
{
    // interface properties and methods
}
 
public class CDisplay : IDisplay
{
    public static IDisplay instance {
        get {
            return CDisplayImplementation.instance;
        }
    }
    // abstract class implementing interface and providing static instance method.
}
 
// class internal to allow other parts of the assembly to access it but prevent access from outside the assembly.
internal class CDisplayImplementation : CDisplay
{
    private static CDisplay s_instance = null;
    public static IDisplay instance {
        get {
            if (null == s_instance)
                s_instance = new CDisplay();
            return s_instance;
        }
    }
    // concrete implementation
 
}
 

but by this seems unnecessarily complicated and I would be inclined to just expose my interface as an abstract class unless I want to inherit more than one interface.