Using interfaces on abstract classes in C#

Question

I'm learning C# coming from C++ and have run into a wall.

I have an abstract class AbstractWidget, an interface IDoesCoolThings, and a class which derives from AbstractWidget called RealWidget:

public interface IDoesCoolThings
{
    void DoCool();
}

public abstract class AbstractWidget : IDoesCoolThings
{
    void IDoesCoolThings.DoCool()
    {
        Console.Write("I did something cool.");
    }
}

public class RealWidget : AbstractWidget
{

}

When I instantiate a RealWidget object and call DoCool() on it, the compiler gives me an error saying

'RealWidget' does not contain a definition for 'DoCool'

I can cast RealWidget object to an IDoesCoolThings and then the call will work, but that seems unnecessary and I also lose polymorphism (AbstractWidget.DoCool() will always be called even if i define RealWidget.DoCool()).

I imagine the solution is simple, but I've tried a variety of things and for the life of me can't figure this one out.

Answer 1

You're running into the issue because you used explicit interface implementation (EII). When a member is explicitly implemented, it can't be accessed through a class instance -- only through an instance of the interface. In your example, that's why you can't call DoCool() unless you cast your instance to IDoesCoolThings .

The solution is to make DoCool() public and remove the explicit interface implementation:

public abstract class AbstractWidget : IDoesCoolThings
{
    public void DoCool()      // DoCool() is part of the abstract class implementation.
    {
        Console.Write("I did something cool.");
    }
}

// ...

var rw = new RealWidget();
rw.DoCool();                  // Works!

In general, you use EII in two cases:

• You have a class that must implement two interfaces, each of which contains a member that has an identical name/signature to another member in the other interface.
• You want to force clients not to depend on the implementation details of your class, but rather on the interface that's being implemented by your class. (This is considered a good practice by some.)

Answer 2

Change your declaration to:

public abstract class AbstractWidget : IDoesCoolThings 
{
    public void DoCool()
    { 
        Console.Write("I did something cool."); 
    }
}

Answer 3

The way you implement the interface is explicit implement void IDoesCoolThings.DoCool(), if you choose implicit implement interface.

public abstract class AbstractWidget : IDoesCoolThings
{
    public void DoCool()
    {
        Console.Write("I did something cool.");
    }
}

Then it will work.

Read this :

C# Interfaces. Implicit implementation versus Explicit implementation

Answer 4

You should do it this way:

public interface IDoesCoolThings 
{
   void DoCool();
}

public abstract class AbstractWidget 
{
   public void DoCool()
   {
      Console.WriteLine("I did something cool.");
   }
}

public class Widget : AbstractWidget, IDoesCoolThings 
{
}

Usage:

var widget = new Widget();
widget.DoCool();