Abstract/Interface question

Question

I'm using abstract classes to enforce implementation of certain methods.   At the same time, I am also requiring derivitives to implement methods of an common interface (this interface is used elsewhere also, so I don't want to change that part).

 

I'm getting inconvenienced by a compiler error, illustrated by the following code:

 

Code Snippet

public interface IMyInterface

{

   void bob();

}

 

public abstract class Foo : IMyInterface

{

}

 

public class Bar : Foo

{

   void bob()

   {

   }

}

 

 

The error is:

error CS0535: 'Foo' does not implement interface member 'IMyInterface.bob()'

 

From my perspective, I don't want class Foo to implement bob(). It's an abstract class, and the responsibility that bob() is implemented should be given to the derived class.  So 1.) Is this an oversight in C#?  and 2) any suggestions on how to get around this?

 

Thanks,

 

Brian

 

 

Answer 1

If you say that Foo will implement IMyInterface, you have to make sure that the slots for the members on IMyInterface exist in Foo.

By declaring the bob() message as abstract, you create such a slot, but because it's abstract, the actual implementation will be forced upon derivatives...

Code Snippet

public abstract class Foo : IMyInterface
{
 public abstract void bob();
}

// we want to create instances of Bar, thus we have to make sure there are no abstract members anymore...

class Bar : Foo

{

 public override void bob() { }

}

Answer 2

 

Ah, that's the trick... I'm wondering if I'm being dense on this, or this is not an obvious thing to do.  In any case, I'm good to go. Thanks, Tim!

Answer 3

No you are not being dense. This is a very strange solution, and I wonder if there is a reason why they wanted it this way. If I do the same thing in Java then I dont have to add abstract stubs to the abstract class.

Answer 4

I agree. I just got stuck on this, I don't see why I need to declare the methods a second time. Seems like more code to fix if I want to make changes later, but no benefit.

-Scott

Answer 5

I disagree my friends.
If you do not want to implement bob() in your abstract class,
the correct way is to declare your interface as abstract interface
and the method abstract method, that's it.
let me know your thoughts.

Answer 6

You cannot implement an abstract interface.  Try compiling the following:

public abstract interface IMyInterface
{
}

It will not compile.

---

David Morton - http://blog.davemorton.net/

Answer 7

Hi Dave,

it does not compile in C#?


then that is the problem with C#.

It is perfectly legal in JAVA... and that is the only reason why one would use an abstract interface.

Answer 8

please see for example:

http://www.ph.ed.ac.uk/~adk/java/qref11/java.sql.Connection.html