Problem with interface implementation in partial classes.

Question

I have a question regarding a problem with L2S, Autogenerated DataContext and the use of Partial Classes. I have abstracted my datacontext and for every table I use, I'm implementing a class with an interface. In the code below you can see I have the Interface and two partial classes. The first class is just there to make sure the class in the auto-generated datacontext inherets Interface. The other autogenerated class makes sure the method from Interface is implemented.

namespace PartialProject.objects
{

public interface Interface
{
    Interface Instance { get; }
}

//To make sure the autogenerated code inherits Interface
public partial class Class : Interface { }

//This is autogenerated
public partial class Class
{
    public Class Instance
    {
        get
        {
            return this.Instance;
        }
    }
}

}

Now my problem is that the method implemented in the autogenerated class gives the following error: -> Property 'Instance' cannot implement property from interface 'PartialProject.objects.Interface'. Type should be 'PartialProjects.objects.Interface'. <-

Any idea how this error can be resolved? Keep in mind that I can't edit anything in the autogenerated code.

Thanks in advance!

Answer 1

Return types aren't covariant in C#. As you can't change the auto-generated code the only solution I see is to change the interface:

public interface Interface<T>
{
    T Instance { get; }
}

And change your partial class accordingly:

public partial class Class : Interface<Class> { }

Answer 2

You can solve this by implementing the interface explicitely:

namespace PartialProject.objects
{
  public interface Interface
  {
    Interface Instance { get; }
  }

  //To make sure the autogenerated code inherits Interface
  public partial class Class : Interface 
  {
    Interface Interface.Instance 
    {
      get
      {
        return Instance;
      }
    }
  }

  //This is autogenerated
  public partial class Class
  {
     public Class Instance
     {
        get
        {
          return this.Instance;
        }
     }
  }
}