Question

I’m leveraging the Doubleton Pattern from the Doubleton Design Pattern on Code Project in my own code. I think it makes things a lot easier since the Singleton only provides one instance, but I get two with this pattern. I was wondering if it would make sense to have it implement an interface so I can inject it into my domain layer.

Answer 1

Despite the usefulness and cleverness of such design, it is not possible. You can’t define a static member in an interface.

public interface IDoubleton
{
    static IDoubleton Instance { get; } // Can't be done
}

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@Pierreten, If you use instance methods, imagine this:

IDoubleton firstInstance;
// Construct firstInstance,
// do some stuff...

// Now comes trouble.
IDoubleton secondInstance = firstInstance.Instance; // what does it mean?
IDoubleton anotherInstance = secondInstance.Instance.Instance.Instance; // what now?

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EDIT: As @Roger Pate rightfully pointed out, my 2nd answer should be merged within this one. As he already had done it, I deleted the other one.