public abstract class Base<T>

{

    protected abstract InnerFooBase<InnerBarBase<T>> GetInnerFoo();

    protected abstract class InnerFooBase<TFoo> where TFoo : InnerBarBase<T>

    {

    }

    protected abstract class InnerBarBase<TBar>

    {

    }

}

​

public class Implementation : Base<string>

{

    protected override InnerFooBase<InnerBarBase<string>> GetInnerFoo()

    {

        return new InnerFoo();

    }

    class InnerFoo : InnerFooBase<InnerBar>

    {

    }

    class InnerBar : InnerBarBase<string>

    {

    }

}