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    internal class A
    {
        public void DoSixth()
        {
        }

        public void DoFirst()
        {
        }
    }

    internal class B : A, IFirstFive
    {
    }

    internal class C : A, ISecondFive
    {
    }

    public class UsageClass
    {
        public void Usage()
        {
            IFirstFive b = new B();
            b.DoFirst();
            // b.DoSixth(); not working
        }
    }

    public interface ISecondFive
    {
        void DoSixth();
        //...
    }

    public interface IFirstFive
    {
        void DoFirst();
        //...
    }