gistfile1.cs

interface I1
{
    void Add(int _fn, int _sn);
}

interface I2
{
    void Add(int _fn, int _sn);
}

abstract class Simple: I2, I1
{
    public abstract void Add(int _fn, int _sn);
    public abstract void Prod(int _fn, int _sn);
    public abstract void Sub(int _fn, int _sn);
    void I2.Add(int _fn, int _sn)
    {
        AddInternal(_fn, _sn);
    }
    
    protected abstract void AddInternal(int _fn, int _sn);
}

class Test : Simple
{
    public override void Add(int _fn, int _sn)
    {
    }
    public override void Prod(int _fn, int _sn)
    {
    }
    public override void Sub(int _fn, int _sn)
    {
    }

    protected override void AddInternal(int _fn, int _sn)
    {
        Console.WriteLine(_fn + _sn);
    }
}