Do you mean you want Class C to be the base class for A & B in that case.
public abstract class C
{
public abstract void Method1();
public abstract void Method2();
}
public class A : C
{
public override void Method1()
{
throw new NotImplementedException();
}
public override void Method2()
{
throw new NotImplementedException();
}
}
public class B : C
{
public override void Method1()
{
throw new NotImplementedException();
}
public override void Method2()
{
throw new NotImplementedException();
}
}