public virtual void A()
{
    ...
}

public abstract void B();

    public abstract class Test
    {
        public virtual void A()
        {
            Console.WriteLine("Test A...");
        }
        public abstract void B();
    }

    public class TestA : Test
    {
        //public override void A()
        //{//    Console.WriteLine("TestA A...");
        //}

        //public new void A()
        //{
        //    Console.WriteLine("TestA A...");
        //}

        public override void B()
        {
            Console.WriteLine("TestA B...");
        }
    }