public abstract class ParentClass

{

    public string CommonMethod()

    {

        return "CommonMethod";

    }

​

    public abstract string Method3();

    public virtual string Method1()

    {

        return "Default Method 1";

    }

​

    public virtual string Method2()

    {

        return "Default Method 2";

    }

}

​

public class ClassA : ParentClass

{

    public override string Method1()

    {

        return "Class A - Method1";

    }

​

    public override string Method2()

    {

        return "Class A - Method2";

    }

​

    public override string Method3()

    {

        return "Class A - Method3";

    }

}

​

public class ClassB : ParentClass

{

    public override string Method1()

    {

        return "Class B - Method1";

    }

​

    public override string Method2()

    {

        return "Class B - Method2";

    }

​

    public override string Method3()

    {

        return "Class B - Method3";

    }

}

​

public static class MainClass

{

    public static void Main()

    {

        ParentClass instA = new ClassA();

        instA.CommonMethod();

        instA.Method1();

        instA.Method2();

        ClassB instB = new ClassB();

        instB.CommonMethod();

        instB.Method1();

        instB.Method2();

    }

}