C#

abstract class CBase
{
    public override abstract string ToString();
}

class CDeriv1 : CBase
{
    public override string ToString()
    {
        return "Deriv1";
    }
}

class Program
{
    static void Main(string[] args)
    {
        CBase nod = new Deriv1();
    }
}

Keith Barrow wrote:

It is already declared as virtual in object,

CDeriv1 foo = new CDeriv1();
CBase myInstance = foo;

static int weekNum1(DateTime T)
{
    return 1 + (int)T.Day / 7;
}

static int weekNum2(DateTime T)
{
    int weekNum = 1;

    DateTime w = new DateTime(T.Year, T.Month, 2);

    while (w <= T)
    {
        if (w.DayOfWeek == DayOfWeek.Sunday)
            weekNum++;

        w = w.AddDays(1.0);
    }

    return weekNum;
}