example of virtual and override

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace WindowsFormsApplication23
{
    class DML
    {
     protected int a, b;
       public DML(int x, int y)
       {
           a = x;
           b = y;
       }
       public virtual int add()
       {
           return a + b;
       }
    }
    class DML1:DML
    {
        public DML1(int m,int n)
           :base(m,n)
        { }
        public override int add()
        {
            return a + b + 5;
        }
    }
}

private void button1_Click_1(object sender, EventArgs e)
        {
            DML o = new DML(10,15);
           textBox1.Text = o.add().ToString();
        }
out put
-------------------------
25
-------------------------

private void button1_Click_1(object sender, EventArgs e)
        {
            DML o = new DML1(10,15);
           textBox1.Text = o.add().ToString();
        }
out put
-------------------------
30
-------------------------