Why base class object refers its own methods, although referring to memory address of child class

 
using System;
using System.Collections.Generic;
using System.Text;
namespace ConsoleApplication1
{
       class basecalss
    {
        public  void virtual_method()
        {
            Console.WriteLine("Virtual in baseclass");
        }
    }
    class derivedclass1 : basecalss
    {
        public  void virtual_method()
        {
            Console.WriteLine("Override in dervied class1");
        }
    }
   
    class implement_override  
    {
        public static void Main()
        {
            basecalss baseobj;
            derivedclass1 derobj1 = new derivedclass1();
            baseobj = derobj1;
            baseobj.virtual_method();
            Console.ReadLine();
                 
        }
    }
}
-------------------------------------------------------