why overriding  in the following  situation?
   
  suppose i have like this
   
   
  class a 
  {
  public virtual int add(int x ,int y)
  {
  retunt x+y;
  }
   
  }
   
  class a:b 
  {
  public Override int add(int x ,int y)
  {
  /// some logic here 
  }
   
  }
  in the above situation i can get the child and parent  add() method by creating object on them then why i go for Overriding ?
   
  is there is any performance issues are there ?like memory allocation...etc?
   
  Plz help me ...........