how to overwrite a method of inbuild control in c#.net

Well, you can't override any of the 6 overloads of the ToolTip.Show() method because they're not virtual. Nor can you hide them because the method you're proposing to add to the custom Tooltip class doesn't have the same signature as any of the others.

What you can do is to add another overload:

public class CustomToolTip : ToolTip

{

public void Show(string text, string someOtherString)

{

// code

}

// any other code

}

However, what good a Show method is without any control to display it for, I don't know. I assume you have some good reason for wanting to do this.