Invalidates a specific region of the control and causes a paint message to be sent to the control. Optionally, invalidates the child controls assigned to the control.
Calling the Control.Invalidate(System.Drawing.Region) method does not force a synchronous paint; to force a synchronous paint, call the Control.Update method after calling the Control.Invalidate(System.Drawing.Region) method. When this method is called with no parameters, the entire client area is added to the update region.