The InvalidateRect API. From MSDN: The InvalidateRect function adds a rectangle to the specified window's update region. The update region represents the portion of the window's client area that must be redrawn.

Public Overloads Declare Function InvalidateRect Lib "User32" Alias "InvalidateRect" (ByVal hWnd As Int32, ByRef lpRect As RECT, ByVal bErase As Boolean) As Boolean

Public Overloads Declare Function InvalidateRect Lib "User32" Alias "InvalidateRect" (ByVal hWnd As Int32, ByVal lpRect As IntPtr, ByVal bErase As Boolean) As Boolean

Access PInvoke.net directly from VS: