static extern bool ScreenToClient(IntPtr hWnd, ref POINT lpPoint);

    Private Shared Function ScreenToClient(ByVal hWnd As IntPtr, ByRef lpPoint As POINT) As Boolean

ScreenToClient(hWnd, ref topLeft);

ScreenToClient(hWnd, ref bottomRight);

The last bit of the above example is incorrect. The correct client left/top will be retrieved but the width/height will be inaccurate. This function reports the distance of any given screen coordinates from the origin of our hwnd's ClientRect. In this way it allows us to deduce the location of the ClientRect origin given any other point. It does not allow us to deduce the location of any other point of the ClientRect, just the origin. The above example errs by trying to use this to deduce the distance from the ClientRect origin to the WindowRect bottom/right. This is not helpful because it will not give the accurate height and width of the ClientRect since the WindowRect bottom/right will be further out and different than the yet unknown ClientRect. The origin is deducible because that's where the function reports from, but this is not true of any other point in the ClientRect. So the proper way to use this is to make only the first call to ScreenToClient and then call GetClientRect to retrieve the ClientRect dimensions and add them to the origin.