Please note that this does NOT work properly with Windows Vista Aero (this includes Windows 10), and reports faulty values. This is largely due to Aero incorporating additional invisible borders which are used to "resize" the window using the cursor. In this instance, the developer should look into using DwmGetWindowAttribute (dwmapi.dll) with DWMWA_EXTENDED_FRAME_BOUNDS.

static extern int DwmGetWindowAttribute(IntPtr hwnd, DWMWINDOWATTRIBUTE dwAttribute, out bool pvAttribute, int cbAttribute);

static extern int DwmGetWindowAttribute(IntPtr hwnd, DWMWINDOWATTRIBUTE dwAttribute, out RECT pvAttribute, int cbAttribute);

Declare Function DwmGetWindowAttribute Lib "dwmapi.dll" (TODO) As TODO

    static extern int DwmGetWindowAttribute(IntPtr hWnd, DwmWindowAttribute dwAttribute, out RECT lpRect, int cbAttribute);

Enum DwmGetWindowAttribute