Turn off the monitor in windows - winapi

Question

I need to turn off my monitor in code. I googled and found some code that should do it, but when I run this, nothing at all happens, and I just get a 0 WinAPI result. What am I doing wrong?
class Program
{
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
[DllImport("user32.dll")]
private static extern IntPtr GetDesktopWindow();
const int SC_MONITORPOWER = 0xF170;
const int WM_SYSCOMMAND = 0x0112;
const int MONITOR_OFF = 2;
static void Main(string[] args)
{
Console.WriteLine(
SendMessage(
GetDesktopWindow(),
WM_SYSCOMMAND,
(IntPtr) SC_MONITORPOWER,
(IntPtr) MONITOR_OFF));
Console.WriteLine("Hello World!");
}
}

Answer 1

From Fumbling around in the dark and stumbling across the wrong solution:
the desktop window is a very special window and as a rule should be avoided, since it won't behave like windows created by applications. In particular, the author tried to post a message to the desktop window. This used to work in the historically open world of the window manager, but security and robustness concerns have come to take priority over compatibility.
The real solution is to create your own window and send it the message, anything else is a hack.
If you don't mind hacks, at least try to find a suitable window:
[DllImport("user32.dll", SetLastError = false)]
public static extern IntPtr GetForegroundWindow();
[DllImport("user32.dll")]
static extern IntPtr PostMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
const int SC_MONITORPOWER = 0xF170;
const int WM_SYSCOMMAND = 0x0112;
const int MONITOR_OFF = 2;
static void Main(string[] args)
{
IntPtr w;
for (; IntPtr.Zero == (w = GetForegroundWindow());) System.Threading.Thread.Sleep(1000);
PostMessage(w, WM_SYSCOMMAND, (IntPtr) SC_MONITORPOWER, (IntPtr) MONITOR_OFF);
}
And because it is a hack there are where times it might not work. The window you are borrowing might be destroyed before you post the message. You might not have the right to send messages to it (UAC). Or it might decide to not pass this message to DefWindowProc.
Another slightly better hack is to create a temporary window:
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
[DllImport("user32.dll")]
static extern int DestroyWindow(IntPtr hWnd);
[DllImport("user32.dll", SetLastError=true)]
public static extern IntPtr CreateWindowEx(uint dwExStyle, string lpClassName, IntPtr cap, uint dwStyle, int x, int y, int nWidth, int nHeight, IntPtr hWndParent, IntPtr hMenu, IntPtr hInstance, IntPtr lpParam);
const int SC_MONITORPOWER = 0xF170;
const int WM_SYSCOMMAND = 0x0112;
const int MONITOR_OFF = 2;
static void Main(string[] args)
{
IntPtr w = CreateWindowEx(0, "Button", IntPtr.Zero, 0, 0, 0, 0, 0, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero);
if (w != IntPtr.Zero)
{
SendMessage(w, WM_SYSCOMMAND, (IntPtr) SC_MONITORPOWER, (IntPtr) MONITOR_OFF);
DestroyWindow(w);
}
}
This is still somewhat of a hack because it never actually pumps messages.