shut off monitor will flash display,by use SendMessage command.

Question

SendMessage function Win32 API Windows and Messages

Similar problems monitor-will-flash-before-shut-off-or-monitor-will.html

screen recording onedrive

using System;  
using System.Runtime.InteropServices;  
  
namespace ConsoleApp  
{  
    internal class Program  
    {  
        static void Main(string[] args)  
        {  
            const uint WM_SYSCOMMAND = 0x0112;  
  
            const uint SC_MONITORPOWER = 0xF170;  
  
            IntPtr HWND_BROADCAST = new IntPtr(0xffff);  
  
            // Shut down  
            SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, 2);  
  
            // Open the  
            //SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, -1);  
        }  
  
        [DllImport("user32.dll")]  
        public static extern IntPtr SendMessage(IntPtr hWnd, uint msg, uint wParam, int lParam);  
    }  
}  

SendMessage function (winuser.h)Win32 API Windows and Messages

Similar problems monitor-will-flash-before-shut-off-or-monitor-will.html

screen recording onedrive

using System;  
using System.Runtime.InteropServices;  
  
namespace ConsoleApp  
{  
    internal class Program  
    {  
        static void Main(string[] args)  
        {  
            const uint WM_SYSCOMMAND = 0x0112;  
  
            const uint SC_MONITORPOWER = 0xF170;  
  
            IntPtr HWND_BROADCAST = new IntPtr(0xffff);  
  
            // Shut down  
            SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, 2);  
  
            // Open the  
            //SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, -1);  
        }  
  
        [DllImport("user32.dll")]  
        public static extern IntPtr SendMessage(IntPtr hWnd, uint msg, uint wParam, int lParam);  
    }  
}  
 

Answer 1

I still cannot reproduce this...
Maybe you can test by sending the message only to the Desktop :

    DefWindowProc(GetDesktopWindow(), WM_SYSCOMMAND, (int)SC_MONITORPOWER, (IntPtr)2);  

with :

    [DllImport("User32.dll", SetLastError = false)]  
    public static extern IntPtr GetDesktopWindow();  
      
    [DllImport("User32.dll", SetLastError = true, CharSet = CharSet.Auto)]  
    public static extern int DefWindowProc(IntPtr hWnd, uint uMsg, int wParam, IntPtr lParam);

Answer 2

I also encountered this problem. The solution is not to use HWND_BROADCAST, but to create another window, send the message and then destroy the 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);
    }
}