C# winapi mouse_event

Question

        static uint DOWN = 0x0002;
        static uint UP = 0x0004;    
        [DllImport("user32.dll")]
        static extern void mouse_event(uint dwFlags, int dx, int dy, uint dwData, int dwExtraInfo); 
        
        static void Click()
        {
            mouse_event(DOWN, 800, 800, 0, 0);
            Thread.Sleep(100);
            mouse_event(UP, 800, 800, 0, 0);
        }

Why it does not click on my coordinates 800 800, it clicks where my mouse is

Answer 1

If you want to click specified point instead of relative movement, the MOUSEEVENTF_ABSOLUTE flag is required. And in order for making the movement from current position to target position, the MOUSEEVENTF_MOVE flag is required.

So the code will like this:

    static uint DOWN = 0x0002;
    static uint UP = 0x0004;
    static uint MOUSEEVENTF_ABSOLUTE = 0x8000;
    static uint MOUSEEVENTF_MOVE = 0x0001;
    [DllImport("user32.dll")]
    static extern void mouse_event(uint dwFlags, int dx, int dy, uint dwData, int dwExtraInfo);

    static void Click()
    {
        mouse_event(DOWN | MOUSEEVENTF_ABSOLUTE | MOUSEEVENTF_MOVE, 800, 800, 0, 0);
        Thread.Sleep(100);
        mouse_event(UP | MOUSEEVENTF_ABSOLUTE | MOUSEEVENTF_MOVE, 800, 800, 0, 0);
    }

And note:

If MOUSEEVENTF_ABSOLUTE value is specified, dx and dy contain normalized absolute coordinates between 0 and 65,535. The event procedure maps these coordinates onto the display surface. Coordinate (0,0) maps onto the upper-left corner of the display surface, (65535,65535) maps onto the lower-right corner.

So if you want to click the specified pixel of the screen you can convert absolute coordinate to screen pixel like this:

    static int SM_CXSCREEN = 0;
    static int SM_CYSCREEN = 1;

    [DllImport("user32.dll")]
    static extern int GetSystemMetrics(int nIndex);

    static void Click()
    {
        int sx = GetSystemMetrics(SM_CXSCREEN);
        int sy = GetSystemMetrics(SM_CYSCREEN);

        int x = 800 * 65536 / sx;
        int y = 800 * 65536 / sy;

        mouse_event(DOWN | MOUSEEVENTF_ABSOLUTE | MOUSEEVENTF_MOVE, x, y, 0, 0);
        Thread.Sleep(100);
        mouse_event(UP | MOUSEEVENTF_ABSOLUTE | MOUSEEVENTF_MOVE, x, y, 0, 0);
    }

Note mouse_event function has been superseded. Use SendInput instead.