SendInput doesn't perform click mouse button unless I move cursor

SendInput doesn't perform click mouse button unless I move cursor.

I would appreciate a help on this one, as I seems cannot wrap my head around it.

I have a program that perform mouse click on foreground window, in which I am using SendInput to emulate left mouse click. The issue is, that if I move cursor to clicking position than SendInput will make the click, however if I don't move cursor than no click happens even trough I do pass x and y points to the MouseInputData. I would like to perform left mouse click without the need of actually moving the cursor at all.

Bellow is the class I have (it fairly simple and stright forward)

namespace StackSolution.Classes
{
    public static class SendInputClass
    {

        [DllImport("user32.dll", SetLastError = true)]
        static extern uint SendInput(uint nInputs, ref INPUT pInputs, int cbSize);

        [DllImport("user32.dll")]
        static extern bool SetCursorPos(int X, int Y);

        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool GetCursorPos(out Point lpPoint);



        [StructLayout(LayoutKind.Sequential)]
        struct INPUT
        {
            public SendInputEventType type;
            public MouseKeybdhardwareInputUnion mkhi;
        }
        [StructLayout(LayoutKind.Explicit)]
        struct MouseKeybdhardwareInputUnion
        {
            [FieldOffset(0)]
            public MouseInputData mi;

            [FieldOffset(0)]
            public KEYBDINPUT ki;

            [FieldOffset(0)]
            public HARDWAREINPUT hi;
        }
        [StructLayout(LayoutKind.Sequential)]
        struct KEYBDINPUT
        {
            public ushort wVk;
            public ushort wScan;
            public uint dwFlags;
            public uint time;
            public IntPtr dwExtraInfo;
        }
        [StructLayout(LayoutKind.Sequential)]
        struct HARDWAREINPUT
        {         
            public int uMsg;
            public short wParamL;
            public short wParamH;
        }
        struct MouseInputData
        {
            public int dx;
            public int dy;
            public uint mouseData;
            public MouseEventFlags dwFlags;
            public uint time;
            public IntPtr dwExtraInfo;
        }
        [Flags]
        enum MouseEventFlags : uint
        {
            MOUSEEVENTF_MOVE = 0x0001,
            MOUSEEVENTF_LEFTDOWN = 0x0002,
            MOUSEEVENTF_LEFTUP = 0x0004,
            MOUSEEVENTF_RIGHTDOWN = 0x0008,
            MOUSEEVENTF_RIGHTUP = 0x0010,
            MOUSEEVENTF_MIDDLEDOWN = 0x0020,
            MOUSEEVENTF_MIDDLEUP = 0x0040,
            MOUSEEVENTF_XDOWN = 0x0080,
            MOUSEEVENTF_XUP = 0x0100,
            MOUSEEVENTF_WHEEL = 0x0800,
            MOUSEEVENTF_VIRTUALDESK = 0x4000,
            MOUSEEVENTF_ABSOLUTE = 0x8000
        }
        enum SendInputEventType : int
        {
            InputMouse,
            InputKeyboard,
            InputHardware
        }

        public static void ClickLeftMouseButton(int x, int y)
        {
            INPUT mouseInput = new INPUT();
            mouseInput.type = SendInputEventType.InputMouse;
            mouseInput.mkhi.mi.dx = x;
            mouseInput.mkhi.mi.dy = y;
            mouseInput.mkhi.mi.mouseData = 0;

            //getting current cursor location
            Point p;
            if (GetCursorPos(out p))
                SetCursorPos(x, y);


            mouseInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENTF_LEFTDOWN;
            SendInput(1, ref mouseInput, Marshal.SizeOf(new INPUT()));

            mouseInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENTF_LEFTUP;
            SendInput(1, ref mouseInput, Marshal.SizeOf(new INPUT()));

            //returning cursor to previous position
            SetCursorPos(p.X, p.Y);
        }    
    }
  }

Same ClickLeftMouseButton function will not click if I remove getting cursor position like that.

   public static void ClickLeftMouseButton(int x, int y)
        {
            INPUT mouseInput = new INPUT();
            mouseInput.type = SendInputEventType.InputMouse;
            mouseInput.mkhi.mi.dx = x;
            mouseInput.mkhi.mi.dy = y;
            mouseInput.mkhi.mi.mouseData = 0;                     


            mouseInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENTF_LEFTDOWN;
            SendInput(1, ref mouseInput, Marshal.SizeOf(new INPUT()));

            mouseInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENTF_LEFTUP;
            SendInput(1, ref mouseInput, Marshal.SizeOf(new INPUT()));            
        }   

Thank you in advance.

There are a few things you should consider when using the SendInput function.

If you do not specify the MOUSEEVENTF_ABSOLUTE flag then dx and dy (MouseInputData structure) are relative coordinates to the current mouse position. If you do specify MOUSEEVENTF_ABSOLUTE then dx and dy are absolute coordinates between 0 and 65535. So if your x and y coordinates are screen coordinates you should use the following function to calculate dx and dy:

enum SystemMetric
{
  SM_CXSCREEN = 0,
  SM_CYSCREEN = 1,
}

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

int CalculateAbsoluteCoordinateX(int x)
{
  return (x * 65536) / GetSystemMetrics(SystemMetric.SM_CXSCREEN);
}

int CalculateAbsoluteCoordinateY(int y)
{
  return (y * 65536) / GetSystemMetrics(SystemMetric.SM_CYSCREEN);
}

Furthermore before you send the MOUSEDOWN and MOUSEUP events to via SendInput you have to move the mouse to the control you want to click on:

public static void ClickLeftMouseButton(int x, int y)
{
  INPUT mouseInput = new INPUT();
  mouseInput.type = SendInputEventType.InputMouse;
  mouseInput.mkhi.mi.dx = CalculateAbsoluteCoordinateX(x);
  mouseInput.mkhi.mi.dy = CalculateAbsoluteCoordinateY(y);
  mouseInput.mkhi.mi.mouseData = 0;                     


  mouseInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENTF_MOVE |MouseEventFlags.MOUSEEVENTF_ABSOLUTE;
  SendInput(1, ref mouseInput, Marshal.SizeOf(new INPUT()));

  mouseInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENTF_LEFTDOWN;
  SendInput(1, ref mouseInput, Marshal.SizeOf(new INPUT()));

  mouseInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENTF_LEFTUP;
  SendInput(1, ref mouseInput, Marshal.SizeOf(new INPUT()));            
} 

The above code assumes that x and y are screen pixel coordinates. You can calculate those coordinates for a button (the target) on a winform by using the following code:

 Point screenCoordsCentre=
button1.PointToScreen(new Point(button1.Width/2, button1.Height/2));

Hope, this helps.