programmatically mouse click in another window

You can’t do that by sending messages, instead use SendInput Windows API.

Call method ClickOnPoint, this is an example from form click event, so this.handle is form handle, note that these are client coordinates on window witch handle is send, you can easily change this and send screen coordinates, and in that case you don’t need handle or ClientToScreen call below.

ClickOnPoint(this.Handle, new Point(375, 340));

UPDATE: using SendInput now, tnx Tom.

btw. I used only declarations needed for this sample, for anything more there is a nice library : Windows Input Simulator (C# SendInput Wrapper – Simulate Keyboard and Mouse)

  public class ClickOnPointTool
  {

    [DllImport("user32.dll")]
    static extern bool ClientToScreen(IntPtr hWnd, ref Point lpPoint);

    [DllImport("user32.dll")]
    internal static extern uint SendInput(uint nInputs, [MarshalAs(UnmanagedType.LPArray), In] INPUT[] pInputs,  int cbSize);

#pragma warning disable 649
    internal struct INPUT
    {
      public UInt32 Type;
      public MOUSEKEYBDHARDWAREINPUT Data;
    }

    [StructLayout(LayoutKind.Explicit)]
    internal struct MOUSEKEYBDHARDWAREINPUT
    {
      [FieldOffset(0)]
      public MOUSEINPUT Mouse;
    }

    internal struct MOUSEINPUT
    {
      public Int32 X;
      public Int32 Y;
      public UInt32 MouseData;
      public UInt32 Flags;
      public UInt32 Time;
      public IntPtr ExtraInfo;
    }

#pragma warning restore 649


    public static void ClickOnPoint(IntPtr wndHandle , Point clientPoint)
    {
      var oldPos = Cursor.Position;

      /// get screen coordinates
      ClientToScreen(wndHandle, ref clientPoint);

      /// set cursor on coords, and press mouse
      Cursor.Position = new Point(clientPoint.X, clientPoint.Y);

      var inputMouseDown = new INPUT();
      inputMouseDown.Type = 0; /// input type mouse
      inputMouseDown.Data.Mouse.Flags = 0x0002; /// left button down

      var inputMouseUp = new INPUT();
      inputMouseUp.Type = 0; /// input type mouse
      inputMouseUp.Data.Mouse.Flags = 0x0004; /// left button up

      var inputs = new INPUT[] { inputMouseDown, inputMouseUp };
      SendInput((uint)inputs.Length, inputs, Marshal.SizeOf(typeof(INPUT)));

      /// return mouse 
      Cursor.Position = oldPos;
    }

  }