Cannot send a new scroll direction using SendInput in a LowLevelMouseProc call

Question

I call the code below and it connects to WM_MOUSEWHEEL accordingly. The problem I am facing is that even if I send a scroll message in one direction (-120), the scroll wheel works as expected. I set breakpoints and ifs in it and work as expected.

private delegate IntPtr LowLevelMouseProc(int nCode, IntPtr wParam, IntPtr lParam); private static IntPtr HookCallback( int nCode, IntPtr wParam, IntPtr lParam) { if (nCode >= 0 && MouseMessages.WM_MOUSEWHEEL == (MouseMessages)wParam) { MSLLHOOKSTRUCT hookStruct = (MSLLHOOKSTRUCT)Marshal.PtrToStructure(lParam, typeof(MSLLHOOKSTRUCT)); Console.WriteLine(hookStruct.mouseData); INPUT[] input; input = new INPUT[1]; input[0].type = INPUT_MOUSE; input[0].mi.dx = 0; input[0].mi.dy = 0; input[0].mi.dwFlags = MOUSEEVENTF_WHEEL; input[0].mi.time = 0; input[0].mi.dwExtraInfo = 0; input[0].mi.mouseData = -120; SendInput(1, input, Marshal.SizeOf(input)); return (IntPtr)1; 

Answer 1

Finally it turned out with Phil :

 using System; using System.Diagnostics; using System.Windows.Forms; using System.Runtime.InteropServices; namespace EnableMacScrolling { class InterceptMouse { const int INPUT_MOUSE = 0; const int MOUSEEVENTF_WHEEL = 0x0800; const int WH_MOUSE_LL = 14; private static LowLevelMouseProc _proc = HookCallback; private static IntPtr _hookID = IntPtr.Zero; public static void Main() { _hookID = SetHook(_proc); if (_hookID == null) { MessageBox.Show("SetWindowsHookEx Failed"); return; } Application.Run(); UnhookWindowsHookEx(_hookID); } private static IntPtr SetHook(LowLevelMouseProc proc) { using (Process curProcess = Process.GetCurrentProcess()) using (ProcessModule curModule = curProcess.MainModule) { return SetWindowsHookEx(WH_MOUSE_LL, proc, GetModuleHandle(curModule.ModuleName), 0); } } private delegate IntPtr LowLevelMouseProc(int nCode, IntPtr wParam, IntPtr lParam); private static IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam) { if (nCode >= 0 && MouseMessages.WM_MOUSEWHEEL == (MouseMessages)wParam) { MSLLHOOKSTRUCT hookStruct = (MSLLHOOKSTRUCT)Marshal.PtrToStructure(lParam, typeof(MSLLHOOKSTRUCT)); Console.WriteLine(hookStruct.mouseData); if (hookStruct.flags != -1) //prevents recursive call to self { INPUT input; input = new INPUT(); input.type = INPUT_MOUSE; input.mi.dx = 0; input.mi.dy = 0; input.mi.dwFlags = MOUSEEVENTF_WHEEL; input.mi.time = 0; input.mi.dwExtraInfo = 0; input.mi.mouseData = -(hookStruct.mouseData >> 16); try { SendInput(1, ref input, Marshal.SizeOf(input)); } catch (Exception e) { System.Diagnostics.Debug.WriteLine(e.Message); } return (IntPtr)1; } } return CallNextHookEx(_hookID, nCode, wParam, lParam); } private enum MouseMessages { WM_MOUSEWHEEL = 0x020A } [StructLayout(LayoutKind.Sequential)] private struct POINT { public int x; public int y; } [StructLayout(LayoutKind.Sequential)] private struct MSLLHOOKSTRUCT { public POINT pt; public int mouseData; public int flags; public int time; public IntPtr dwExtraInfo; } public struct INPUT { public int type; public MOUSEINPUT mi; } [StructLayout(LayoutKind.Sequential)] public struct MOUSEINPUT { public int dx; public int dy; public int mouseData; public uint dwFlags; public int time; public int dwExtraInfo; } [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)] private static extern IntPtr SetWindowsHookEx(int idHook, LowLevelMouseProc lpfn, IntPtr hMod, uint dwThreadId); [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)] [return: MarshalAs(UnmanagedType.Bool)] private static extern bool UnhookWindowsHookEx(IntPtr hhk); [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)] private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam); [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)] private static extern IntPtr GetModuleHandle(string lpModuleName); [DllImport("User32.dll", SetLastError = true)] public static extern int SendInput(int nInputs, ref INPUT pInputs, int cbSize); } }