Question

In my C# app I want to do the keyhook but so, that the method invoked when the key is pressed is invoked in another thread. In this way it will not happen, that a key is omitted by my program, because my application was busy with something else. How can I make it in another thread? I have found information that, depending on the thread in which I create the hook, the method invoked when the key is pressed will be invoked in that thread, but that does not happen. Regardless of the thread in which I hooked it, this method is always invoked in the main thread. Please help. This is the outline of my code:

int HookProc(int code, int wParam, ref lParamStruct lParam)
{
    // ... this should be invoked in another thread - thread th, which I use to call SetWindowsHook
}
void SetWindowsHook()
{
    lpMsg msg = new lpMsg();

    PeekMessage(out msg, new IntPtr(0), 0, 0, PM_NOREMOVE);

    hhook = SetWindowsHookEx(WH_KEYBOARD_LL, HookProc, new IntPtr(0), 0);

    while (GetMessage(out msg, new IntPtr(0), 0, 0))
    {
        DefWindowProc(msg.hwnd, msg.message, msg.wParam, msg.lParam);
        if (abortThreadFlag)
            break;

    }
    UnhookWindowsHookEx(hhook);
}

SetWindowsHook is invoked in this way, but id doesn’t force the method to be invoked in another thread:

 Thread th = new Thread(SetWindowsHook);
 th.Start()

Answer 1

The point of the message loop is to give Windows an opportunity to call your hook procedure. There is no other way for it to call your code. It cannot break in and force your thread to run code, that would cause horrible re-entrancy problems. The hook procedure can only run when your code calls GetMessage(). And will run in the context of that thread. You can verify this by looking at the return value of GetCurrentThreadId().

Using a thread is otherwise no fix for a slow hook procedure. Windows won’t dispatch any other input events until the hook procedure returns. Make it slow enough and Windows will kill the hook forcibly to recover. Consider using a worker thread that you feed from a thread-safe queue instead, allowing your hook procedure to quickly return.