Why Does My WPF C# Global Keyboard Hook Stop Working, and How Can I Fix It?

Using Global Keyboard Hook (WH_KEYBOARD_LL) in WPF / C#

To address the issue of the keyboard listener ceasing to work after a period of time, it's essential to understand the cause of the problem. The root of the issue lies in the way the callback delegate is created and used within the SetHook method.

The following code snippet demonstrates the issue:

public static IntPtr SetHook(LowLevelKeyboardProc proc)
{
    using (Process curProcess = Process.GetCurrentProcess())
    using (ProcessModule curModule = curProcess.MainModule)
    {
        return SetWindowsHookEx(WH_KEYBOARD_LL, proc,
            GetModuleHandle(curModule.ModuleName), 0);
    }
}

In this code, the callback delegate is created inline within the SetHook method call and assigned to the proc parameter. However, this delegate is not assigned to any other variable or property within the class or instance, meaning there is no reference to it that prevents it from being garbage collected.

When the delegate is garbage collected, the function pointer that was passed to SetWindowsHookEx becomes invalid, and the callback can no longer be invoked. This is why the listener stops working after a while.

To resolve this issue, it's necessary to keep a reference to the delegate alive as long as the hook is in place. This can be achieved by assigning the delegate to a field or property within the class or instance. For example:

private static LowLevelKeyboardProc _proc;

public static IntPtr SetHook(LowLevelKeyboardProc proc)
{
    _proc = proc;
    using (Process curProcess = Process.GetCurrentProcess())
    using (ProcessModule curModule = curProcess.MainModule)
    {
        return SetWindowsHookEx(WH_KEYBOARD_LL, _proc,
            GetModuleHandle(curModule.ModuleName), 0);
    }
}

By assigning the delegate to the _proc field, we ensure that it remains alive as long as the SetHook method is called. This prevents the garbage collector from collecting the delegate and ensures that the callback continues to be invoked.

The above is the detailed content of Why Does My WPF C# Global Keyboard Hook Stop Working, and How Can I Fix It?. For more information, please follow other related articles on the PHP Chinese website!