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

Using Global Keyboard Hook (WH_KEYBOARD_LL) in WPF / C#: Troubleshooting Callback Disappearing

In the provided code, the global keyboard hook is established using the SetHook method. However, there is a potential issue where the hook delegate, defined inline in the SetHook call, is not properly referenced and can be garbage collected. This leads to the callback function failing, and no further keyboard events are received.

Solution:

To resolve this issue, it is necessary to explicitly create the callback delegate and keep a reference to it within the application. This can be done by declaring the delegate variable outside the SetHook method and passing it as an argument:

public class App : Application
{
    private KeyboardListener KListener;
    private LowLevelKeyboardProc hookCallback;

    private void Application_Startup(object sender, StartupEventArgs e)
    {
        hookCallback = HookCallback;
        KListener = new KeyboardListener(hookCallback);
        KListener.KeyDown += new RawKeyEventHandler(KListener_KeyDown);
    }

    private void Application_Exit(object sender, ExitEventArgs e)
    {
        KListener.Dispose();
    }
}

In this updated code:

1. The LowLevelKeyboardProc delegate is declared as a private field named hookCallback.
2. In the Application_Startup method, the hookCallback delegate is passed as an argument to the KeyboardListener constructor.
3. The KListener instance is created and references the callback using the instance constructor.

By maintaining a reference to the delegate, the callback function will remain in scope and will continue to receive keyboard events as expected, resolving the issue where the hook stops working after a period of time.

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