Why Does My WPF WH_KEYBOARD_LL Hook Stop Working After Some Keystrokes, and How Can I Fix It?

Implementing WH_KEYBOARD_LL Hook in WPF Applications: Addressing the Intermittent Shutdown Issue

Introduction

The WH_KEYBOARD_LL hook, a low-level keyboard hook, enables you to capture and handle keyboard inputs in a WPF application. However, some users have reported an issue where the hook ceases to function after a period of keystrokes. To resolve this problem, let's delve into the underlying cause and provide a solution.

The Problem

The issue arises from a threading problem. When creating the low-level keyboard hook callback delegate, it's done inline within the SetHook method call. Unfortunately, this delegate is immediately eligible for garbage collection since no reference is maintained. Once the delegate is collected, no more callbacks are received.

The Solution

To prevent the delegate from being garbage collected, we need to keep a reference to it while the hook is active. This can be achieved by storing the delegate in a field or property of the class:

private HookCallback callback;
...

public KeyboardListener()
{
    callback = new HookCallback(HookCallbackInner);
    hookId = InterceptKeys.SetHook(callback);
}

By assigning the callback to a field, the delegate is kept alive for the duration of the hook's existence. This ensures that callbacks continue to be received even after multiple keystrokes.

Additional Considerations

To further enhance the stability of the keyboard hook, consider the following recommendations:

• Use the LowLevelKeyboardProc delegate type from the System.Windows.Input namespace instead of creating a custom one. This ensures compatibility with WPF's input system.
• Check the return value of SetHook. If it returns zero, it indicates that the hook was not installed successfully.
• Handle exceptions gracefully in your callback method to prevent crashes.

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