Why Does My Global Keyboard Hook in C# Stop Working After Sustained Keystrokes?

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

The provided code aims to establish a global keyboard hook using the WH_KEYBOARD_LL hook to capture and process keyboard events. While the implementation essentially achieves its intended functionality, a crucial issue arises: after a period of sustained keystrokes, the event handling ceases abruptly.

This behavior suggests a potential threading problem. To resolve this, we need to ensure that the callback delegate remains alive as long as the hook is in place.

Problem Analysis

The callback delegate, HookCallback, is defined inline within the SetHook method. Consequently, the delegate is eligible for garbage collection once the method exits. When this occurs, the hook will stop receiving callbacks, leading to the observed termination of event handling.

Solution

To rectify this, we need to keep a reference to the delegate and ensure its lifetime coincides with the hook's existence. Here's the revised code that addresses the issue:

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;
using System.Windows.Input;

namespace MYCOMPANYHERE.WPF.KeyboardHelper
{
    public class KeyboardListener : IDisposable
    {
        private static IntPtr hookId = IntPtr.Zero;
        private InterceptKeys.LowLevelKeyboardProc callback;

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

        [MethodImpl(MethodImplOptions.NoInlining)]
        private IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
        {
            try
            {
                return HookCallbackInner(nCode, wParam, lParam);
            }
            catch
            {
                Console.WriteLine("There was some error somewhere...");
            }
            return InterceptKeys.CallNextHookEx(hookId, nCode, wParam, lParam);
        }

        private IntPtr HookCallbackInner(int nCode, IntPtr wParam, IntPtr lParam)
        {
            // ... (original code here)

            return InterceptKeys.CallNextHookEx(hookId, nCode, wParam, lParam);
        }

        // ... (rest of the class)
    }
}

In this updated code, the LowLevelKeyboardProc delegate (callback) is now a field of the KeyboardListener class. By maintaining this reference, we ensure that the delegate remains alive and available for callback invocations throughout the hook's lifetime.

By addressing the threading issue in this manner, the global keyboard hook will continue to capture and process keyboard events reliably, even under relentless keystroke scenarios.

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