Why Does My WPF Global Keyboard Hook (WH_KEYBOARD_LL) Stop Working After Rapid Keystrokes?

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

This question discusses the issue of a global keyboard hook stopping working after repeatedly hitting keys.

Problem:

A global keyboard hook implemented using WH_KEYBOARD_LL stops receiving key events after a period of intense keystrokes. No errors are thrown, and it occurs regardless of where the keystrokes occur.

Suspected Cause:

Threading issues are suspected to be the underlying problem.

Code Sample for Keyboard Hook:

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;

        // Delegate to hook callback (method that will handle key events)
        private static InterceptKeys.LowLevelKeyboardProc hookCallback;

        [MethodImpl(MethodImplOptions.NoInlining)]
        private IntPtr HookCallback(
            int nCode, IntPtr wParam, IntPtr lParam)
        {
            // Handle key events here
            return InterceptKeys.CallNextHookEx(hookId, nCode, wParam, lParam);
        }

        public event RawKeyEventHandler KeyDown;
        public event RawKeyEventHandler KeyUp;

        public KeyboardListener()
        {
            // Create and assign the hook callback delegate
            hookCallback = (nCode, wParam, lParam) =>
            {
                HookCallbackInner(nCode, wParam, lParam);
                return InterceptKeys.CallNextHookEx(hookId, nCode, wParam, lParam);
            };

            // Set the hook using the delegate as a parameter
            hookId = InterceptKeys.SetHook(hookCallback);
        }

        // Remaining code not shown for brevity
    }

    internal static class InterceptKeys
    {
        public delegate IntPtr LowLevelKeyboardProc(
            int nCode, IntPtr wParam, IntPtr lParam);

        // Hook-related constants
        public const int WH_KEYBOARD_LL = 13;
        public const int WM_KEYDOWN = 0x0100;
        public const int WM_KEYUP = 0x0101;

        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);
            }
        }

        // Other code not shown for brevity
    }

    // Event args and handlers for key events
    public delegate void RawKeyEventHandler(object sender, RawKeyEventArgs args);
}

Solution:

As suggested by the answer, the delegate to the callback method is created inline, which can lead to it being garbage-collected. To keep the delegate alive and prevent this issue, it's necessary to assign the delegate to a member variable.

// Assign the hook callback delegate to a member variable
private InterceptKeys.LowLevelKeyboardProc hookCallback;

By doing this, the delegate will be kept alive for the lifetime of the KeyboardListener object. This should resolve the issue where the keyboard hook stops working after a period of time.

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