


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.
The above is the detailed content of Why Does My WPF Global Keyboard Hook (WH_KEYBOARD_LL) Stop Working After Rapid Keystrokes?. For more information, please follow other related articles on the PHP Chinese website!

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