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Hooks not working, garbage collection RRS feed

  • Question

  • Hey guys, 

    It seems a lot of people have this problem, but it has never been answered before as far as I can tell. I am using a global hook to get keyboard input for my c# application. I have this code:

    using System;
    using System.Collections.Generic;
    using System.Text;
    using System.Runtime.InteropServices;
    using System.Windows.Forms;
    
    namespace Utilities {
    	/// <summary>
    	/// A class that manages a global low level keyboard hook
    	/// </summary>
    	class globalKeyboardHook {
    		#region Constant, Structure and Delegate Definitions
    		/// <summary>
    		/// defines the callback type for the hook
    		/// </summary>
    		public delegate int keyboardHookProc(int code, int wParam, ref keyboardHookStruct lParam);
            private keyboardHookProc _keyboardHookProc; //!
    
    
            public struct keyboardHookStruct {
    			public int vkCode;
    			public int scanCode;
    			public int flags;
    			public int time;
    			public int dwExtraInfo;
    		}
    
    		const int WH_KEYBOARD_LL = 13;
    		const int WM_KEYDOWN = 0x100;
    		const int WM_KEYUP = 0x101;
    		const int WM_SYSKEYDOWN = 0x104;
    		const int WM_SYSKEYUP = 0x105;
    		#endregion
    
    		#region Instance Variables
    		/// <summary>
    		/// The collections of keys to watch for
    		/// </summary>
    		public List<Keys> HookedKeys = new List<Keys>();
    		/// <summary>
    		/// Handle to the hook, need this to unhook and call the next hook
    		/// </summary>
    		IntPtr hhook = IntPtr.Zero;
    		#endregion
    
    		#region Events
    		/// <summary>
    		/// Occurs when one of the hooked keys is pressed
    		/// </summary>
    		public event KeyEventHandler KeyDown;
    		/// <summary>
    		/// Occurs when one of the hooked keys is released
    		/// </summary>
    		public event KeyEventHandler KeyUp;
    		#endregion
    
    		#region Constructors and Destructors
    		/// <summary>
    		/// Initializes a new instance of the <see cref="globalKeyboardHook"/> class and installs the keyboard hook.
    		/// </summary>
    		public globalKeyboardHook() {
    			hook();
    		}
    
    		/// <summary>
    		/// Releases unmanaged resources and performs other cleanup operations before the
    		/// <see cref="globalKeyboardHook"/> is reclaimed by garbage collection and uninstalls the keyboard hook.
    		/// </summary>
    		~globalKeyboardHook() {
    			unhook();
    		}
            #endregion
    
            #region Public Methods
            /// <summary>
            /// Installs the global hook
            /// </summary>
             keyboardHookProc hookDelegate;
    
            public static void hook() {
                hookDelegate = new keyboardHookProc(hookProc);
                IntPtr hInstance = LoadLibrary("User32");
                hhook = SetWindowsHookEx(WH_KEYBOARD_LL, _keyboardHookProc, hInstance, 0);
            }
    
    		/// <summary>
    		/// Uninstalls the global hook
    		/// </summary>
    		public void unhook() {
    			UnhookWindowsHookEx(hhook);
    		}
    
    		/// <summary>
    		/// The callback for the keyboard hook
    		/// </summary>
    		/// <param name="code">The hook code, if it isn't >= 0, the function shouldn't do anyting</param>
    		/// <param name="wParam">The event type</param>
    		/// <param name="lParam">The keyhook event information</param>
    		/// <returns></returns>
    		public int hookProc(int code, int wParam, ref keyboardHookStruct lParam) {
    			if (code >= 0) {
    				Keys key = (Keys)lParam.vkCode;
    				if (HookedKeys.Contains(key)) {
    					KeyEventArgs kea = new KeyEventArgs(key);
    					if ((wParam == WM_KEYDOWN || wParam == WM_SYSKEYDOWN) && (KeyDown != null)) {
    						KeyDown(this, kea) ;
    					} else if ((wParam == WM_KEYUP || wParam == WM_SYSKEYUP) && (KeyUp != null)) {
    						KeyUp(this, kea);
    					}
    					if (kea.Handled)
    						return 1;
    				}
    			}
    			return CallNextHookEx(hhook, code, wParam, ref lParam);
    		}
    		#endregion
    
    		#region DLL imports
    		/// <summary>
    		/// Sets the windows hook, do the desired event, one of hInstance or threadId must be non-null
    		/// </summary>
    		/// <param name="idHook">The id of the event you want to hook</param>
    		/// <param name="callback">The callback.</param>
    		/// <param name="hInstance">The handle you want to attach the event to, can be null</param>
    		/// <param name="threadId">The thread you want to attach the event to, can be null</param>
    		/// <returns>a handle to the desired hook</returns>
    		[DllImport("user32.dll")]
    		static extern IntPtr SetWindowsHookEx(int idHook, keyboardHookProc callback, IntPtr hInstance, uint threadId);
    
    		/// <summary>
    		/// Unhooks the windows hook.
    		/// </summary>
    		/// <param name="hInstance">The hook handle that was returned from SetWindowsHookEx</param>
    		/// <returns>True if successful, false otherwise</returns>
    		[DllImport("user32.dll")]
    		static extern bool UnhookWindowsHookEx(IntPtr hInstance);
    
    		/// <summary>
    		/// Calls the next hook.
    		/// </summary>
    		/// <param name="idHook">The hook id</param>
    		/// <param name="nCode">The hook code</param>
    		/// <param name="wParam">The wparam.</param>
    		/// <param name="lParam">The lparam.</param>
    		/// <returns></returns>
    		[DllImport("user32.dll")]
    		static extern int CallNextHookEx(IntPtr idHook, int nCode, int wParam, ref keyboardHookStruct lParam);
    
    		/// <summary>
    		/// Loads the library.
    		/// </summary>
    		/// <param name="lpFileName">Name of the library</param>
    		/// <returns>A handle to the library</returns>
    		[DllImport("kernel32.dll")]
    		static extern IntPtr LoadLibrary(string lpFileName);
    		#endregion
    	}
    }
    


    (I know thats a lot, but I thought i'd include it in case). 

    Anyway, when I run my program I get this error:

    Managed Debugging Assistant 'CallbackOnCollectedDelegate' : 'A callback was made on a garbage collected delegate of type 'SlashA006!Utilities.globalKeyboardHook+keyboardHookProc::Invoke'. This may cause application crashes, corruption and data loss. When passing delegates to unmanaged code, they must be kept alive by the managed application until it is guaranteed that they will never be called.'

    So, like I said, this problem seems persistent. I found people with it here:

    https://social.msdn.microsoft.com/Forums/en-US/75bbf475-5dc5-4014-8533-9c901ba782b7/stop-garbage-collection-on-unmanaged-code-in-c?forum=csharpgeneral

    https://www.codeproject.com/Articles/19004/A-Simple-C-Global-Low-Level-Keyboard-Hook

    as well as other places, but no one has an answer that works. I would be so appreciative of someone could lend a hand. Thanks!

    Tuesday, May 8, 2018 1:50 AM

All replies

  • Have you tried to keep the globalKeyboardHook object (created with new in some other parts) in some static variable, or take other measures to make sure that the hookDelegate member is not deleted by Garbage Collector (GC)?

    It is also unclear how the _keyboardHookProc is initialised; maybe it can be deleted since you have hookDelegate.


    Tuesday, May 8, 2018 5:06 AM

  •   public static void hook() {
                hookDelegate = new keyboardHookProc(hookProc);
                IntPtr hInstance = LoadLibrary("User32");
                hhook = SetWindowsHookEx(WH_KEYBOARD_LL, _keyboardHookProc, hInstance, 0);
            }

    _keyboardHookProc is entirely uninitialized.  How did you expect this to work?  I suspect that very first statement was supposed to be assigned to _keyboardHookProc, and not hookDelegate.

    The hInstance is supposed to be the HINSTANCE of your executable, not the handle of an arbitrary DLL.  You can get the handle by calling GetModuleHandle(nullptr).


    Tim Roberts, Driver MVP Providenza & Boekelheide, Inc.

    Tuesday, May 8, 2018 5:16 AM
  • I'll try that
    Tuesday, May 8, 2018 10:35 PM