CodeProject: A Simple C# Global Low Level Keyboard Hook. Free source code and programming help

This article discusses a class that I wrote that wraps a global low level keyboard hook. The sample hooks the A and B keys, just for demonstration.

Background

I was trying to find a way for an application that I am writing to restore itself when a combination of keys was pressed. This was born from searching around for the answer.

Using the Code

First download the source, and add globalKeyboardHook.cs to your project. Then add...

... to the top of the file you are going to use it in. Next add an instance of globalKeyboardHook to your class:

When a globalKeyboardHook is constructed, it automatically installs the hook, so all there is left to do is add some keys for it to watch, and define some event handlers for the KeyDown and KeyUp events. I usually do this on the main form's load event handler, like this:

Here I have chosen to watch for the A and B keys, and defined handlers for the KeyUp and KeyDown events that both log to a listbox called lstLog. So whenever the user presses the A or B keys, no matter what has focus, the application will be notified. Setting e.Handled to true makes it so no other notifications for this event go out, in the sample, this effectively stops the user from typing an A or B. This can be useful in ensuring that key combinations are not also typed out when used.

You can add hooks for as many keys as you would like, just add them like above. Don't get frustrated if you add a hook for a key and it doesn't work, many of them, like Keys.Shift show up as other more specific keys, like Keys.LShiftKey or Keys.RShiftKey. Keys.Alt shows up as Keys.LMenu or Keys.RMenu, Keys.Control shows up as Keys.LControl or Keys.RControl, just to name a few.

If you would like to hook or unhook the keyboard hook at any point, just call your globalKeyboardHook's hook and unhook methods, like so:

Points of Interest

The bulk of the work in this code is done in the globalKeyboardHook class, although it is a fairly simple piece of code itself. The hardest part of doing this was finding the correct parameters for SetWindowsHookEx.

The first parameter WH_KEYBOARD_LL is just saying that we want to hook the low level keyboard events, hookProc is the callback for the event, hInstance is a handle to User32.dll, where this event is first processed (I think). The last parameter is if you want to hook a specific thread, then you would just pass a thread id instead of using the hInstance.

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Callback Error

Voulnet

6:38 29 Oct '09

Hello, I'm using your great hook class, and it's been responding great, until I needed to use Settings in Visual Studio!
Basically, I allow the user to save his settings; the settings of the modified key functions.
Just adding this line in the code generates an annoying error:

if (Properties.Settings.Default.DisableAll == false) { }

The error I'm getting is:

A callback was made on a garbage collected delegate of type 'key preview!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.

This is really stopping me from further progress, and I am completely unable to do anything about it, even MSDN experts are not capable to providing solutions!

Re: Callback Error

Voulnet

7:32 29 Oct '09

Never mind, I found the solution here: Solution .
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Great

Ravenet

8:24 9 Oct '09

Hi I have a question in my application , when i used ur code, how i contact you? Cheers,Earn and Enjoy RRave MCTS,MCPD http://ravesoft.blogspot.com
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Alt + Q

a.xyz.b.1

8:40 6 Jul '09

Great Code, thanks!

I was trying to capture Alt+Q. I was able to get "Alt" through Control.ModifierKeys. However, when casting lParam.vkCode to (Keys), I constantly got "MButton | Space | F17", and vkCode was always 164. (I was expecting to get "Q")

I tried with other key combinations, stil getting the same thing. Does anyone knoow what could have caused this and how to resolve it?

Thanks a lot!

Stops Capturing Keys

Nate Shoffner

13:23 24 May '09

As another user has posted below, at random keystroke intervals, it stops capturing them? Any idea as to why?
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Re: Stops Capturing Keys

berni2704

4:27 16 Oct '09

i havre the same problem, after 7 times pressing my hotkey, the proggramm just stops hooking the key.... any ideas why?
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transfor it to wpf

sonicred

2:09 2 Apr '09

hi i'm trying to get it to work in wpf. i have only one problem that is the keys.. using system.windows.input.key the problem is in here: Key key = (Key)lParam.vkCode; i get a code that is for windows form keys.. what do i have to change, to work with the right key in wpf..
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Re: transfor it to wpf

StormySpike

8:55 2 Apr '09

Use `Key k = KeyInterop.KeyFromVirtualKey(lParam.vkCode);`
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Re: transfor it to wpf

sonicred

14:48 3 Apr '09

thx that did work can you explain a little bit why this ..?
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Re: transfor it to wpf

StormySpike

14:47 20 Apr '09

It's just WPF's built in converter for virtual key codes to it's Key enum, no magic there
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Re: transfor it to wpf

panoylhs

5:42 22 Sep '09

I added your class to my wpf project but I get an exception with the following message:

A callback was made on a garbage collected delegate of type 'BingIt!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.

and then nullreferenceexception...Any ideas? Frown

Thanks for sharing your usefull code Smile

unhook does not work, and when i capture the keyboard you cant type :(

mysquad

6:26 13 Mar '09

So i cant make it unhook the keyboard, and when i press a button only the program registers it Frown

. Any way to fix this?


using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using Utilities;
using System.IO;

namespace KeyLog
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        globalKeyboardHook g_hk = new globalKeyboardHook();

        bool load = false;

        private void Form1_Load(object sender, EventArgs e)
        {

        }
        void g_hk_KeyDown(object sender, KeyEventArgs e)
        {
            log_view.Items.Add("Down\t" + e.KeyCode.ToString());
            e.Handled = true;
        }
        private void btn_load_Click(object sender, EventArgs e)
        {
            if (load == false)
            {
                g_hk.hook();
                foreach (object key in Enum.GetValues(typeof(Keys))) g_hk.HookedKeys.Add((Keys)key);

                g_hk.KeyDown += new KeyEventHandler(g_hk_KeyDown);
                load = true;
            }
        }
        private void btn_dump_Click(object sender, EventArgs e)
        {
            string time = DateTime.Now.Hour.ToString() + DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString() + DateTime.Now.Millisecond.ToString();
            string dump = " ";
            for (int i = 0; log_view.Items.Count > i; i++)
            {
                log_view.SelectedIndex = i;
                dump += log_view.Text.ToString() + Environment.NewLine;
            }
            StreamWriter d_ump = new StreamWriter(@Application.StartupPath + "\\" + time + ".txt");
            d_ump.Write(dump);
            d_ump.Close();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            g_hk.unhook();
        }
    }
}

Re: unhook does not work, and when i capture the keyboard you cant type :(

StormySpike

15:39 13 Mar '09

Try setting e.Handled to false, that should cause it to pass the key event on.
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MDA error garbage collected delegate

AzraelKans

14:26 2 Feb '09

Im getting the following error while trying to use this class in my app


A callback was made on a garbage collected delegate of type 'TestFlashCommunication!Utilities.globalKeyboardHook+keyboardHookProc::Invoke'. This may cause application crashes, corruption and data loss.

How can I prevent this from happening?

Re: MDA error garbage collected delegate

StormySpike

15:45 13 Mar '09

Look at the CallbackOnCollectedDelegate Comment, it gives a fix
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Re: MDA error garbage collected delegate

weso

16:54 18 Aug '09

I have the same problem and can't find the solution. Please Help!
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Re: MDA error garbage collected delegate

Member 2516506

11:31 4 Oct '09

This is how I modified the code:

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 delegateReference; // Needed to ensure the GC will not remove the delegate
.
.
.
public void hook()
{
delegateReference = hookProc;
IntPtr hInstance = LoadLibrary("User32");
hhook = SetWindowsHookEx(WH_KEYBOARD_LL, delegateReference, hInstance, 0);
}

Re: MDA error garbage collected delegate

avihaydar10

23:21 12 Oct '09

Still not working!
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Re: MDA error garbage collected delegate

kendolew

6:43 23 Oct '09

I fell into the same problem, apparently it's really confusing. The work around has been explained in this thread before by another guy:

Replace the hook process with a delegate that you've assigned a method. This will fix the problem.

regards,
Kenny Liew

Excerpt from the past posts:

keyboardHookProc khp;

public GlobalKeyboardHook()
{
//assign a method to the delegate, which is hookProc in the example
khp = new keyboardHookProc(hookProc);
hook();
}

//And I modified this function:
public void hook()
{
IntPtr hInstance = LoadLibrary("User32");
hhook = SetWindowsHookEx(WH_KEYBOARD_LL, khp, hInstance, 0);
//notice that the hookProc has been replaced by khp instead
}

5.00/5

How to use this for capturing all the keys?

Member 4754326

10:46 28 Jan '09

I want to use all they keys to be going through my keyboard hook application, and not just A and B. How can I do that? (Except some key combination like Ctrl. Shift Esc. etc. ) Thanks !!
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Re: How to use this for capturing all the keys?

StormySpike

15:41 13 Mar '09

Add them to the HookedKeys collection, you can use the Enum class to get all of the values of the Keys enumeration
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Re: How to use this for capturing all the keys?

jwainer

15:33 6 Aug '09

I did it by adding the following:

private bool _hookAll = false;
and
public bool HookAllKeys { get { return _hookAll; } set { _hookAll = value; } }

and then modify the hookProc function as such:

	
					Keys key = (Keys)lParam.vkCode;
					if(_hookAll ? true : 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;
					}

Hope this helps

Not capturing after random number of key strokes !

Member 4754326

21:38 11 Jan '09

Hi,
I used your example to capture keystrokes. It captures them globally without any problem, but only upto a certain number of key strokes(which is random). For example, sometimes when I have pressed 280 keys it stops capturing. Sometimes 236, 182 rarely, but even 8 !! I know it might not be related to these numbers at all. (I'm running Windows Vista) but what makes it stop capturing key strokes?

Thanks !

1.00/5

My vote of 1

Member 4072865

9:29 20 Dec '08

глючное говно
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My vote of 1

Member 4072865

8:00 20 Dec '08

Critically buggy implementation
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Last Visit: 1:34 21 Nov '09 Last Update: 1:34 21 Nov '09

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Introduction

Background

Using the Code

Points of Interest

History