When discussing keyboard hooking, this article gets thrown around a lot.
So I tied the class that it suggested:
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);
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>
public void hook() {
IntPtr hInstance = LoadLibrary("User32");
hhook = SetWindowsHookEx(WH_KEYBOARD_LL, hookProc, 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
}
}
It works for a bit, but after using the computer for a while, it will throw a System.NullReferenceException on a random key press. Especially around key combinations.
What could be causing this and how can it be fixed?
EDIT: This is the code I am using to start the hook:
globalKeyboardHook globalKeyboardHook = new globalKeyboardHook();
private void Form1_Load(object sender, EventArgs e)
{
globalKeyboardHook.KeyDown += gkh_KeyDown;
globalKeyboardHook.KeyUp += gkh_KeyUp;
globalKeyboardHook.hook();
}
And this is the complete error I am getting:
An unhandled exception of type 'System.NullReferenceException' occurred in System.Windows.Forms.dll Additional information: Object reference not set to an instance of an object.
