API.UnhookWindowsHookEx Problem?

Raysefo

Hi,

I am getting key strokes from keyboard. First I start listening keystrokes as follows:

Process _this = Process.GetCurrentProcess();             ProcessModule mod = _this.MainModule;             hd = HookFunc;             kh = _kh;               hhk = API.SetWindowsHookEx(13, hd, API.GetModuleHandle(mod.ModuleName), 0);

Then according to some condition I stop listening keystrokes as follows:

return API.UnhookWindowsHookEx(hhk);

The problem is, when I want to listen to keystrokes again, It does NOT capture them.

Any help please?

Best Regards.

7 Replies

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Raysefo14y ago
Thank you guys for your help. I decided not to stop hooking so there is no problem right now.
Sam Hobbs14y ago
Okay, that explains things.
Vulpes14y ago
No, you didn't imply that the GC was causing the problem with Cenk's code but it was the first thing that occurred to me when formulating my original reply to him though I then dismissed it for the reason given in my last post.
Sam Hobbs14y ago
Yes, the garbage collection does not cause the problem in this thread. Did I say anything implying it is?

Note that the garbage collection issue does not happen to me until I have at least typed a couple dozen characters.
Vulpes14y ago
When using hooks from C#, the hook procedure delegate can get clobbered by the GC if you don't keep it in scope but there's no chance of that here as you've assigned it to a static field of the form.

Puzzlingly, your model of the app is much the same as mine though I'm not experiencing the same problem :-/

I don't think it can be the reason why Cenk's app is not working as he's reassigning the delegate to 'hd' each time he's setting the hook.

Sam Hobbs14y ago

The following works for me; I can set and remove the hook multiple times. Your problem exists with something you are not showing. It is likely a simple mistake, because we usually look for more complicated explanations.

Except after a few dozen keys, the garbage collector will eventually clobber something that the unmanaged code needs. I do not know what, so I will leave that for you to figure out.

 using  System;

 using  System.Diagnostics;

 using  System.Runtime.InteropServices;

 using  System.Windows.Forms;

 namespace  _155960

 public  partial  class  Form1  : Form

 private  static  IntPtr  hhk = IntPtr .Zero;

 private  static  API .LowLevelKeyboardProc  _proc = HookFunc;

Process  _this = null ;

 ProcessModule  mod = null ;

 public  Form1()

 InitializeComponent();

 _this = Process .GetCurrentProcess();

 mod = _this.MainModule;

 private  void  button1_Click(object  sender, EventArgs  e)

 if  (hhk == IntPtr .Zero)

 button1.Text = "Unset" ;

 hhk = API .SetWindowsHookEx(13, HookFunc, API .GetModuleHandle(mod.ModuleName), 0);

 if  (hhk == IntPtr .Zero)

 MessageBox .Show("Not Hooked" );

 else

 button1.Text = "Set" ;

 bool  rv = API .UnhookWindowsHookEx(hhk);

 hhk = IntPtr .Zero;

 if  (!rv)

 MessageBox .Show("Not Unhooked" );

 private  static  IntPtr  HookFunc(int  nCode, IntPtr  wParam, IntPtr  lParam)

 if  (nCode >= 0 && wParam == (IntPtr )API .WM_KEYDOWN)

 Keys  vkCode = (Keys )Marshal .ReadInt32(lParam);

 Debug .WriteLine("Key: {0}" , vkCode.ToString());

 return  API .CallNextHookEx(hhk, nCode, wParam, lParam);

 public  static  class  API

 [DllImport ("user32.dll" , CharSet = CharSet .Auto, SetLastError = true )]

 public  static  extern  IntPtr  SetWindowsHookEx(int  idHook, LowLevelKeyboardProc  lpfn, IntPtr  hMod, uint  dwThreadId);

 [DllImport ("user32.dll" , CharSet = CharSet .Auto, SetLastError = true )]

 [return : MarshalAs (UnmanagedType .Bool)]

 public  static  extern  bool  UnhookWindowsHookEx(IntPtr  hhk);

 [DllImport ("user32.dll" , CharSet = CharSet .Auto, SetLastError = true )]

 public  static  extern  IntPtr  CallNextHookEx(IntPtr  hhk, int  nCode, IntPtr  wParam, IntPtr  lParam);

 [DllImport ("kernel32.dll" , CharSet = CharSet .Auto, SetLastError = true )]

 public  static  extern  IntPtr  GetModuleHandle(string  lpModuleName);

 public  const  int  WH_KEYBOARD_LL = 13;

 public  const  int  WM_KEYDOWN = 0x0100;

 public  delegate  IntPtr  LowLevelKeyboardProc (int  nCode, IntPtr  wParam, IntPtr  lParam);

Vulpes14y ago
I can't think of any obvious reason why calling that code again to set the keyboard hook wouldn't work.

Are you sure that there's nothing in the hook function, HookFunc, which would prevent you from trapping the keystrokes a second time?

If there isn't, I'd make sure that the call to SetWindowsHookEx is in fact succeeding by checking the return value. A return value of IntPtr.Zero indicates failure.

As you may know, it's possible to get the reason for failure by setting the SetLastError flag to True in the DllImport attribute for the unmanaged function (probably done automatically if you're using an API wrapper) and then calling the managed method Marshal.GetLastWin32Error() to get the error code.