API.UnhookWindowsHookEx Problem?

Question

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.

Raysefo · Answer

Thank you guys for your help. I decided not to stop hooking so there is no problem right now.

Sam Hobbs · Answer

Okay that explains things.

Vulpes · Answer

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 Hobbs · Answer

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.

Vulpes · Answer

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 Hobbs · Answer

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); } }

Vulpes · Answer

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.