Access violation reading location 0x0000000c.

Question

In my application I disable the Start menu using 'LowLevelKeyboardProc'. But i get the following error when i pressing the tab key continuously. The Error..

Unhandled exception at 0x00352d58 in Timer_soosai.exe: 0xC0000005: Access violation reading location 0x0000000c.

00352D58  mov         eax,dword ptr [ecx+0Ch] /*This is the line which

causes the error in disassemply.*/

The Code:

[DllImport("user32", EntryPoint = "SetWindowsHookExA", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
        public static extern int SetWindowsHookEx(int idHook, LowLevelKeyboardProcDelegate lpfn, int hMod, int dwThreadId);
        [DllImport("user32", EntryPoint = "UnhookWindowsHookEx", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
        public static extern int UnhookWindowsHookEx(int hHook);
        public delegate int LowLevelKeyboardProcDelegate(int nCode, int wParam, ref KBDLLHOOKSTRUCT lParam);
        [DllImport("user32", EntryPoint = "CallNextHookEx", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
        public static extern int CallNextHookEx(int hHook, int nCode, int wParam, ref KBDLLHOOKSTRUCT lParam);
        public const int WH_KEYBOARD_LL = 13;

        /*code needed to disable start menu*/
        [DllImport("user32.dll")]
        private static extern int FindWindow(string className, string windowText);
        [DllImport("user32.dll")]
        private static extern int ShowWindow(int hwnd, int command);

        private const int SW_HIDE = 0;
        private const int SW_SHOW = 1;
        public struct KBDLLHOOKSTRUCT
        {
            public int vkCode;
            public int scanCode;
            public int flags;
            public int time;
            public int dwExtraInfo;
        }
        public static int intLLKey;

        public int LowLevelKeyboardProc(int nCode, int wParam, ref KBDLLHOOKSTRUCT lParam)
        {
            bool blnEat = false;

            switch (wParam)
            {
                case 256:
                case 257:
                case 260:
                case 261:
                    //Alt+Tab, Alt+Esc, Ctrl+Esc, Windows Key,
                    blnEat = ((lParam.vkCode == 9) && (lParam.flags == 32)) | ((lParam.vkCode == 27) && (lParam.flags == 32)) | ((lParam.vkCode == 27) && (lParam.flags == 0)) | ((lParam.vkCode == 91) && (lParam.flags == 1)) | ((lParam.vkCode == 92) && (lParam.flags == 1)) | ((lParam.vkCode == 73) && (lParam.flags == 0));
                    break;
            }

            if (blnEat == true)
            {
                return 1;
            }
            else
            {
                return CallNextHookEx(0, nCode, wParam, ref lParam);
            }
        }
        public void KillStartMenu()
        {
            int hwnd = FindWindow("Shell_TrayWnd", "");
            ShowWindow(hwnd, SW_HIDE);
        }

    private void Login_Load(object sender, EventArgs e)
        {
            intLLKey = SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc, System.Runtime.InteropServices.Marshal.GetHINSTANCE(System.Reflection.Assembly.GetExecutingAssembly().GetModules()[0]).ToInt32(), 0);

        }

Can anyone please help me in this case. Many thanks in advance..

Answer 1

Looks like you're dereferencing a NULL (i.e., a zero pointer) somewhere. While a NULL isn't formally a zero, it is in practice. (It's not an actual dereference of a zero pointer because the code is assuming there's a struct starting there and is accessing the 4th word of that struct.)

Check for anywhere where you're calling the code with a null or zero parameter. Also consider making all calls into the low-level API private and wrap them with safe versions that guard all the arguments. I can't diagnose much more than that; you don't provide quite enough info yet (well, for me to do so at least; someone more experienced may be able to help).

Answer 2

Your P/Invoke declarations are all wrong, using "int" where IntPtr is required. This code is indeed going to bomb on a 64-bit version of Windows. Get them fixed by getting the proper declarations from pinvoke.net