Hello, I just wonder... I mainly use .NET but now I started to investigate WINAPI calls. For example I am using this piece of code to hook to the API functions. It starts freezing, when I try to debug the application... using System; using System.Diagnostics; using System.Runtime.InteropServices; using System.Threading; using System.Windows.Forms; public class Keyboard { private const int WH_KEYBOARD_LL = 13; private const int WM_KEYDOWN = 0x0100; private static LowLevelKeyboardProc _proc = HookCallback; private static IntPtr _hookID = IntPtr.Zero; public static event Action<Keys,bool, bool> KeyDown; public static void Hook() { new Thread(new ThreadStart(()=> { _hookID = SetHook(_proc); Application.Run(); })).Start(); } public static void Unhook() { UnhookWindowsHookEx(_hookID); } private static IntPtr SetHook(LowLevelKeyboardProc proc) { using (Process curProcess = Process.GetCurrentProcess()) using (ProcessModule curModule = curProcess.MainModule) { return SetWindowsHookEx(WH_KEYBOARD_LL, proc, GetModuleHandle(curModule.ModuleName), 0); } } private delegate IntPtr LowLevelKeyboardProc( int nCode, IntPtr wParam, IntPtr lParam); private static IntPtr HookCallback( int nCode, IntPtr wParam, IntPtr lParam) { if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN) { int vkCode = Marshal.ReadInt32(lParam); Keys k = (Keys) vkCode; if (KeyDown != null) { KeyDown.BeginInvoke(k, IsKeyPressed(VirtualKeyStates.VK_CONTROL), IsKeyPressed(VirtualKeyStates.VK_SHIFT),null,null); } } return CallNextHookEx(_hookID, nCode, wParam, lParam); } private static bool IsKeyPressed(VirtualKeyStates virtualKeyStates) { return (GetKeyState(virtualKeyStates) & (1 << 7))==128; } [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)] private static extern IntPtr SetWindowsHookEx(int idHook, LowLevelKeyboardProc lpfn, IntPtr hMod, uint dwThreadId); [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)] [return: MarshalAs(UnmanagedType.Bool)] private static extern bool UnhookWindowsHookEx(IntPtr hhk); [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)] private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam); [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)] private static extern IntPtr GetModuleHandle(string lpModuleName); [DllImport("user32.dll")] static extern short GetKeyState(VirtualKeyStates nVirtKey); } enum VirtualKeyStates : int { VK_LBUTTON = 0x01, VK_RBUTTON = 0x02, VK_CANCEL = 0x03, VK_MBUTTON = 0x04, // VK_XBUTTON1 = 0x05, VK_XBUTTON2 = 0x06, // VK_BACK = 0x08, VK_TAB = 0x09, // VK_CLEAR = 0x0C, VK_RETURN = 0x0D, // VK_SHIFT = 0x10, VK_CONTROL = 0x11, VK_MENU = 0x12, VK_PAUSE = 0x13, VK_CAPITAL = 0x14, // VK_KANA = 0x15, VK_HANGEUL = 0x15, /* old name - should be here for compatibility */ VK_HANGUL = 0x15, VK_JUNJA = 0x17, VK_FINAL = 0x18, VK_HANJA = 0x19, VK_KANJI = 0x19, // VK_ESCAPE = 0x1B, // VK_CONVERT = 0x1C, VK_NONCONVERT = 0x1D, VK_ACCEPT = 0x1E, VK_MODECHANGE = 0x1F, // VK_SPACE = 0x20, VK_PRIOR = 0x21, VK_NEXT = 0x22, VK_END = 0x23, VK_HOME = 0x24, VK_LEFT = 0x25, VK_UP = 0x26, VK_RIGHT = 0x27, VK_DOWN = 0x28, VK_SELECT = 0x29, VK_PRINT = 0x2A, VK_EXECUTE = 0x2B, VK_SNAPSHOT = 0x2C, VK_INSERT = 0x2D, VK_DELETE = 0x2E, VK_HELP = 0x2F, // VK_LWIN = 0x5B, VK_RWIN = 0x5C, VK_APPS = 0x5D, // VK_SLEEP = 0x5F, // VK_NUMPAD0 = 0x60, VK_NUMPAD1 = 0x61, VK_NUMPAD2 = 0x62, VK_NUMPAD3 = 0x63, VK_NUMPAD4 = 0x64, VK_NUMPAD5 = 0x65, VK_NUMPAD6 = 0x66, VK_NUMPAD7 = 0x67, VK_NUMPAD8 = 0x68, VK_NUMPAD9 = 0x69, VK_MULTIPLY = 0x6A, VK_ADD = 0x6B, VK_SEPARATOR = 0x6C, VK_SUBTRACT = 0x6D, VK_DECIMAL = 0x6E, VK_DIVIDE = 0x6F, VK_F1 = 0x70, VK_F2 = 0x71, VK_F3 = 0x72, VK_F4 = 0x73, VK_F5 = 0x74, VK_F6 = 0x75, VK_F7 = 0x76, VK_F8 = 0x77, VK_F9 = 0x78, VK_F10 = 0x79, VK_F11 = 0x7A, VK_F12 = 0x7B, VK_F13 = 0x7C, VK_F14 = 0x7D, VK_F15 = 0x7E, VK_F16 = 0x7F, VK_F17 = 0x80, VK_F18 = 0x81, VK_F19 = 0x82, VK_F20 = 0x83, VK_F21 = 0x84, VK_F22 = 0x85, VK_F23 = 0x86, VK_F24 = 0x87, // VK_NUMLOCK = 0x90, VK_SCROLL = 0x91, // VK_OEM_NEC_EQUAL = 0x92, // '=' key on numpad // VK_OEM_FJ_JISHO = 0x92, // 'Dictionary' key VK_OEM_FJ_MASSHOU = 0x93, // 'Unregister word' key VK_OEM_FJ_TOUROKU = 0x94, // 'Register word' key VK_OEM_FJ_LOYA = 0x95, // 'Left OYAYUBI' key VK_OEM_FJ_ROYA = 0x96, // 'Right OYAYUBI' key // VK_LSHIFT = 0xA0, VK_RSHIFT = 0xA1, VK_LCONTROL = 0xA2, VK_RCONTROL = 0xA3, VK_LMENU = 0xA4, VK_RMENU = 0xA5, // VK_BROWSER_BACK = 0xA6, VK_BROWSER_FORWARD = 0xA7, VK_BROWSER_REFRESH = 0xA8, VK_BROWSER_STOP = 0xA9, VK_BROWSER_SEARCH = 0xAA, VK_BROWSER_FAVORITES = 0xAB, VK_BROWSER_HOME = 0xAC, // VK_VOLUME_MUTE = 0xAD, VK_VOLUME_DOWN = 0xAE, VK_VOLUME_UP = 0xAF, VK_MEDIA_NEXT_TRACK = 0xB0, VK_MEDIA_PREV_TRACK = 0xB1, VK_MEDIA_STOP = 0xB2, VK_MEDIA_PLAY_PAUSE = 0xB3, VK_LAUNCH_MAIL = 0xB4, VK_LAUNCH_MEDIA_SELECT = 0xB5, VK_LAUNCH_APP1 = 0xB6, VK_LAUNCH_APP2 = 0xB7, // VK_OEM_1 = 0xBA, // ';:' for US VK_OEM_PLUS = 0xBB, // '+' any country VK_OEM_COMMA = 0xBC, // ',' any country VK_OEM_MINUS = 0xBD, // '-' any country VK_OEM_PERIOD = 0xBE, // '.' any country VK_OEM_2 = 0xBF, // '/?' for US VK_OEM_3 = 0xC0, // '`~' for US // VK_OEM_4 = 0xDB, // '[{' for US VK_OEM_5 = 0xDC, // '\|' for US VK_OEM_6 = 0xDD, // ']}' for US VK_OEM_7 = 0xDE, // ''"' for US VK_OEM_8 = 0xDF, // VK_OEM_AX = 0xE1, // 'AX' key on Japanese AX kbd VK_OEM_102 = 0xE2, // "<>" or "\|" on RT 102-key kbd. VK_ICO_HELP = 0xE3, // Help key on ICO VK_ICO_00 = 0xE4, // 00 key on ICO // VK_PROCESSKEY = 0xE5, // VK_ICO_CLEAR = 0xE6, // VK_PACKET = 0xE7, // VK_OEM_RESET = 0xE9, VK_OEM_JUMP = 0xEA, VK_OEM_PA1 = 0xEB, VK_OEM_PA2 = 0xEC, VK_OEM_PA3 = 0xED, VK_OEM_WSCTRL = 0xEE, VK_OEM_CUSEL = 0xEF, VK_OEM_ATTN = 0xF0, VK_OEM_FINISH = 0xF1, VK_OEM_COPY = 0xF2, VK_OEM_AUTO = 0xF3, VK_OEM_ENLW = 0xF4, VK_OEM_BACKTAB = 0xF5, // VK_ATTN = 0xF6, VK_CRSEL = 0xF7, VK_EXSEL = 0xF8, VK_EREOF = 0xF9, VK_PLAY = 0xFA, VK_ZOOM = 0xFB, VK_NONAME = 0xFC, VK_PA1 = 0xFD, VK_OEM_CLEAR = 0xFE } It works well even if you put messagebox into the event or something that blocks execution. But it gets bad if you try to put breakpoint into the event. Why? I mean event is not run in the same thread that the windows hook is. That means that It shouldn't block HookCallback. It does however... I would really like to know why is this happening. My theory is that Visual Studio when breaking execution temporarily stops all threads and that means that HookCallback is blocked... Is there any book or valuable resource that would explain concepts behind all of this threading?