要实现一个屏幕键盘，需要监听所有键盘事件，无论窗体是否被激活。因此需要一个全局的钩子，也就
是系统范围的钩子。
什么是钩子（Hook）
钩子（Hook）是Windows提供的一种消息处理机制平台，是指在程序正常运行中接受信息之前预先启动的函数，用来检查和修改传 给该程序的信息，（钩子）实际上是一个处理消息的程序段，通过系统调用，把它挂入系统。每当特定的消息发出，在没有到达目的窗口前，钩子程序就先捕获该消 息，亦即钩子函数先得到控制权。这时钩子函数即可以加工处理（改变）该消息，也可以不作处理而继续传递该消息，还可以强制结束消息的传递。注意：安装钩子 函数将会影响系统的性能。监测“系统范围事件”的系统钩子特别明显。因为系统在处理所有的相关事件时都将调用您的钩子函数，这样您的系统将会明显的减慢。 所以应谨慎使用，用完后立即卸载。还有，由于您可以预先截获其它进程的消息，所以一旦您的钩子函数出了问题的话必将影响其它的进程。
钩子的作用范围
一共有两种范围（类型）的钩子，局部的和远程的。局部钩子仅钩挂自己进程的事件。远程的钩子还可以将钩挂其它进程发生的事件。远程的 钩子又有两种： 基于线程的钩子将捕获其它进程中某一特定线程的事件。简言之，就是可以用来观察其它进程中的某一特定线程将发生的事件。系统范围的钩子将捕捉系统中所有进 程将发生的事件消息。
Hook 类型
Windows共有14种Hooks，每一种类型的Hook可以使应用程序能够监视不同类型的系统消息处理机制。下面描述所有可以利用的Hook类型的发生时机。详细内容可以查阅MSDN，这里只介绍我们将要用到的两种类型的钩子。
（1）WH_KEYBOARD_LL Hook
WH_KEYBOARD_LL Hook监视输入到线程消息队列中的键盘消息。
（2）WH_MOUSE_LL Hook
WH_MOUSE_LL Hook监视输入到线程消息队列中的鼠标消息。
下面的 class 把 API 调用封装起来以便调用。
1// NativeMethods.cs
2using System;
3using System.Runtime.InteropServices;
4using System.Drawing;
5
6namespace CnBlogs.Youzai.ScreenKeyboard {
7 [StructLayout(LayoutKind.Sequential)]
8 internal struct MOUSEINPUT {
9 public int dx;
10 public int dy;
11 public int mouseData;
12 public int dwFlags;
13 public int time;
14 public IntPtr dwExtraInfo;
15 }
16
17 [StructLayout(LayoutKind.Sequential)]
18 internal struct KEYBDINPUT {
19 public short wVk;
20 public short wScan;
21 public int dwFlags;
22 public int time;
23 public IntPtr dwExtraInfo;
24 }
25
26 [StructLayout(LayoutKind.Explicit)]
27 internal struct Input {
28 [FieldOffset(0)]
29 public int type;
30 [FieldOffset(4)]
31 public MOUSEINPUT mi;
32 [FieldOffset(4)]
33 public KEYBDINPUT ki;
34 [FieldOffset(4)]
35 public HARDWAREINPUT hi;
36 }
37
38 [StructLayout(LayoutKind.Sequential)]
39 internal struct HARDWAREINPUT {
40 public int uMsg;
41 public short wParamL;
42 public short wParamH;
43 }
44
45 internal class INPUT {
46 public const int MOUSE = 0;
47 public const int KEYBOARD = 1;
48 public const int HARDWARE = 2;
49 }
50
51 internal static class NativeMethods {
52 [DllImport("User32.dll", CharSet = CharSet.Auto, SetLastError = false)]
53 internal static extern IntPtr GetWindowLong(IntPtr hWnd, int nIndex);
54
55 [DllImport("User32.dll", CharSet = CharSet.Auto, SetLastError = false)]
56 internal static extern IntPtr SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
57
58 [DllImport("User32.dll", EntryPoint = "SendInput", CharSet = CharSet.Auto)]
59 internal static extern UInt32 SendInput(UInt32 nInputs, Input[] pInputs, Int32 cbSize);
60
61 [DllImport("Kernel32.dll", EntryPoint = "GetTickCount", CharSet = CharSet.Auto)]
62 internal static extern int GetTickCount();
63
64 [DllImport("User32.dll", EntryPoint = "GetKeyState", CharSet = CharSet.Auto)]
65 internal static extern short GetKeyState(int nVirtKey);
66
67 [DllImport("User32.dll", EntryPoint = "SendMessage", CharSet = CharSet.Auto)]
68 internal static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);
69 }
70}
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安装钩子
使用SetWindowsHookEx函数（API函数），指定一个Hook类型、自己的Hook过程是全局还是局部Hook，同时 给出Hook过程的进入点，就可以轻松的安装自己的Hook过程。SetWindowsHookEx总是将你的Hook函数放置在Hook链的顶端。你可 以使用CallNextHookEx函数将系统消息传递给Hook链中的下一个函数。
对于某些类型的Hook，系统将向该类的所有Hook函数发送消息，这时，Hook函数中的CallNextHookEx语句将被忽略。全局（远 程钩子）Hook函数可以拦截系统中所有线程的某个特定的消息，为了安装一个全局Hook过程，必须在应用程序外建立一个DLL并将该Hook函数封装到 其中， 应用程序在安装全局Hook过程时必须先得到该DLL模块的句柄。将Dll名传递给LoadLibrary 函数，就会得到该DLL模块的句柄；得到该句柄 后，使用GetProcAddress函数可以得到Hook过程的地址。最后，使用SetWindowsHookEx将 Hook过程的首址嵌入相应的Hook链中，SetWindowsHookEx传递一个模块句柄，它为Hook过程的进入点，线程标识符置为0，该 Hook过程同系统中的所有线程关联。如果是安装局部Hook此时该Hook函数可以放置在DLL中，也可以放置在应用程序的模块段。在C#中通过平台调用（前文已经介绍过）来调用API函数。
1 public void Start(bool installMouseHook, bool installKeyboardHook) {
2 if (hMouseHook == IntPtr.Zero && installMouseHook) {
3 MouseHookProcedure = new HookProc(MouseHookProc);
4 hMouseHook = SetWindowsHookEx(
5 WH_MOUSE_LL,
6 MouseHookProcedure,
7 Marshal.GetHINSTANCE(
8 Assembly.GetExecutingAssembly().GetModules()[0]),
9 0
10 );
11
12 if (hMouseHook == IntPtr.Zero) {
13 int errorCode = Marshal.GetLastWin32Error();
14 Stop(true, false, false);
15
16 throw new Win32Exception(errorCode);
17 }
18 }
19
20 if (hKeyboardHook == IntPtr.Zero && installKeyboardHook) {
21 KeyboardHookProcedure = new HookProc(KeyboardHookProc);
22 //install hook
23 hKeyboardHook = SetWindowsHookEx(
24 WH_KEYBOARD_LL,
25 KeyboardHookProcedure,
26 Marshal.GetHINSTANCE(
27 Assembly.GetExecutingAssembly().GetModules()[0]),
28 0);
29 // If SetWindowsHookEx fails.
30 if (hKeyboardHook == IntPtr.Zero) {
31 // Returns the error code returned by the last
32 // unmanaged function called using platform invoke
33 // that has the DllImportAttribute.SetLastError flag set.
34 int errorCode = Marshal.GetLastWin32Error();
35 //do cleanup
36 Stop(false, true, false);
37 //Initializes and throws a new instance of the
38 // Win32Exception class with the specified error.
39 throw new Win32Exception(errorCode);
40 }
41 }
42 }
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使用完钩子后，要进行卸载，这个可以写在析构函数中。
1
2 public void Stop() {
3 this.Stop(true, true, true);
4 }
5
6 public void Stop(bool uninstallMouseHook, bool uninstallKeyboardHook,
7 bool throwExceptions) {
8 // if mouse hook set and must be uninstalled
9 if (hMouseHook != IntPtr.Zero && uninstallMouseHook) {
10 // uninstall hook
11 bool retMouse = UnhookWindowsHookEx(hMouseHook);
12 // reset invalid handle
13 hMouseHook = IntPtr.Zero;
14 // if failed and exception must be thrown
15 if (retMouse == false && throwExceptions) {
16 // Returns the error code returned by the last unmanaged function
17 // called using platform invoke that has the DllImportAttribute.
18 // SetLastError flag set.
19 int errorCode = Marshal.GetLastWin32Error();
20 // Initializes and throws a new instance of the Win32Exception class
21 // with the specified error.
22 throw new Win32Exception(errorCode);
23 }
24 }
25
26 // if keyboard hook set and must be uninstalled
27 if (hKeyboardHook != IntPtr.Zero && uninstallKeyboardHook) {
28 // uninstall hook
29 bool retKeyboard = UnhookWindowsHookEx(hKeyboardHook);
30 // reset invalid handle
31 hKeyboardHook = IntPtr.Zero;
32 // if failed and exception must be thrown
33 if (retKeyboard == false && throwExceptions) {
34 // Returns the error code returned by the last unmanaged function
35 // called using platform invoke that has the DllImportAttribute.
36 // SetLastError flag set.
37 int errorCode = Marshal.GetLastWin32Error();
38 // Initializes and throws a new instance of the Win32Exception class
39 // with the specified error.
40 throw new Win32Exception(errorCode);
41 }
42 }
43 }
44
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将这个文件编译成一个dll，即可在应用程序中调用。通过它提供的事件，便可监听所有的键盘事件。
但是，这只能监听键盘事件，没有键盘的情况下，怎么会有键盘事件？其实很简单，通过SendInput
API函数提供虚拟键盘代码的调用即可模拟键盘输入。下面的代码模拟一个 KeyDown 和 KeyUp 过程，
把他们连接起来就是一次按键过程。
1 private void SendKeyDown(short key) {
2 Input[] input = new Input[1];
3 input[0].type = INPUT.KEYBOARD;
4 input[0].ki.wVk = key;
5 input[0].ki.time = NativeMethods.GetTickCount();
6
7 if (NativeMethods.SendInput((uint)input.Length, input, Marshal.SizeOf(input[0]))
8 < input.Length) {
9 throw new Win32Exception(Marshal.GetLastWin32Error());
10 }
11 }
12
13 private void SendKeyUp(short key) {
14 Input[] input = new Input[1];
15 input[0].type = INPUT.KEYBOARD;
16 input[0].ki.wVk = key;
17 input[0].ki.dwFlags = KeyboardConstaint.KEYEVENTF_KEYUP;
18 input[0].ki.time = NativeMethods.GetTickCount();
19
20 if (NativeMethods.SendInput((uint)input.Length, input, Marshal.SizeOf(input[0]))
21 < input.Length) {
22 throw new Win32Exception(Marshal.GetLastWin32Error());
23 }
24 }
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自己实现一个 KeyBoardButton 控件用作按钮，用 Visual Studio 或者 SharpDevelop 为屏幕键盘设计 UI，然后
在这些 Button 的 Click 事件里面模拟一个按键过程。
1
2 private void ButtonOnClick(object sender, EventArgs e) {
3 KeyboardButton btnKey = sender as KeyboardButton;
4 if (btnKey == null) {
5 return;
6 }
7
8 SendKeyCommand(btnKey);
9 }
10
11 private void SendKeyCommand(KeyboardButton keyButton) {
12 short key = keyButton.VKCode;
13 if (combinationVKButtonsMap.ContainsKey(key)) {
14 if (keyButton.Checked) {
15 SendKeyUp(key);
16 } else {
17 SendKeyDown(key);
18 }
19 } else {
20 SendKeyDown(key);
21 SendKeyUp(key);
22 }
23 }
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其中 combinationVKButtonsMap 是一个 IDictionary>, key 存储的是VK_SHIFT, VK_CONTROL 等组合键的键盘码。左右两个按钮对应同一个键盘码，因此需要放在一个 List 里。
标准键盘上的每一个键都有虚拟键码( VK_CODE)与之对应。还有一些其他的常量，
把它写在一个静态 class 里吧。
1 // KeyboardConstaint.cs
2 internal static class KeyboardConstaint {
3 internal static readonly short VK_F1 = 0x70;
4 internal static readonly short VK_F2 = 0x71;
5 internal static readonly short VK_F3 = 0x72;
6 internal static readonly short VK_F4 = 0x73;
7 internal static readonly short VK_F5 = 0x74;
8 internal static readonly short VK_F6 = 0x75;
9 internal static readonly short VK_F7 = 0x76;
10 internal static readonly short VK_F8 = 0x77;
11 internal static readonly short VK_F9 = 0x78;
12 internal static readonly short VK_F10 = 0x79;
13 internal static readonly short VK_F11 = 0x7A;
14 internal static readonly short VK_F12 = 0x7B;
15
16 internal static readonly short VK_LEFT = 0x25;
17 internal static readonly short VK_UP = 0x26;
18 internal static readonly short VK_RIGHT = 0x27;
19 internal static readonly short VK_DOWN = 0x28;
20
21 internal static readonly short VK_NONE = 0x00;
22 internal static readonly short VK_ESCAPE = 0x1B;
23 internal static readonly short VK_EXECUTE = 0x2B;
24 internal static readonly short VK_CANCEL = 0x03;
25 internal static readonly short VK_RETURN = 0x0D;
26 internal static readonly short VK_ACCEPT = 0x1E;
27 internal static readonly short VK_BACK = 0x08;
28 internal static readonly short VK_TAB = 0x09;
29 internal static readonly short VK_DELETE = 0x2E;
30 internal static readonly short VK_CAPITAL = 0x14;
31 internal static readonly short VK_NUMLOCK = 0x90;
32 internal static readonly short VK_SPACE = 0x20;
33 internal static readonly short VK_DECIMAL = 0x6E;
34 internal static readonly short VK_SUBTRACT = 0x6D;
35
36 internal static readonly short VK_ADD = 0x6B;
37 internal static readonly short VK_DIVIDE = 0x6F;
38 internal static readonly short VK_MULTIPLY = 0x6A;
39 internal static readonly short VK_INSERT = 0x2D;
40
41 internal static readonly short VK_OEM_1 = 0xBA; // ';:' for US
42 internal static readonly short VK_OEM_PLUS = 0xBB; // '+' any country
43
44 internal static readonly short VK_OEM_MINUS = 0xBD; // '-' any country
45
46 internal static readonly short VK_OEM_2 = 0xBF; // '/?' for US
47 internal static readonly short VK_OEM_3 = 0xC0; // '`~' for US
48 internal static readonly short VK_OEM_4 = 0xDB; // '[{' for US
49 internal static readonly short VK_OEM_5 = 0xDC; // '|' for US
50 internal static readonly short VK_OEM_6 = 0xDD; // ']}' for US
51 internal static readonly short VK_OEM_7 = 0xDE; // ''"' for US
52 internal static readonly short VK_OEM_PERIOD = 0xBE; // '.>' any country
53 internal static readonly short VK_OEM_COMMA = 0xBC; // ',<' any country
54 internal static readonly short VK_SHIFT = 0x10;
55 internal static readonly short VK_CONTROL = 0x11;
56 internal static readonly short VK_MENU = 0x12;
57 internal static readonly short VK_LWIN = 0x5B;
58 internal static readonly short VK_RWIN = 0x5C;
59 internal static readonly short VK_APPS = 0x5D;
60
61 internal static readonly short VK_LSHIFT = 0xA0;
62 internal static readonly short VK_RSHIFT = 0xA1;
63 internal static readonly short VK_LCONTROL = 0xA2;
64 internal static readonly short VK_RCONTROL = 0xA3;
65 internal static readonly short VK_LMENU = 0xA4;
66 internal static readonly short VK_RMENU = 0xA5;
67
68 internal static readonly short VK_SNAPSHOT = 0x2C;
69 internal static readonly short VK_SCROLL = 0x91;
70 internal static readonly short VK_PAUSE = 0x13;
71 internal static readonly short VK_HOME = 0x24;
72
73 internal static readonly short VK_NEXT = 0x22;
74 internal static readonly short VK_PRIOR = 0x21;
75 internal static readonly short VK_END = 0x23;
76
77 internal static readonly short VK_NUMPAD0 = 0x60;
78 internal static readonly short VK_NUMPAD1 = 0x61;
79 internal static readonly short VK_NUMPAD2 = 0x62;
80 internal static readonly short VK_NUMPAD3 = 0x63;
81 internal static readonly short VK_NUMPAD4 = 0x64;
82 internal static readonly short VK_NUMPAD5 = 0x65;
83 internal static readonly short VK_NUMPAD5NOTHING = 0x0C;
84 internal static readonly short VK_NUMPAD6 = 0x66;
85 internal static readonly short VK_NUMPAD7 = 0x67;
86 internal static readonly short VK_NUMPAD8 = 0x68;
87 internal static readonly short VK_NUMPAD9 = 0x69;
88
89 internal static readonly short KEYEVENTF_EXTENDEDKEY = 0x0001;
90 internal static readonly short KEYEVENTF_KEYUP = 0x0002;
91
92 internal static readonly int GWL_EXSTYLE = -20;
93 internal static readonly int WS_DISABLED = 0X8000000;
94 internal static readonly int WM_SETFOCUS = 0X0007;
95 }
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屏幕键盘必须是一个不能获得输入焦点的窗体，在这个窗体的构造函数里，可以安装
一个全局鼠标钩子，再通过调用 SetWindowLong API 函数完成。
1UserActivityHook hook = new UserActivityHook(true, true);
2hook.MouseActivity += HookOnMouseActivity;
3
4private void HookOnMouseActivity(object sener, HookEx.MouseExEventArgs e) {
5 Point location = e.Location;
6
7 if (e.Button == MouseButtons.Left) {
8 Rectangle captionRect = new Rectangle(this.Location, new Size(this.Width,
9 SystemInformation.CaptionHeight));
10 if (captionRect.Contains(location)) {
11 NativeMethods.SetWindowLong(this.Handle, KeyboardConstaint.GWL_EXSTYLE,
12 (int)NativeMethods.GetWindowLong(this.Handle, KeyboardConstaint.GWL_EXSTYLE)
13 & (~KeyboardConstaint.WS_DISABLED));
14 NativeMethods.SendMessage(this.Handle, KeyboardConstaint.WM_SETFOCUS, IntPtr.Zero, IntPtr.Zero);
15 } else {
16 NativeMethods.SetWindowLong(this.Handle, KeyboardConstaint.GWL_EXSTYLE,
17 (int)NativeMethods.GetWindowLong(this.Handle, KeyboardConstaint.GWL_EXSTYLE) |
18 KeyboardConstaint.WS_DISABLED);
19 }
20 }
21}
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鼠标单击标题栏，让屏幕键盘可以接收焦点，并激活，单击其他部分则不激活窗体（如果激活了，其他程序必然取消激活，输入就无法进行了），这样才可以进行输入，并且保证了可以拖动窗体到其他位置。
至此，一个屏幕键盘程序差不多完成了，能够实现与实际键盘完全同步。至于窗体，按键重绘，以及 Num Lock， Caps Lock，Scroll Lock 等键盘灯的模拟，这里就不讲了，如果有兴趣，可以下载完整的代码。
说明：本程序参考了 Jeffrey Richter 先生的著作 CLR via C#， Second Edition， MSDN 以及一些网络资料。
这是微软技术的一贯特点，使用简单。但是如果要深入的话，还是要投入不少精力的