要實現一個屏幕鍵盤，需要監聽所有鍵盤事件，無論窗體是否被激活。因此需要一個全局的鉤子，也就是系統范圍的鉤子。
什麼是鉤子（Hook）
鉤子（Hook）是windows提供的一種消息處理機制平台，是指在程序正常運行中接受信息之前預先啟動的函數，用來檢查和修改傳給該程序的信息，（鉤子）實際上是一個處理消息的程序段，通過系統調用，把它掛入系統。每當特定的消息發出，在沒有到達目的窗口前，鉤子程序就先捕獲該消息，亦即鉤子函數先得到控制權。這時鉤子函數即可以加工處理（改變）該消息，也可以不作處理而繼續傳遞該消息，還可以強制結束消息的傳遞。注意：安裝鉤子函數將會影響系統的性能。監測“系統范圍事件”的系統鉤子特別明顯。因為系統在處理所有的相關事件時都將調用您的鉤子函數，這樣您的系統將會明顯的減慢。所以應謹慎使用，用完後立即卸載。還有，由於您可以預先截獲其它進程的消息，所以一旦您的鉤子函數出了問題的話必將影響其它的進程。
鉤子的作用范圍
一共有兩種范圍（類型）的鉤子，局部的和遠程的。局部鉤子僅鉤掛自己進程的事件。遠程的鉤子還可以將鉤掛其它進程發生的事件。遠程的鉤子又有兩種： 基於線程的鉤子將捕獲其它進程中某一特定線程的事件。簡言之，就是可以用來觀察其它進程中的某一特定線程將發生的事件。系統范圍的鉤子將捕捉系統中所有進程將發生的事件消息。
Hook 類型
windows共有14種Hooks，每一種類型的Hook可以使應用程序能夠監視不同類型的系統消息處理機制。下面描述所有可以利用的Hook類型的發生時機。詳細內容可以查閱MSDN，這裡只介紹我們將要用到的兩種類型的鉤子。
（1）WH_KEYBOARD_LL Hook
WH_KEYBOARD_LL Hook監視輸入到線程消息隊列中的鍵盤消息。
（2）WH_MOUSE_LL Hook
WH_MOUSE_LL Hook監視輸入到線程消息隊列中的鼠標消息。
下面的 class 把 API 調用封裝起來以便調用。
1 // NativeMethods.cs
2 using system;
3 using system.Runtime.InteropServices;
4 using system.Drawing;
5
6 namespace 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}
安裝鉤子
使用SetWindowsHookEx函數（API函數），指定一個Hook類型、自己的Hook過程是全局還是局部Hook，同時給出Hook過程的進入點，就可以輕松的安裝自己的Hook過程。SetWindowsHookEx總是將你的Hook函數放置在Hook鏈的頂端。你可以使用CallNextHookEx函數將系統消息傳遞給Hook鏈中的下一個函數。
對於某些類型的Hook，系統將向該類的所有Hook函數發送消息，這時，Hook函數中的CallNextHookEx語句將被忽略。全局（遠程鉤子）Hook函數可以攔截系統中所有線程的某個特定的消息，為了安裝一個全局Hook過程，必須在應用程序外建立一個DLL並將該Hook函數封裝到其中， 應用程序在安裝全局Hook過程時必須先得到該DLL模塊的句柄。將Dll名傳遞給LoadLibrary 函數，就會得到該DLL模塊的句柄；得到該句柄 後，使用GetProcAddress函數可以得到Hook過程的地址。最後，使用SetWindowsHookEx將 Hook過程的首址嵌入相應的Hook鏈中，SetWindowsHookEx傳遞一個模塊句柄，它為Hook過程的進入點，線程標識符置為0，該Hook過程同系統中的所有線程關聯。如果是安裝局部Hook此時該Hook函數可以放置在DLL中，也可以放置在應用程序的模塊段。在C#中通過平台調用（前文已經介紹過）來調用API函數。
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 }
使用完鉤子後，要進行卸載，這個可以寫在析構函數中。
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
將這個文件編譯成一個dll，即可在應用程序中調用。通過它提供的事件，便可監聽所有的鍵盤事件。
但是，這只能監聽鍵盤事件，沒有鍵盤的情況下，怎麼會有鍵盤事件？其實很簡單，通過SendInput
API函數提供虛擬鍵盤代碼的調用即可模擬鍵盤輸入。下面的代碼模擬一個 KeyDown 和 KeyUp 過程，
把他們連接起來就是一次按鍵過程。
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 }
自己實現一個 KeyBoardButton 控件用作按鈕，用 Visual Studio 或者 SharpDevelop 為屏幕鍵盤設計 UI，然後在這些 Button 的 Click 事件裡面模擬一個按鍵過程。
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 }
其中 combinationVKButtonsMap 是一個 IDictionary>, key 存儲的是VK_SHIFT, VK_CONTROL 等組合鍵的鍵盤碼。左右兩個按鈕對應同一個鍵盤碼，因此需要放在一個 List 裡。
標准鍵盤上的每一個鍵都有虛擬鍵碼( VK_CODE)與之對應。還有一些其他的常量，
把它寫在一個靜態 class 裡吧。
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 }
屏幕鍵盤必須是一個不能獲得輸入焦點的窗體，在這個窗體的構造函數裡，可以安裝
一個全局鼠標鉤子，再通過調用 SetWindowLong API 函數完成。
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}
鼠標單擊標題欄，讓屏幕鍵盤可以接收焦點，並激活，單擊其他部分則不激活窗體（如果激活了，其他程序必然取消激活，輸入就無法進行了），這樣才可以進行輸入，並且保證了可以拖動窗體到其他位置。
至此，一個屏幕鍵盤程序差不多完成了，能夠實現與實際鍵盤完全同步。至於窗體，按鍵重繪，以及 Num Lock， Caps Lock，Scroll Lock 等鍵盤燈的模擬，這裡就不講了，如果有興趣，可以下載完整的代碼。
說明：本程序參考了 Jeffrey Richter 先生的著作 CLR via C#， Second Edition， MSDN 以及一些網絡資料。
這是微軟技術的一貫特點，使用簡單。但是如果要深入的話，還是要投入不少精力的。
