Another approach using a Windows Hook instead of subclassing XNA's window

http://www.gamedev.net/community/forums/topic.asp?topic_id=543581
http://www.benryves.com/index.php?module=journal&mode=filtered&single_post=3024426

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/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * XnaTextInput.TextInputHandler - benryves@benryves.com                                     *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * This is quick and very, VERY dirty.                                                       *
 * It uses Win32 message hooks to grab messages (as we don't get a nicely wrapped WndProc).  *
 * I couldn't get WH_KEYBOARD to work (accessing the data via its pointer resulted in access *
 * violation exceptions), nor could I get WH_CALLWNDPROC to work.                            *
 * Maybe someone who actually knows what they're  doing can work something out that's not so *
 * kludgy.                                                                                   *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * This quite obviously relies on a Win32 nastiness, so this is for Windows XNA games only!  *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

#region Using Statements
using System;
using System.Runtime.InteropServices;
using System.Windows.Forms; // This class exposes WinForms-style key events.
#endregion

namespace XnaTextInput {

	/// <summary>
	/// A class to provide text input capabilities to an XNA application via Win32 hooks.
	/// </summary>
	class TextInputHandler : IDisposable {

		#region Win32

		/// <summary>
		/// Types of hook that can be installed using the SetWindwsHookEx function.
		/// </summary>
		public enum HookId {
			WH_CALLWNDPROC = 4,
			WH_CALLWNDPROCRET = 12,
			WH_CBT = 5,
			WH_DEBUG = 9,
			WH_FOREGROUNDIDLE = 11,
			WH_GETMESSAGE = 3,
			WH_HARDWARE = 8,
			WH_JOURNALPLAYBACK = 1,
			WH_JOURNALRECORD = 0,
			WH_KEYBOARD = 2,
			WH_KEYBOARD_LL = 13,
			WH_MAX = 11,
			WH_MAXHOOK = WH_MAX,
			WH_MIN = -1,
			WH_MINHOOK = WH_MIN,
			WH_MOUSE_LL = 14,
			WH_MSGFILTER = -1,
			WH_SHELL = 10,
			WH_SYSMSGFILTER = 6,
		};

		/// <summary>
		/// Window message types.
		/// </summary>
		/// <remarks>Heavily abridged, naturally.</remarks>
		public enum WindowMessage {
			WM_KEYDOWN = 0x100,
			WM_KEYUP = 0x101,
			WM_CHAR = 0x102,
		};

		/// <summary>
		/// A delegate used to create a hook callback.
		/// </summary>
		public delegate int GetMsgProc(int nCode, int wParam, ref Message msg);

		/// <summary>
		/// Install an application-defined hook procedure into a hook chain.
		/// </summary>
		/// <param name="idHook">Specifies the type of hook procedure to be installed.</param>
		/// <param name="lpfn">Pointer to the hook procedure.</param>
		/// <param name="hmod">Handle to the DLL containing the hook procedure pointed to by the lpfn parameter.</param>
		/// <param name="dwThreadId">Specifies the identifier of the thread with which the hook procedure is to be associated.</param>
		/// <returns>If the function succeeds, the return value is the handle to the hook procedure. Otherwise returns 0.</returns>
		[DllImport("user32.dll", EntryPoint = "SetWindowsHookExA")]
		public static extern IntPtr SetWindowsHookEx(HookId idHook, GetMsgProc lpfn, IntPtr hmod, int dwThreadId);

		/// <summary>
		/// Removes a hook procedure installed in a hook chain by the SetWindowsHookEx function. 
		/// </summary>
		/// <param name="hHook">Handle to the hook to be removed. This parameter is a hook handle obtained by a previous call to SetWindowsHookEx.</param>
		/// <returns>If the function fails, the return value is zero. To get extended error information, call GetLastError.</returns>
		[DllImport("user32.dll")]
		public static extern int UnhookWindowsHookEx(IntPtr hHook);

		/// <summary>
		/// Passes the hook information to the next hook procedure in the current hook chain.
		/// </summary>
		/// <param name="hHook">Ignored.</param>
		/// <param name="ncode">Specifies the hook code passed to the current hook procedure.</param>
		/// <param name="wParam">Specifies the wParam value passed to the current hook procedure.</param>
		/// <param name="lParam">Specifies the lParam value passed to the current hook procedure.</param>
		/// <returns>This value is returned by the next hook procedure in the chain.</returns>
		[DllImport("user32.dll")]
		public static extern int CallNextHookEx(int hHook, int ncode, int wParam, ref Message lParam);

		/// <summary>
		/// Translates virtual-key messages into character messages.
		/// </summary>
		/// <param name="lpMsg">Pointer to an Message structure that contains message information retrieved from the calling thread's message queue.</param>
		/// <returns>If the message is translated (that is, a character message is posted to the thread's message queue), the return value is true.</returns>
		[DllImport("user32.dll")]
		public static extern bool TranslateMessage(ref Message lpMsg);


		/// <summary>
		/// Retrieves the thread identifier of the calling thread.
		/// </summary>
		/// <returns>The thread identifier of the calling thread.</returns>
		[DllImport("kernel32.dll")]
		public static extern int GetCurrentThreadId();

		#endregion

		#region Hook management and class construction.

		/// <summary>Handle for the created hook.</summary>
		private readonly IntPtr HookHandle;

		private readonly GetMsgProc ProcessMessagesCallback;

		/// <summary>Create an instance of the TextInputHandler.</summary>
		/// <param name="whnd">Handle of the window you wish to receive messages (and thus keyboard input) from.</param>
		public TextInputHandler(IntPtr whnd) {
			// Create the delegate callback:
			this.ProcessMessagesCallback = new GetMsgProc(ProcessMessages);
			// Create the keyboard hook:
			this.HookHandle = SetWindowsHookEx(HookId.WH_GETMESSAGE, this.ProcessMessagesCallback, IntPtr.Zero, GetCurrentThreadId());
		}

		public void Dispose() {
			// Remove the hook.
			if (this.HookHandle != IntPtr.Zero) UnhookWindowsHookEx(this.HookHandle);
		}

		#endregion

		#region Message processing

		private int ProcessMessages(int nCode, int wParam, ref Message msg) {
			// Check if we must process this message (and whether it has been retrieved via GetMessage):
			if (nCode == 0 && wParam == 1) {

					// We need character input, so use TranslateMessage to generate WM_CHAR messages.
					TranslateMessage(ref msg);

					// If it's one of the keyboard-related messages, raise an event for it:
					switch ((WindowMessage)msg.Msg) {
						case WindowMessage.WM_CHAR:
							this.OnKeyPress(new KeyPressEventArgs((char)msg.WParam));
							break;
						case WindowMessage.WM_KEYDOWN:
							this.OnKeyDown(new KeyEventArgs((Keys)msg.WParam));
							break;
						case WindowMessage.WM_KEYUP:
							this.OnKeyUp(new KeyEventArgs((Keys)msg.WParam));
							break;
					}

			}

			// Call next hook in chain:
			return CallNextHookEx(0, nCode, wParam, ref msg);
		}

		#endregion

		#region Events

		public event KeyEventHandler KeyUp;
		protected virtual void OnKeyUp(KeyEventArgs e) {
			if (this.KeyUp != null) this.KeyUp(this, e);
		}

		public event KeyEventHandler KeyDown;
		protected virtual void OnKeyDown(KeyEventArgs e) {
			if (this.KeyDown != null) this.KeyDown(this, e);
		}

		public event KeyPressEventHandler KeyPress;
		protected virtual void OnKeyPress(KeyPressEventArgs e) {
			if (this.KeyPress != null) this.KeyPress(this, e);
		}

		#endregion
	}
}