Subclassing controls in .NETCF 2.0, part 1

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The .NET Compact Framework is just that. It is compact. We have to make tradeoffs between functionality and size. Sometimes, developers want more than we provide in the box. In our v1 product we didn't expose the Handle property on Control. It made it very difficult for developers to do the sort of customization they wanted to do. In addition, we didn't provide a way for native code to call into managed code via a delegate.

On v2, we have fixed those issues. We still don't provide all of the properties, methods and events that our desktop counterpart does. We're the Compact framework, right? By calling down into native code, however, we can use some of the Win32 API to extend some of the provided .NETCF controls. The code provided today will lay the groundwork for the next post where we will add a "missing" event to the TreeView control.

That said, here is the "WndProcHooker" class that is the main "guts" to this sample.

using

System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Forms;

namespace SubclassSample
{
    class WndProcHooker
{
        /// <summary>
        /// The callback used when a hooked window's message map contains the
        /// hooked message
        /// </summary>
        /// <param name="hwnd">The handle to the window for which the message
        /// was received</param>
        /// <param name="wParam">The message's parameters (part 1)</param>
        /// <param name="lParam">The message's parameters (part 2)</param>
        /// <param name="handled">The invoked function sets this to true if it
        /// handled the message. If the value is false when the callback
        /// returns, the next window procedure in the wndproc chain is
        /// called</param>
        /// <returns>A value specified for the given message in the MSDN
        /// documentation</returns>
        public delegate int WndProcCallback(
            IntPtr hwnd, uint msg, uint wParam, int lParam, ref bool handled);

        /// <summary>
        /// This is the global list of all the window procedures we have
        /// hooked. The key is an hwnd. The value is a HookedProcInformation
        /// object which contains a pointer to the old wndproc and a map of
        /// messages/callbacks for the window specified. Controls whose handles
        /// have been created go into this dictionary.
        /// </summary>
        private static Dictionary<IntPtr, HookedProcInformation> hwndDict =
            new Dictionary<IntPtr, HookedProcInformation>();

        /// <summary>
        /// See <see>hwndDict</see>. The key is a control and the value is a
        /// HookedProcInformation. Controls whose handles have not been created
        /// go into this dictionary. When the HandleCreated event for the
        /// control is fired the control is moved into <see>hwndDict</see>.
        /// </summary>
        private static Dictionary<Control, HookedProcInformation> ctlDict =
            new Dictionary<Control, HookedProcInformation>();

        /// <summary>
        /// Makes a connection between a message on a specified window handle
        /// and the callback to be called when that message is received. If the
        /// window was not previously hooked it is added to the global list of
        /// all the window procedures hooked.
        /// </summary>
        /// <param name="ctl">The control whose wndproc we are hooking</param>
        /// <param name="callback">The method to call when the specified
        /// message is received for the specified window</param>
        /// <param name="msg">The message we are hooking.</param>
        public static void HookWndProc(
            Control ctl, WndProcCallback callback, uint msg)
{
            HookedProcInformation hpi = null;
            if (ctlDict.ContainsKey(ctl))
hpi = ctlDict[ctl];
            else if (hwndDict.ContainsKey(ctl.Handle))
hpi = hwndDict[ctl.Handle];
            if (hpi == null)
{
                // We havne't seen this control before. Create a new
                // HookedProcInformation for it
                hpi = new HookedProcInformation(ctl,
                    new Win32.WndProc(WndProcHooker.WindowProc));
ctl.HandleCreated += new EventHandler(ctl_HandleCreated);
                ctl.HandleDestroyed += new EventHandler(ctl_HandleDestroyed);
ctl.Disposed += new EventHandler(ctl_Disposed);

                // If the handle has already been created set the hook. If it
                // hasn't been created yet, the hook will get set in the
                // ctl_HandleCreated event handler
                if (ctl.Handle != IntPtr.Zero)
                    hpi.SetHook();
}

            // stick hpi into the correct dictionary
            if (ctl.Handle == IntPtr.Zero)
                ctlDict[ctl] = hpi;
            else
hwndDict[ctl.Handle] = hpi;

            // add the message/callback into the message map
hpi.messageMap[msg] = callback;
}

        /// <summary>
        /// The event handler called when a control is disposed.
        /// </summary>
        /// <param name="sender">The object that raised this event</param>
        /// <param name="e">The arguments for this event</param>
        static void ctl_Disposed(object sender, EventArgs e)
{
            Control ctl = sender as Control;
            if (ctlDict.ContainsKey(ctl))
ctlDict.Remove(ctl);
            else
System.Diagnostics.Debug.Assert(false);
}

        /// <summary>
        /// The event handler called when a control's handle is destroyed.
        /// We remove the HookedProcInformation from <see>hwndDict</see> and
        /// put it back into <see>ctlDict</see> in case the control get re-
        /// created and we still want to hook its messages.
        /// </summary>
        /// <param name="sender">The object that raised this event</param>
        /// <param name="e">The arguments for this event</param>
        static void ctl_HandleDestroyed(object sender, EventArgs e)
{
            // When the handle for a control is destroyed, we want to
            // unhook its wndproc and update our lists
            Control ctl = sender as Control;
            if (hwndDict.ContainsKey(ctl.Handle))
{
                HookedProcInformation hpi = hwndDict[ctl.Handle];
UnhookWndProc(ctl, false);
}
            else
                System.Diagnostics.Debug.Assert(false);
}

        /// <summary>
        /// The event handler called when a control's handle is created. We
        /// call SetHook() on the associated HookedProcInformation object and
        /// move it from <see>ctlDict</see> to <see>hwndDict</see>.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        static void ctl_HandleCreated(object sender, EventArgs e)
{
            Control ctl = sender as Control;
            if (ctlDict.ContainsKey(ctl))
{
                HookedProcInformation hpi = ctlDict[ctl];
hwndDict[ctl.Handle] = hpi;
ctlDict.Remove(ctl);
hpi.SetHook();
}
            else
System.Diagnostics.Debug.Assert(false);
}

        /// <summary>
        /// This is a generic wndproc. It is the callback for all hooked
        /// windows. If we get into this function, we look up the hwnd in the
        /// global list of all hooked windows to get its message map. If the
        /// message received is present in the message map, its callback is
        /// invoked with the parameters listed here.
        /// </summary>
        /// <param name="hwnd">The handle to the window that received the
        /// message</param>
        /// <param name="msg">The message</param>
        /// <param name="wParam">The message's parameters (part 1)</param>
        /// <param name="lParam">The messages's parameters (part 2)</param>
        /// <returns>If the callback handled the message, the callback's return
        /// value is returned form this function. If the callback didn't handle
        /// the message, the message is forwarded on to the previous wndproc.
        /// </returns>
        private static int WindowProc(
            IntPtr hwnd, uint msg, uint wParam, int lParam)
{
            if (hwndDict.ContainsKey(hwnd))
{
                HookedProcInformation hpi = hwndDict[hwnd];
                if (hpi.messageMap.ContainsKey(msg))
{
                    WndProcCallback callback = hpi.messageMap[msg];
                    bool handled = false;
                    int retval = callback(hwnd, msg, wParam, lParam, ref handled);
                    if (handled)
                        return retval;
}

                // if we didn't hook the message passed or we did, but the
                // callback didn't set the handled property to true, call
                // the original window procedure
                return hpi.CallOldWindowProc(hwnd, msg, wParam, lParam);
}

System.Diagnostics.Debug.Assert(
                false, "WindowProc called for hwnd we don't know about");
            return Win32.DefWindowProc(hwnd, msg, wParam, lParam);
}

        /// <summary>
        /// This method removes the specified message from the message map for
        /// the specified hwnd.
        /// </summary>
        /// <param name="ctl">The control whose message we are unhooking
        /// </param>
        /// <param name="msg">The message no longer want to hook</param>
        public static void UnhookWndProc(Control ctl, uint msg)
{
            // look for the HookedProcInformation in the control and hwnd
            // dictionaries
            HookedProcInformation hpi = null;
            if (ctlDict.ContainsKey(ctl))
hpi = ctlDict[ctl];
            else if (hwndDict.ContainsKey(ctl.Handle))
hpi = hwndDict[ctl.Handle];

            // if we couldn't find a HookedProcInformation, throw
            if (hpi == null)
                throw new ArgumentException("No hook exists for this control");

            // look for the message we are removing in the messageMap
            if (hpi.messageMap.ContainsKey(msg))
hpi.messageMap.Remove(msg);
            else
                // if we couldn't find the message, throw
                throw new ArgumentException(
                    string.Format(
                        "No hook exists for message ({0}) on this control",
msg));
}

        /// <summary>
        /// Restores the previous wndproc for the specified window.
        /// </summary>
        /// <param name="ctl">The control whose wndproc we no longer want to
        /// hook</param>
        /// <param name="disposing">if true we remove don't readd the
        /// HookedProcInformation
        /// back into ctlDict</param>
        public static void UnhookWndProc(Control ctl, bool disposing)
{
            HookedProcInformation hpi = null;
            if (ctlDict.ContainsKey(ctl))
hpi = ctlDict[ctl];
            else if (hwndDict.ContainsKey(ctl.Handle))
hpi = hwndDict[ctl.Handle];

            if (hpi == null)
                throw new ArgumentException("No hook exists for this control");

            // If we found our HookedProcInformation in ctlDict and we are
            // disposing remove it from ctlDict
            if (ctlDict.ContainsKey(ctl) && disposing)
ctlDict.Remove(ctl);

            // If we found our HookedProcInformation in hwndDict, remove it
            // and if we are not disposing stick it in ctlDict
            if (hwndDict.ContainsKey(ctl.Handle))
{
hpi.Unhook();
hwndDict.Remove(ctl.Handle);
                if (!disposing)
ctlDict[ctl] = hpi;
}
}

        /// <summary>
        /// This class remembers the old window procedure for the specified
        /// window handle and also provides the message map for the messages
        /// hooked on that window.
        /// </summary>
        class HookedProcInformation
{
            /// <summary>
            /// The message map for the window
            /// </summary>
            public Dictionary<uint, WndProcCallback> messageMap;

            /// <summary>
            /// The old window procedure for the window
            /// </summary>
                private IntPtr oldWndProc;

            /// <summary>
            /// The delegate that gets called in place of this window's
            /// wndproc.
            /// </summary>
            private Win32.WndProc newWndProc;

            /// <summary>
            /// Control whose wndproc we are hooking
            /// </summary>
            private Control control;

            /// <summary>
            /// Constructs a new HookedProcInformation object
            /// </summary>
            /// <param name="ctl">The handle to the window being hooked</param>
            /// <param name="wndproc">The window procedure to replace the
            /// original one for the control</param>
            public HookedProcInformation(Control ctl, Win32.WndProc wndproc)
{
control = ctl;
newWndProc = wndproc;
messageMap = new Dictionary<uint, WndProcCallback>();
}

            /// <summary>
            /// Replaces the windows procedure for <see>control</see> with the
            /// one specified in the constructor.
            /// </summary>
            public void SetHook()
{
                IntPtr hwnd = control.Handle;
                if (hwnd == IntPtr.Zero)
                throw new InvalidOperationException(
                    "Handle for control has not been created");

oldWndProc = Win32.SetWindowLong(hwnd, Win32.GWL_WNDPROC,
                    Marshal.GetFunctionPointerForDelegate(newWndProc));
}

            /// <summary>
            /// Restores the original window procedure for the control.
            /// </summary>
            public void Unhook()
{
                IntPtr hwnd = control.Handle;
                if (hwnd == IntPtr.Zero)
                    throw new InvalidOperationException(
                        "Handle for control has not been created");

                Win32.SetWindowLong(hwnd, Win32.GWL_WNDPROC, oldWndProc);
}

            /// <summary>
            /// Calls the original window procedure of the control with the
            /// arguments provided.
            /// </summary>
            /// <param name="hwnd">The handle of the window that received the
            /// message</param>
            /// <param name="msg">The message</param>
            /// <param name="wParam">The message's arguments (part 1)</param>
            /// <param name="lParam">The message's arguments (part 2)</param>
            /// <returns>The value returned by the control's original wndproc
            /// </returns>
            public int CallOldWindowProc(
                    IntPtr hwnd, uint msg, uint wParam, int lParam)
{
                    return Win32.CallWindowProc(
oldWndProc, hwnd, msg, wParam, lParam);
}
}
}
}

Here are some of the PInvokes, structures and constants you will need for this sample. I include them all in the sealed class called Win32:

using

System;
using System.Drawing;
using System.Runtime.InteropServices;

/// <summary>
/// Contains managed wrappers or implementations of Win32 structs, delegates,
/// constants and PInvokes that are useful for this sample.
///
/// See the documentation on MSDN for more information on the elements provided
/// in this file.
/// </summary>
public sealed class Win32
{
/// <summary>
/// A callback to a Win32 window procedure (wndproc)
/// </summary>
/// <param name="hwnd">The handle of the window receiving a message</param>
/// <param name="msg">The message</param>
/// <param name="wParam">The message's parameters (part 1)</param>
/// <param name="lParam">The message's parameters (part 2)</param>
/// <returns>A integer as described for the given message in MSDN</returns>
public delegate int WndProc(IntPtr hwnd, uint msg, uint wParam, int lParam);

#if DESKTOP
[DllImport("user32.dll")]
#else
[DllImport("coredll.dll")]
#endif
public extern static int DefWindowProc(
IntPtr hwnd, uint msg, uint wParam, int lParam);

#if DESKTOP
[DllImport("user32.dll")]
#else
[DllImport("coredll.dll")]
#endif
public extern static IntPtr SetWindowLong(
IntPtr hwnd, int nIndex, IntPtr dwNewLong);

public const int GWL_WNDPROC = -4;

#if DESKTOP
[DllImport("user32.dll")]
#else
[DllImport("coredll.dll")]
#endif
public extern static int CallWindowProc(
IntPtr lpPrevWndFunc, IntPtr hwnd, uint msg, uint wParam, int lParam);
}

Next time we will use the WndProcHooker class to extend the .NETCF supplied TreeView control to add the NodeMouseClicked event.

That's it for now.
Tim

This posting is provided "AS IS" with no warranties, and confers no rights.

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