Export (0) Print
Expand All

How to: Use a Class for Hooking Windows Procedures 

This sample program assigns a Windows procedure, WndProc, to a window when it receives a particular Windows message. This action occurs when the window's handle, hwnd, is mapped to a collection handles to respond when that Windows message is received. The following how-to topics use this class:

This example program is described in detail in Subclassing Controls with a Managed Window Procedure.

Example

public class WndProcHooker
{

    // The WndProcCallback method is used when a hooked
    // window's message map contains the hooked message.
    // Parameters:
    // hwnd - The handle to the window for which the message
    // was received.
    // wParam - The message's parameters (part 1).
    // lParam - The message's parameters (part 2).
    // 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.
    //
    // Returns a value specified for the given message.
    public delegate int WndProcCallback(
        IntPtr hwnd, uint msg, uint wParam, int lParam, ref bool handled);

    // 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
    // message's callbacks for the window specified. Controls whose handles
    // have been created go into this dictionary.
    private static Dictionary<IntPtr, HookedProcInformation> hwndDict =
        new Dictionary<IntPtr, HookedProcInformation>();

    // The key for this dictionary 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 hwndDict.
    private static Dictionary<Control, HookedProcInformation> ctlDict =
        new Dictionary<Control, HookedProcInformation>();

    // 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.
    // Parameters:
    // ctl - The control whose wndproc we are hooking.
    // callback - The method to call when the specified.
    // message is received for the specified window.
    // msg - The message being hooked.
    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)
        {
            // If new control, 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;
    }

    // The event handler called when a control is disposed.
    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);
    }

    // The event handler called when a control's handle is destroyed.
    // We remove the HookedProcInformation from hwndDict and
    // put it back into ctlDict in case the control get re-
    // created and we still want to hook its messages.
    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);
    }

    // The event handler called when a control's handle is created. We
    // call SetHook() on the associated HookedProcInformation object and
    // move it from ctlDict to hwndDict.
    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);
    }

    // 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.
    // Parameters:
    // hwnd - The handle to the window that received the
    // message
    // msg - The message
    // wParam - The message's parameters (part 1)
    // lParam - The messages's parameters (part 2)
    // Returns 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.
    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 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);
    }

    // This method removes the specified message from the message map for
    // the specified hwnd.
    public static void UnhookWndProc(Control ctl, uint msg)
    {
        // Look for the HookedProcInformation in the
        // ctrDict and hwndDict 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));
    }

    // Restores the previous wndproc for the specified window.
    // Parameters:
    // ctl - The control whose wndproc we no longer want to hook.
    // disposing - True if HookedProcInformation is not
    //   read back into ctlDict.
    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;
        }
    }

    // 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.
    class HookedProcInformation
    {

        // The message map for the window.
        public Dictionary<uint, WndProcCallback> messageMap;

        // The old window procedure for the window.
        private IntPtr oldWndProc;

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

        // Control whose wndproc is being hooked.
        private Control control;

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

        // Replaces the windows procedure for control with the
        // one specified in the constructor.
        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));
        }

        // Restores the original window procedure for the control.
        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);
        }

        // Calls the original window procedure of the control with the
        // arguments provided.
        // Parameters:
        // hwnd - The handle of the window that received the
        // message
        // msg - The message
        // wParam - The message's arguments (part 1)
        // lParam - The message's arguments (part 2)
        // Returns the value returned by the control's original wndproc.
        public int CallOldWindowProc(
            IntPtr hwnd, uint msg, uint wParam, int lParam)
        {
            return Win32.CallWindowProc(
                oldWndProc, hwnd, msg, wParam, lParam);
        }
    }
}

See Also

Community Additions

ADD
Show:
© 2015 Microsoft