How to: Use a Class for Hooking Windows Procedures
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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
Tasks
How to: Use a Helper Class for Platform Invokes
How to: Subclass a TreeView by Using Native Callbacks
How to: Subclass a Button by Using Native Callbacks
Concepts
Subclassing Controls with a Managed Window Procedure
.NET Compact Framework How-to Topics