delphiaccess-violationwindows-messagesobject-destruction

How to prevent message handling after the control's destruction

There are two instances of the following component types:

  1. TfrmTimeSliceStructure, which is a direct descendant of TFrame.
  2. THKSDBVirtualStringTree, which is a direct descendant of TDBVirtualStringTree (from FIBPlus), which itself is a direct descendant of Mike Lischke's TVirtualStringTree class.

The THKSDBVirtualStringTree component is used as a child control on TfrmTimeSliceStructure.

On a double click - under certain conditions - the frame shall be destroyed.

For this I do a PostMessage call with a custom message code WM_USER + 4 (this is hexadecimal $0404), so that the destruction is delayed until all current messages are handled completely.

Though, in many cases an access violation occurs, as the THKSDBVirtualStringTree component is still handling messages after its own destruction.

I expected that there is no message handling happening after a control's destruction.

How can I prevent messages to be handled by a control that has already been destroyed?

Below, you can see the debugger's output. In both classes, I added a message logging in the method WndProc to output the recieved message code. In the first line, one can see that my custom message code WM_USER + 4 has been recieved.

Some lines later, there are two lines Instance of class THKSDBVirtualStringTree is going to be destroyed. and Instance of class THKSDBVirtualStringTree has been destroyed.. In between these two lines, no messages are recieved.
After these lines, there are still some messages handled. Eventually, this leads to the access violation at the end. From their message codes I can see, that those messages are control messages, because CM_BASE = $B000;.

Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $0404 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $0405 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $8001 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $0200 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $0202 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $0215 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $02A3 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B014 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $B014 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $8001 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $8001 Prozess Memory.exe (20916)
Thread-Start: Thread-ID: 6260. Prozess Memory.exe (20916)
Thread-Start: Thread-ID: 21148. Prozess Memory.exe (20916)
Thread-Ende: Thread-ID: 6260. Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $0002 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $000E Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $0272 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $0002 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $000E Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $0082 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class TfrmTimeSliceStructure recieved message $0082 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree is going to be destroyed. Prozess Memory.exe (20916)
Thread-Ende: Thread-ID: 22156. Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree has been destroyed. Prozess Memory.exe (20916)
Thread-Start: Thread-ID: 5672. Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B028 Prozess Memory.exe (20916)
Thread-Start: Thread-ID: 9244. Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B009 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B008 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B023 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B03D Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B050 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B058 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B011 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B03B Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B03B Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B00D Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B022 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B009 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B008 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B023 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B035 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B03D Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B050 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B058 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B011 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B009 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B035 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B034 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B009 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B03B Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B008 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B00E Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B034 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B008 Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B00E Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B034 Prozess Memory.exe (20916)
Thread-Ende: Thread-ID: 5672. Prozess Memory.exe (20916)
Thread-Ende: Thread-ID: 21148. Prozess Memory.exe (20916)
Debug-Ausgabe: Instance of class THKSDBVirtualStringTree recieved message $B007 Prozess Memory.exe (20916)
Erste Gelegenheit für Exception bei $01819981. Exception-Klasse $C0000005 mit Meldung 'access violation at 0x01819981: read of address 0x00000050'. Prozess Memory.exe (20916)
Solution

  • The problem was caused by an explicit call to Application.ProcessMessages somewhere in our OnNodeDblClick event handler code.

    Unfortunately, we still have some of these calls in our code base. :-(

    Let me tell you how I found out the problem's cause:

    As you can see in the question, I tried these two approaches:

    1. Override the control's WndProc and log every processed message with OutputDebugString.
    2. Check, whether the control has already been destroyed, by adding one call to OutputDebugString to the beginning and another to the end of your control's destructor.

    Later, it came to my mind to...

    1. ...check, whether the initiating user interaction (here: a double click) has been processed completely. This is to be done with OutputDebugString, too.
      In my case, the second mouse-down was not completely handled before the access violation occured.

    I could then conclude, that somewhere the message queue is prematurely processed. So, I had to find out, where Application.ProcessMessages is called.

    1. Therefore, I added a breakpoint in this method. Because a normal breakpoint would have broken the message processing. I used another breakpoint type, which - I have to confess - I used the first time at all.
      In the breakpoint's advanced settings I deselected Break and selected Log Call Stack instead. I decided to log only two stack frames, because I just wanted to know who the direct caller of Application.ProcessMessages is.

    After another round of reproduction, I had an extensive debugger log containing all those handled message codes and lines for each call to the mouse-down handler, the object's destructor and every call to Application.ProcessMessages.

    1. Now, I could infer that the first call to Application.ProcessMessages after the beginning of the unfinished mouse-down handler and before the reception of my custom event WM_USER + 4 has to be the one, which breaks the code.