Access violation when accessing a COM object from .Net
- by Groo
Dear Sirs,
I am sorry if the post is too long, but I would be happy if someone would at least point read the bolded titles, and point me in the right direction. I am having this problem for couple of days, but was unable to found the answer on the net. These are the things I have found out so far.
1. "Access violation" exception crushes my managed application
My C# WinForms app sometimes closes with an "Access violation" exception ("Attempted to read or write protected memory"), right in the moment when selecting a TabPage in a windows form TabControl. From the stack trace (try/catch around Application.Run) I can see that the exception happens at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg), called inside UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData).
-- Message: Attempted to read or write protected memory.
This is often an indication that other memory is corrupt.
-- Stack trace:
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager
.System.Windows.Forms.UnsafeNativeMethods
.IMsoComponentManager.FPushMessageLoop
(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext
.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext
.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(ApplicationContext context)
at MyApp.Program.Main()
2. The faulting module seems to be a COM object (ChartFX Client Server 6.2)
Using WinDbg (with SoS loaded), I caught it on the unmanaged side, inside ChartFX.ClientServer.Core.dll (that's a COM charting component we are using):
(ca84.c98c): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=00000000 ebx=06e67c38 ecx=06e67c38 edx=000018c6 esi=06e7df30 edi=317a9e80
eip=31666110 esp=0015e040 ebp=0015e08c iopl=0 nv up ei pl zr na pe nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010246
ChartFX_ClientServer_Core!Ordinal5507+0x97b7:
31666110 8a404d mov al,byte ptr [eax+4Dh] ds:0023:0000004d=??
[edit:]
I also wasn't able to get the unmamanged stack details from WinDbg (it said "Stack unwind info not available"):
0:000 kP
ChildEBP RetAddr
WARNING: Stack unwind information not available. Following frames may be wrong.
0015e08c 3166288b ChartFX_ClientServer_Core!Ordinal5507+0x97b7
0015e394 3165a921 ChartFX_ClientServer_Core!Ordinal5507+0x5f32
0015e480 31678685 ChartFX_ClientServer_Core!Ordinal5496+0x26a
0015e568 3167bef4 ChartFX_ClientServer_Core!Ordinal5492+0x975
0015e668 316a356b ChartFX_ClientServer_Core!Ordinal5492+0x41e4
0015e77c 31709496 ChartFX_ClientServer_Core!Ordinal443+0x5745
0015e7d0 31707f70 ChartFX_ClientServer_Core!Ordinal2584+0x3cdc
0015e7f8 3170817d ChartFX_ClientServer_Core!Ordinal2584+0x27b6
0015e81c 3162fd76 ChartFX_ClientServer_Core!Ordinal2584+0x29c3
0015e86c 7719f8d2 ChartFX_ClientServer_Core!Ordinal899+0x6b6
0015e898 7719f794 USER32!GetMessageW+0x93
0015e910 771a06f6 USER32!GetWindowLongW+0x115
0015e940 771a069c USER32!CallWindowProcW+0x75
0015e960 747fcef4 USER32!CallWindowProcW+0x1b
0015e97c 747fd073 comctl32!Ordinal377+0x5c
0015e9e0 747fd027 comctl32!DefSubclassProc+0x92
0015ea04 747fd4e6 comctl32!DefSubclassProc+0x46
0015ea20 747fd073 comctl32!DefSubclassProc+0x505
0015ea84 747fd118 comctl32!DefSubclassProc+0x92
0015eae4 7719f8d2 comctl32!DefSubclassProc+0x137
3. Bug is not easy to reproduce (although it can be provoked usually in less than 5 min.)
I have several Chart instances in several TabPages, and this usually happens while I am switching the tabs. I still don't know how to reproduce it, besides switching those tabs for several minutes before it happens, so I cannot use our source control to reliably find the build which didn't have this problem. I am accessing the charts through the managed AxChart wrapper class (derived from AxHost), which was created by VS designer automatically.
4. What should be my next step?
If someone could point me to the next step I should do to find the actual cause, I would be very grateful. Experimenting (removing and returning code) does not do much good, because I don't know how to reproduce it, so it would take large amounts of time on each iteration just to convince myself that the bug is still there.
I have found that people often suggest something like "switching compiler optimizations", but since the exception is not thrown deterministically, I don't want to simply rearrange some bytes and hope that it never returns.
Thanks a lot in advance!
Best regards,
Groo
