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In the output, we can see that we still have 3000 objects on the finalization queue that are waiting to be cleaned up What is preventing these objects from being cleaned up After all, a forced GC should guarantee that the finalizer thread in the process wakes up and starts invoking the Finalize methods on each object in a serialized fashion Let s take a look at the finalizer thread to make sure it is in a valid state:
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0:007> !threads ThreadCount: 3 UnstartedThread: 0 BackgroundThread: 2 PendingThread: 0 DeadThread: 0 Hosted Runtime: no PreEmptive GC Alloc Lock ID OSID ThreadOBJ State GC Context Domain Count APT Exception 0 1 42f8 004669a0 a020 Enabled 028f5f98 : 028f740c 00431288 0 MTA 2 2 3bd8 00468868 b220 Enabled 00000000:00000000 00431288 0 MTA (Finalizer) 3 3 2904 00497248 2007220 Enabled 00000000:00000000 00431288 0 STA 0:007> ~2s eax=feeefeee ebx=ffffffff ecx=0000032f edx=7fffffff esi=00000000 edi=00000000 eip=77629a94 esp=0404ee18 ebp=0404ee88 iopl=0 nv up ei pl zr na pe nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000246 ntdll!KiFastSystemCallRet: 77629a94 c3 ret 0:002> k ChildEBP RetAddr
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0404ee14 77629254 ntdll!KiFastSystemCallRet 0404ee18 765ac244 ntdll!ZwWaitForSingleObject+0xc 0404ee88 765ac1b2 KERNEL32!WaitForSingleObjectEx+0xbe 0404ee9c 774a1f07 KERNEL32!WaitForSingleObject+0x12 0404eec0 775a7f3e ole32!GetToSTA+0xad 0404eeec 775a89d4 ole32!CRpcChannelBuffer::SwitchAptAndDispatchCall+0x132 0404efcc 774be6f5 ole32!CRpcChannelBuffer::SendReceive2+0xef 0404f044 774a0f4f ole32!CAptRpcChnl::SendReceive+0xaf 0404f098 7625364e ole32!CCtxComChnl::SendReceive+0x95 0404f0b0 762536af RPCRT4!NdrProxySendReceive+0x43 0404f0bc 762533e2 RPCRT4!NdrpProxySendReceive+0xc 0404f534 762535f4 RPCRT4!NdrClientCall2+0x5e9 0404f558 761ee20e RPCRT4!ObjectStublessClient+0x6f 0404f568 774a2ed7 RPCRT4!ObjectStubless+0xf 0404f5f0 774f2833 ole32!CObjectContext::InternalContextCallback+0x128 0404f640 7a0bf7db ole32!CObjectContext::ContextCallback+0x87 0404f68c 7a0c04b2 mscorwks!CtxEntry::EnterContextOle32BugAware+0x2b 0404f7ac 7a0bfa13 mscorwks!CtxEntry::EnterContext+0x322 0404f7c0 7a0bfe6f mscorwks!IUnkEntry::MarshalIUnknownToStreamCallback+0x34 0404f808 79ff0959 mscorwks!IUnkEntry::UnmarshalIUnknownForCurrContext+0x45 0404f814 79f831ce mscorwks!IUnkEntry::GetIUnknownForCurrContext+0x31 0404f840 79f823cc mscorwks!RCW::SafeQueryInterfaceRemoteAware+0x16 0404f884 79f82353 mscorwks!RCW::GetComIPForMethodTableFromCache+0x6b 0404f894 79f8243f mscorwks!RCW::GetComIPFromRCW+0x25 0404f8d4 79f82519 mscorwks!ComObject::GetComIPFromRCW+0x40 0404f93c 79f836ce mscorwks!ComObject::GetComIPFromRCWEx+0x66 0404f988 000ca3ee mscorwks!ComObject::StaticGetComIPFromRCWEx+0x50 WARNING: Frame IP not in any known module Following frames may be wrong 0404f9f8 79fbcca7 0xca3ee 0404fa18 79fbcca7 mscorwks!MethodTable::SetObjCreateDelegate+0xc5 0404fa7c 79fbcc15 mscorwks!MethodTable::SetObjCreateDelegate+0xc5 0404fa9c 79fbcbb7 mscorwks!MethodTable::CallFinalizer+0x76 0404fab0 79f6acb6 mscorwks!SVR::CallFinalizer+0xb2 0404fb00 79f6abf7 mscorwks!WKS::GCHeap::TraceGCSegments+0x170 0404fb88 79fb99d6 mscorwks!WKS::GCHeap::TraceGCSegments+0x2b6 0404fba0 79ef3207 mscorwks!WKS::GCHeap::FinalizerThreadWorker+0xe7 0404fbb4 79ef31a3 mscorwks!Thread::DoADCallBack+0x32a 0404fc48 79ef30c3 mscorwks!Thread::ShouldChangeAbortToUnload+0xe3 0404fc84 79fb9643 mscorwks!Thread::ShouldChangeAbortToUnload+0x30a 0404fcac 79fb960d mscorwks!ManagedThreadBase_NoADTransition+0x32 0404fcbc 79fba09b mscorwks!ManagedThreadBase::FinalizerBase+0xd 0404fcf4 79f95a2e mscorwks!WKS::GCHeap::FinalizerThreadStart+0xbb 0404fd90 765a4911 mscorwks!Thread::intermediateThreadProc+0x49 0404fd9c 7760e4b6 KERNEL32!BaseThreadInitThunk+0xe 0404fddc 7760e489 ntdll!__RtlUserThreadStart+0x23 0404fdf4 00000000 ntdll!_RtlUserThreadStart+0x1b
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Debugging COM Interop Finalization
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By using the threads command, we can quickly identify which thread in the process corresponds to the finalizer thread (debugger thread id 2) After we know the thread, we can switch to that thread context and dump out the callstack From the callstack, we can identify that we are in fact looking at a finalizer thread by looking for a frame that contains FinalizerThreadWorker It seems that our particular finalizer thread is in a waiting state (as can be identified by the top frames) There is nothing unusual for a finalizer thread to be in a waiting state per se as it s quite common for it to wait until something is placed on the finalizer queue The key difference between a properly waiting finalizer thread and ours is that we have a frame with ole32!GetToSTA This implies that the finalizer thread is making a call to an STA COM object and is simply getting stuck Let s take a look at the source code in Listing 7-7 and see if we can spot any objects that have finalizers that call into a COM object The Data type seems to have a finalizer that calls into the BasicMathClass COM object We can verify that this is the object that is being finalized by simply looking at the managed call stack for the finalizer thread:
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0:002> !ClrStack OS Thread Id: 0x3bd8 (2) ESP EIP 0404f8f8 77629a94 [GCFrame: 0404f8f8] 0404f9d8 77629a94 [ComPlusMethodFrameStandaloneCleanup: 0404f9d8] COMInteropBasicMathClassAdd(Int32, Int32, Int32 ByRef) 0404f9f0 007602e5 AdvancedNETDebugging7DataFinalize()
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Before we continue with the debug session, it is important to understand how the BasicMathClass COM object was instantiated The Main method started an STA helper thread, which in turn created the COM object After it was created, the helper thread simply went to sleep for an extended period of time Seems like a fairly trivial instantiation The key, however, lies not in how the COM object itself was created, but how the thread that instantiated the COM object was created As mentioned earlier, the thread was created in the STA, which means that all calls into the COM object will be serialized by COM The way that this serialization works, under the covers, is by creating a hidden window and placing messages on the windows message queue that correspond to the method invocations The STA thread picks up the windows messages in a serialized fashion and executes the COM methods In order for the STA thread to get the windows messages, it must ensure to process the messages in the message queue If the thread fails to do so, no messages will be dispatched The picture should now be clear: The STA thread that created the COM object also promptly went to sleep preventing the processing of messages on the message queue, thereby also preventing any calls on the COM object from being
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