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.NET Forum / .NET Framework / Interop / January 2007COM interop multithreading problem : Unable to cast COM object of type 'System.__ComObject' to interface
Hi, I have a DirectShow filter which implements a COM interface called IDVBSubtitle. It is defined like this on the C++ side (simplified version used to debug of course, real interface has more methods :) // C19647D5-A861-4845-97A6-EBD0A135D0BF DEFINE_GUID(IID_IDVBSubtitle, 0xc19647d5, 0xa861, 0x4845, 0x97, 0xa6, 0xeb, 0xd0, 0xa1, 0x35, 0xd0, 0xbf); DECLARE_INTERFACE_( IDVBSubtitle, IUnknown ) { STDMETHOD(Test)(int status) PURE; }; On the C# side it looks like this: [Guid( ... ), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)] public interface IDVBSubtitle { void Test(int status); } When i create an instance of the filter in C# i can call test as much as i like succesfully, but ONLY from within the same thread that created it. If i try to call the Test method from any other thread i get this: Unable to cast COM object of type 'System.__ComObject' to interface type 'MediaPortal.Player.IDVBSubtitle'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{C19647D5-A861-4845-97A6-EBD0A135D0BF}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)). Again, note that this ONLY occurs when the call is made from another thread than the one that created the filter. I have tried both creating a seperate C# thread to call Test, and had a callback from the COM object call Test, both gave the above error. Reading the thread referenced below lead me to believe this problem is related to thread apartment states, but unfortunately didnt give me a solution. I have tried creating the calling thread as both STA and MTA, but it makes do difference. Could someone give me a hint here, been stuck on this for a few days? Reference: http://groups.google.dk/group/microsoft.public.dotnet.languages.csharp/browse_th read/thread/bf1617e0ddcf1d9c/144f5a7cf1ae8165?lnk=st&q=Unable+to+cast+COM+object +of+type%2C+STA%2C+MTA&rnum=1&hl=da#144f5a7cf1ae8165 You definately cannot invoke the COM object from an MTA thread. Ideally you want to just call methods from the STA thread that created the object. IIRC, you should be able to call from one STA thread to another if you have a typelib registered which allows the cirrect marshalling to happen. ziph...@gmail.com wrote: > Hi, > [quoted text clipped - 41 lines] > but it makes do difference. Could someone give me a hint here, been > stuck on this for a few days? > You definately cannot invoke the COM object from an MTA thread. Interesting that you should say that: I checked the Main method of the application im working on (i didnt write that myself). It is tagged with [STAThread], so i suppose all the threads its spawns are STA as well. I tried changing it to MTAThread (which im not too fond of since there is a comment saying another component is dependent on STAThread) and all my problems went away! So it seems that i can indeed call it from an MTA thread. As far as i understand its the apartment mode of the thread that creates the COM object that decides the apartment mode, so when it is created with an MTA thread and only accessed by MTA threads it seems to work. When i previously tried spawning a MTA thread to create it, the other STA threads couldnt access the COM object. > Ideally you want to just call methods from the STA thread that created > the object. That is sadly not possible(or at least not worth the bother), the COM object in question is a DirectShow filter which will be accessed by the graph thread and additionally my own thread. I get the abovementioned error even if the COM object calls back to a managed method and tries to call its own methods (ie it notifies my of a new subtitle, and i call a method to get it, within the callback thread). > IIRC, you should be able to call from one STA thread to > another if you have a typelib registered which allows the cirrect > marshalling to happen. Do you know where more information can be found on this? COM objects have to be specifically coded to support MTA and most do not, so I assumed your problem was calling an STA-oly object from an MTA thread. When you register the object dll, it will show the apratment type supported in the registry, but it seems like you've already determined it supports MTA. As an aside, there is nothing in the .NET SDK that states that threads always spawn threads of the same type. You should explicitly use the thread type you need. Finally, the best resource is a book entitled .NET and COM: The Complete Interoperability Guide. It's easily over 800 pages, if that gives an idea of it's depth of coverage. Tom > > You definately cannot invoke the COM object from an MTA thread. > [quoted text clipped - 27 lines] > > Do you know where more information can be found on this? > When you register the object dll, it will show the > apratment type supported in the registry, but it seems like you've > already determined it supports MTA. Where exactly? I looked under the CLSID in the registry, but it doesnt seem to store such information. As far as i understand DirectShow filters are supposed to be 'free' threaded objects btw. > As an aside, there is nothing in the .NET SDK that states that threads > always spawn threads of the same type. You should explicitly use the > thread type you need. Sure, but even if i create an MTA thread and create the object there, the other threads cannot access it, so there is some significance to the main threads apartment model. I suppose it might be related to the 'Main STA' concept described here: http://www.idevresource.com/com/library/articles/aparts.asp#multiap > Finally, the best resource is a book entitled .NET and COM: The > Complete Interoperability Guide. It's easily over 800 pages, if that > gives an idea of it's depth of coverage. Okay thanks :) > > When you register the object dll, it will show the > > apratment type supported in the registry, but it seems like you've [quoted text clipped - 3 lines] > seem to store such information. As far as i understand DirectShow > filters are supposed to be 'free' threaded objects btw. Drill into InprocServer32\ThreadingModel.
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