From what I understand, COM objects can only be used in the thread they were instantiated in. If that thread dies, the object becomes invalid and you can't use it anymore. The error I face when I attempt to is COM object that has been separated from its underlying RCW cannot be used. The advice I've typically seen to deal with this is just to reinitialize a new COM object for every task you want to run.
The problem is that the COM object I'm using handles communication with another program, and re-establishing the connection with the same instance of that program with a new COM object is tricky. The operations I want to run are long winded and require feedback from the user in between, so including all the operations in a single Task is not really feasible afaik.
What I want to do is build a wrapper around that COM object that puts calls to it into a queue to be invoked by the appropriate thread, that would be compatible with async/await, and I would like to know if that's possible. But to avoid the XY problem I'll ask something different:
What is the most elegant way to keep a COM object functioning, off the main thread, so that I can continue to use it for multiple things?
COM object that has been separated from its underlying RCW cannot be used.
To keep the COM object alive you need to declare a corresponding RCW at a global scope, so the reference counter is not declared when the RCW is swiped from the heap. Also you need to pay attention to any Marshal.ReleaseComObject calls that may lead to the issue you faced with. This method is used to explicitly control the lifetime of a COM object used from managed code. You should use this method to free the underlying COM object that holds references to resources in a timely manner or when objects must be freed in a specific order.
For example, if you deal with Office applications like Outlook, it may detect cross-thread calls and throw exceptions in such cases.
The best solution is create a scheduler which can be called from secondary threads and queue such calls on the main thread to communicate with a COM server. Or just consider extracting all the required information (scalar value that don't involve COM objects) which can be simply consumed from secondary threads and then process it in the way you need.
Related
I am writing a Win32 C++ DLL that uses the COM object(B.dll) which is made in C#.
This dll(A.dll) provides CMyComObject class which creates a COM object and access to it.
Here is my code.
void CMyComObject::CMyComObject()
{
HRESULT result = CoInitialize(NULL);
...
result = CoCreateInstance(CLSID_COMDLL, NULL, CLSCTX_INPROC_SERVER, IID_COMDLL, reinterpret_cast<void**>(&MyComObject));
}
void CMyComObject::~CMyComObject()
{
..
CoUninitialize();
..
}
And then, here is a client program that loads A.dll and access to the COM object.
This program creates several threads which load A.dll and create a COM object concurrently.
In this case, Is this correct to use CoInitialize() function or Should I use CoInitializeEx() function with COINIT_MULTITHREADED parameter?
Or Is there any mistake I did?
(I registered B.dll by commanding "reg_asm.exe B.dll B.tlb /codebase")
Sorry for my poor English.
Thanks.
You are supposed to use CoInitialize[Ex]/CoUninitialize before and after any COM activity on that thread, and your choice between CoInitialize and CoInitializeEx with specific parameters depends on whether you prefer STA or MTA mode for your thread.
Having said that, your initialization:
Does not depend on whether the COM object itself creates any threads
Does not depend on other parts of your application possibly having other COM activity, including similar instantiation of the same COM class
Entirely depends on your COM activity on the thread in question
Does not normally happen in class constructor; it is typical to have COM initialization associated with top level thread code such as before windows message pump or at the very beginning of the thread procedure; putting it into constructor is an easy way to get into collision e.g. with another initialization (esp. using different apartment model) or too early uninitialization.
Summing it all once again, your initialization:
looks good if you are OK with COM single thread apartment model and you don't pass obtained pointer between threads
you would be better off moving CoInitialize and CoUninitialize calls out of constructor and associate it with thread code
be sure to check returned value to detect failures, esp. attempt to initialize mismatching apartment on the thread already having COM initialization
the part you missing is that you have to close all your COM activity before CoUninitialize call, including releasing your MyComObject pointer.
Although there are many questions about COM and STA/MTA (e.g. here), most of them talk about applications which have a UI. I, however, have the following setup:
A console application, which is by default Multi-Threaded Apartment (Main() explicitly has the [MTAThread] attribute).
The main thread spawns some worker threads.
The main thread instantiates a single-threaded COM object.
The main thread calls Console.ReadLine() until the user hits 'q', after which the application terminates.
A few questions:
Numerous places mentions the need of a message pump for COM objects. Do I need to manually create a message-pump for the main thread, or will the CLR create it for me on a new STA thread, as this question suggests?
Just to make sure - assuming the CLR automagically creates the necessary plumbing, can I then use the COM object from any worker thread without the need of explicit synchronization?
Which of the following is better in terms of performance:
Let the CLR take care of the marshaling to and from the COM object.
Explicitly instantiate the object on a separate STA thread, and have other thread communicate with it via e.g. a ConcurrentQueue.
This is done automagically by COM. Since your COM object is single-threaded, COM requires a suitable home for the object to ensures it is used in a thread-safe way. Since your main thread is not friendly enough to provide such guarantees, COM automatically creates another thread and creates the object on that thread. This thread also automatically pumps, nothing you have to do to help. You can see it being created in the debugger. Enable unmanaged debugging and look in the Debug + Windows + Threads window. You'll see the thread getting added when you step over the new call.
Nice and easy, but it does have a few consequences. First off, the COM component needs to provide a proxy/stub implementation. Helper code that knows how to serialize the arguments of a method call so the real method call can be made on another thread. That's usually provided, but not always. You'll get a hard to diagnose E_NOINTERFACE exception if it is missing. Sometimes TYPE_E_LIBNOTREGISTERED, a common install problem.
And most significantly, every call on the COM component will be marshaled. That's slow, a marshaled call is usually around 10,000x slower than a direct call on a method that itself takes very little time. Like a property getter call. That can really bog your program down of course.
An STA thread avoids this and is therefore the recommended way to use a single-threaded component. And yes, it is a requirement for an STA thread to pump a message loop. Application.Run() in a .NET program. It is the message loop that marshals calls from one thread to another in COM. Do note that it doesn't necessarily mean that you must have a message loop. If no call ever needs to marshaled, or in other words, if you make all the calls on the component from the same thread, then the message loop isn't needed. That's typically easy to guarantee, particularly in a console mode app. Not if you create threads yourself of course.
One more nasty detail: a single-threaded COM component sometimes assumes it is created on a thread that pumps. And will use PostMessage() itself, typically when it uses worker threads internally and needs to raise events on the STA thread. That will of course not work correctly anymore when you don't pump. You normally diagnose this by noticing that events are not being raised. The common example of such a component is WebBrowser. Which heavily uses threads internally but raises events on the thread on which it was created. You'll never get the DocumentCompleted event if you don't pump.
So putting [STAThread] on your Main() method might be enough to get happy fast code, even without a call to Application.Run(). Just keep the consequences in mind, seeing a method call deadlock or an event not getting raised is the tell-tale sign that pumping is required.
Yes, it is possible to create a STA COM object from an MTA thread.
In this case, COM (not CLR) will create an implicit STA apartment (a separate COM-owned thread) or re-use the existing one, created ealier. The COM object will be instantiated there, then a thread-safe proxy object (COM marshalling wrapper) will be created for it and returned to the MTA thread. All calls to the object made on the MTA thread will be marshalled by COM to that implicit STA apartment.
This scenario is usually undesirable. It has a lot of shortcomings and may simply not work as expected, if COM is unable to marshal some interfaces of the object. Check this question for more details. Besides, the message pump loop, run by the implicit STA apartment, pumps only a limited number of COM-specific messages. That may also affect the functionality of the COM.
You may try it and it may work well for you. Or, you may run into some unpleasant issues like deadlocks, quite difficult to diagnose.
Here is a closely related question I just recently answered:
StaTaskScheduler and STA thread message pumping
I'd personally prefer to manually control the logic of the inter-thread calls and thread affinity, with something like ThreadAffinityTaskScheduler proposed in my answer.
You may also want to read this: INFO: Descriptions and Workings of OLE Threading Models, highly recommended.
Do I need to manually create a message-pump for the main thread,
No. It is in the MTA therefore no message pump is needed.
or will the CLR create it for me on a new STA thread
If COM creates the thread (because there is no STA in the process) then it also creates the message pump (and a hidden window: can be seen with the SPY++ and similar debugging tools).
COM object from any worker thread without the need of explicit synchronization
Depends.
If the reference to the single threaded object (STO) was created in the MTA then COM will supply the appropriate proxy. This proxy is good for all threads in the MTA.
In any other case the reference will need to be marshalled to ensure it has the correct proxy.
is better in terms of performance
The only answer to this is to test both and compare.
(Remember if you create the thread for the STA and then instantiate the object locally you need to do the message pumping. It is not clear to me that there is any CLR level lightweight message pump—including WinForms just for this certainly isn't.)
NB. The only in depth explanatory coverage of COM and the CLR is .NET and COM: The Complete Interoperability Guide by Adam Nathan (Sams, January 2002). But it is based on .NET 1.1 and now out of print (but there is a Kindle edition and is available via Safari Books Online). Even this book doesn't describe directly what you are trying to do. I would suggest some prototyping.
I've come across this problem several times while developing in C#. I'll be a happily coding along, passing objects to and fro between threads and what not, then all of a sudden I get this familiar error:
"The calling thread cannot access this object because a different
thread owns it."
Well, ok, I've dealt with it before, especially with objects on the GUI thread. You just have to write some extra code to program around that specific problem. But every once in while I come across an object that is by all means ordinary, yet it doesn't like being accessed by different threads.
EDIT I was mistaken in my original post about the object that was causing the access exception. It was NOT IPAddress, instead its System.Printing.PrintQueue. which I was using to obtain the IP address. This is the object that you can't assess from more than 1 thread.
All my classes I've written never have this problem. I don't even know how I'd implement this myself. Would you have to keep a member variable with the thread ID that created you, and then check the current thread against that on every single property and method access? That seems crazy. Why would Microsoft decide that..... "OK... PrintQueue, definitely not sharable among threads. But these other classes.... their good to go."
Why are some objects blocked from multiple thread access?
I think this may explain things fairly well, I think this specifically has to do with COM.
http://msdn.microsoft.com/en-us/library/ms693344%28v=vs.85%29
specifically.
In general, the simplest way to view the COM threading architecture is
to think of all the COM objects in the process as divided into groups
called apartments. A COM object lives in exactly one apartment, in the
sense that its methods can legally be directly called only by a thread
that belongs to that apartment. Any other thread that wants to call
the object must go through a proxy.
There are two types of apartments: single-threaded apartments, and
multithreaded apartments.
Single-threaded apartments consist of exactly one thread, so all COM objects that live in a single-threaded apartment can receive
method calls only from the one thread that belongs to that apartment.
All method calls to a COM object in a single-threaded apartment are
synchronized with the windows message queue for the single-threaded
apartment's thread. A process with a single thread of execution is
simply a special case of this model.
Multithreaded apartments consist of one or more threads, so all COM objects that live in an multithreaded apartment can receive method
calls directly from any of the threads that belong to the
multithreaded apartment. Threads in a multithreaded apartment use a
model called free-threading. Calls to COM objects in a multithreaded
apartment are synchronized by the objects themselves.
Extension to Release COM Object in C#
I noticed that saving the MailItem and releasing is a time consuming task. So, it is safe to do the following ? (pseudo-code below)
Thread 1 (main thread)
- Open 10 (different .msg files) - MailItems [List<MailItem> items]
- user works on them and want to save and close all of them with one click.
- On_save_All_click (runs on main thread)
- Do
- toBeClearedList.addAll(items);
- items.clear() [so that main thread cannot access those items]
- BG_Thread.ExecuteAsyn(toBeClearedList);
- End
Thread 2 (background thread) (input - List<MailItems>)
- foreach(MailItem item in input)
item.save();
System.Runtime.InteropServices.Marshal.ReleaseComObject(item)
- done
I wrote couple of tests and looks like its working; Just want to know if its safe to do so ? "Releasing COM objects in different thread than the one in which it was created"
Thanks
Karephul
When using COM from unmanaged code (C/C++), the rules are pretty strict: you can only call methods on an interface from the same apartment that you acquired the object on. So if you obtain an interface pointer on an STA thread, then only that thread is allowed to call any of the methods. If you obtain an interface pointer on an MTA thread, then only other threads in the same MTA can use that pointer. Any other use that crosses apartments requires that the interface pointer is marshaled to the other apartment.
However, that's unamanged world. .Net adds a whole layer on top of COM which buries a lot of these low-level details, and for the most part, once you get your hands on an interface, you can pass that interface around between threads as much as you please without having to worry about the old threading rules. What's happening here is that it are actually passing around references to an object called a "Runtime Callable Wrapper" (RCW), and it is managing the underlying COM interface, and controlling access to it accordingly. (It takes on the burden of upholding the COM apartment rules so that you don't have to, and that's why it can appear that the old COM threading rules don't apply in .Net: they do, they're just hidden from you.)
So you can safely call Release or other methods from another thread: but be aware that if the original thread was STA, then calling those methods will cause the underlying RCW to marshal the call back to the original owning thread so that it still upholds the underling COM rules. So using a separate thread may not actually get you any performance in the end!
Some articles worth reading that fill in some of the details here:
The mapping between interface pointers and runtime callable wrappers (RCWs) - good overview of some of the details of how RCW's work, but it doesn't say much about threading.
Improving Interop Performance - Marshal.ReleaseComObject - some good notes on when to use or not use ReleaseComObject, and how it works with the RCWs.
cbrumme's WebLog - Apartments and Pumping in the CLR - more internals than you're likely to want to know; this is a couple of CLR releases out of date, but still gives a good insight into how .Net covers over many of the underlying COM issues.
If I remember my COM properly (I always hated that technology, way too complicated), if the COM object is single-threaded (that is, belongs in a single-threaded-apartment), releasing it from another thread isn't going to do you any good - it's just going to execute the actual release code in your main thread.
You are going to see a little difference between your code, and releasing the objects from the main thread in one loop. If you release the objects in one loop, your UI is going to be unresponsive until you release all messages. By using a secondary thread, your UI thread will release one message, then handle other events, then release another, and so on. You can get the same effect by sending yourself a message (or using a Dispatcher if you have a WPF application), and avoid having another thread.
I'm trying to fix problem with "COM object that has been separated from its underlying RCW cannot be used" error, and I think what's causing it is that COM objects are used on a thread that didn't instantiate them.
I'm not allowed to do much refactoring, and since objects should be available on multiple threads I wonder if there is a way to find out if they have been created on current thread before doing something with them that would cause aforementioned error. And, if they haven't, create them.
Also, I'm new to this interop thing, so if someone would be kind enough to help me understand, I'd much appreciate it:
What happens with the COM object once the thread finishes, and why is RCW still available on the other thread even when it doesn't have the COM object in it anymore (why isn't it null?). Also, why would it cause that error and in the same time return true on Marshal.IsCOMObject?
What happens in the following scenario(s) with reference count and the wrapper and the memory:
Create COM object x on the thread A
Pass it and save it on the thread B
Create another x (alternatively, what would happen if it were y?) on the thread C
Pass it and overwrite x on the thread B
What happens with the COM object once the thread finishes
The COM object gets destroyed automatically by COM. Which will produce the 'COM object that has been separated' exception message when another thread continues to use it. You cannot allow the thread to exit.
Clearly you have an single threaded COM server, by far the most common kind. It has affinity to the STA thread on which it was created. COM makes sure to automatically marshal any calls made on another thread to the thread that created the object. That can no longer work when the thread is gone. Also beware that you don't get any concurrency.
Another way to get this exception is by making the mistake of handling reference counts explicitly with Marshal.ReleaseComObject(). Not unlikely either since you should have gotten an MDA warning.