Detect if called from P/Invoke

Question

I'm working with a managed service written in C#.
The service interacts with an unmanaged COM object using P/Invoke.
The service supplies the object with a reference to a callback interface.
The object invokes methods on the interface as and when appropriate.

My question: from any point in the service code at runtime, is it possible to determine whether the current thread is executing due to a P/Invoke callback?

In other words, I basically want to do a stack trace and check if the base call was a P/Invoke call back - without any reference to the callback interface or the names of its methods.

Hope that makes sense!

Edit: The reason I asked the question in general terms is because I wanted to know if there was an answer to the question. I know that the design may be... questionable... but at this point I can't see any alternative solution. Here is some more context for the people that might want to offer solutions to my more specific problem.

The issue I'm dealing with boils down to apartments (STA vs MTA).

The COM object is part of an interface for interacting with hardware devices. The service manages multiple hardware devices (ie. multiple instances of the COM object exist at runtime, using the same callback interface).

The service runs in a MTA. The COM object instances also usually run in a MTA (because the object is instanciated by the service). However, in order to interact with certain hardware drivers the COM object must sometimes run in a STA. In those cases the service uses a wrapper class with an internal STA thread to proxy interaction with the COM object instance in the STA. The wrapper uses the same interfaces as the COM object in order to keep one code path.

Most of the time the wrapper solution works fine. The problem is dealing with callbacks from the COM object, because the service needs to invoke methods on the object during the execution of the callback. If you try to proxy those method calls through the STA thread it results in deadlock, presumably because the calls get blocked on the STA message queue (which won't be processed until the callback is completed).

When the STA instances invoke callbacks on the service, P/Invoke executes these callbacks in MTA threads as we'd expect (because the service is running in a MTA). What I didn't expect was that it is possible to directly invoke methods on the COM object during the callback simply by bypassing the proxy thread. Somehow P/Invoke must realise that the thread originated in an object in a STA.

So the crux of the problem is that when dealing with STA, the same methods must be invoked in different ways depending on the context. If the service wants to invoke a method from its own managed MTA threads then it must go through the STA proxy thread; however, if the service wants to invoke that same method during a callback it must invoke the method directly. If I can distinguish between the two contexts then I have a solution for the problem.

Before you suggest... No, it is not practical/possible to pass all the required information as parameters on the callback to avoid the need to call methods during the callback. No, it is not possible to retrieve the required information after the callback is completed. The COM object's context is lost after the callback completes. It would be impractical to try to preserve it.

Sounds complicated just to know that... why not expose a wrapper which is definitely called and one which is not? — Jay, May 23 '14 at 12:30
That sounds like bad design. Why wouldn't you simply pass a different method reference for the P/Invoke callback? In that, you could call the "actual" callback with a "fromPinvoke" parameter or something like that. — Luaan, May 23 '14 at 12:30
Every managed thread has unmanaged stack frames on the stack. After all, it was started by the operating system. There can be many, you'll at least have two. Reliably stack walking is out, unmanaged code provides no such guarantee. Don't try to solve this problem the hard way when you can do it simply. Of course your callback method can know, all it has to do to let it be known elsewhere is pass a *bool* argument. — Hans Passant, May 23 '14 at 13:28
I've edited to give more context about the problem I'm trying to solve, though I'd still appreciate an answer to the original question. I did want to keep the interfaces identical if possible. Passing an STA/MTA flag through a callback function feels dirty... and I'm not exactly convinced that what I'm intending to do (ie. calling functions on the STA instance directly from the MTA) is "safe" anyway. — music man, May 23 '14 at 13:45
Sound like a duplicate of [How to check apartment state of current thread?](http://stackoverflow.com/questions/2378379/how-to-check-apartment-state-of-current-thread). I agree with @HansPassant that you should pass in bool argument based on the current apartment. And you could get the apartment state via [How to Query the COM Apartment State for the Current Thread](http://stackoverflow.com/questions/15981228/how-to-query-the-com-apartment-state-for-the-current-thread) — Black Frog, May 23 '14 at 13:46
@Black Frog Yes, I'm able to get the apartment of the managed threads, but that doesn't help because they'll always be MTA (because the service is MTA). If I could get the apartment of the unmanaged callback thread that would solve the problem. — music man, May 23 '14 at 13:51
@Luaan the callbacks always come from P/Invoke. The issue would be that I'd have to propagate that extra parameter through nested function calls in multiple classes to get it to the context where I'd actually use it. The execution context has nothing to do with the application context, which is why it feels so dirty to do that, and why it feels like it should be possible to check a property on the thread. But it sounds like I may not have a choice... :( — music man, May 23 '14 at 14:06
@musicman Well, if you don't care about breaking encapsulation, you can simply store a thread local value (for example, using a static field marked with [Threadstatic]) and write and read that as appropriate. I wouldn't recommend it, but it's the path of least friction. — Luaan, May 23 '14 at 14:11

score 0 · Answer 1 · edited May 23 '14 at 13:42

0

Does this help

using System.Diagnostics; 

StackTrace stackTrace = new StackTrace();           // Get call stack
StackFrame[] stackFrames = stackTrace.GetFrames();  // Get method calls (frames)

// write call stack method names
foreach (StackFrame stackFrame in stackFrames)
{
    Console.WriteLine(stackFrame.GetMethod().Name); // Write method name
}

edited May 23 '14 at 13:42

Mo Patel

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answered May 23 '14 at 12:42

Sathish

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Thanks for the suggestion, but unfortunately it doesn't help. It would rely on me comparing method names which I want to avoid. – music man May 23 '14 at 13:09

Detect if called from P/Invoke

1 Answers1