Accessing the C++ COM dll from Webservice

Question

I am using a COM dll from a web service. The COM dll is added as reference. And I am declaring the object as static in Global.asax. I am creating the COM object in the Application_Start.

I have to call the COM dll interface function in each request. I am getting exceptions here as memory corruption.I could see the logs that it happens when simultaneous requests come up. Please let me know what is the best way to do that. How to make it thread safe.?

Answer 1

Try creating a new instance in each request and not use application scope for the object.

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If you are accessing it at application scope(eg through Application_Start) you will need to make sure it is safe for multithreading. I don't know how C++ dlls handle threading but you might be able to manage multithreading at the asp.net level.

For example To manage a simple application level counter the code is something like:

Application.Lock();
Application["SomeGlobalCounter"] =
   (int)Application["SomeGlobalCounter"] + 1;
Application.UnLock();

For more information you might want to see the MSDN page on Application State .

Answer 2

If the COM object is apartment threaded, COM provides the synchronization to enforce a single execution of a method per thread.

Generally, though, COM should be complaining of multiple threads trying to access an instance of an object using the same pointer shared across threads. Having a static variable holding a pointer to the object is probably a bad idea.

Once the COM object shared library is loaded somewhere (in-proc or out-of-proc) by creating an instance, creation of additional instances per thread should be fairly quick. That is, of course, dependent on what types of things that are being done during object construction.