Where can I find more detail on disposal of COM resources in .NET?

Question

The background for my question is this: I am converting a message processing app that uses many COM components from VB6 to C#. Many of the COM components in the application are fine-grained components that are used in high numbers and frequency within message processing loops. I am seeing a massive (and progressively growing) increase in memory usage when processing a set of test messages in the C# app as compared to the VB6 app. I used a memory profiler on the application that confirmed that the high memory usage was due to live instances of COM objects on the application's unmanaged heap. I know that these components are not being "leaked" due to live references because if I put a GC.Collect() at the core of the message processing loop, the memory usage is flat and nearly identical to the VB6 app (although the performance degrades horribly as one would expect).

I have read everything I can find on the multi-generation garbage collector in C#, runtime callable wrappers, unmanaged resource memory allocation, Marshal.ReleaseComObject(), Marshal.FinalReleaseComObject(), etc. None of it explains why the application is holding onto live COM objects in the unmanaged heap when the corresponding RCWs are eligible for garbage collection.

In some articles, I have seen allusions to the possibility that the actual implementation of the garbage collector in C# may involve optimizations such as not performing collection of all eligible objects in a particular generation. If this or something like it were true, it could explain why eligible RCWs and their corresponding COM objects are not collected and destroyed. Another explanation could be if the destruction of a COM object in the unmanaged heap is not directly tied to the collection of its corresponding RCW. I have not found anything that offers this degree of detail on how COM objects are handled in .NET. I need to understand this better because my app's memory usage is currently unacceptable. Any pointers or recommendations would be greatly appreciated.

Edit: I should add that I am quite familiar with finalizers (which are not documented to exist on RCWs) and the IDisposable interface (which RCWs do not implement). To the best of my understanding, Marshal.ReleaseComObject() is the proper method of explicitly "disposing" of a COM reference. I was careful to add that statement for every known usage of a COM object in my app and it resulted in no difference in memory usage.

Further, it is not clear why a lack of disposal or finalization code could be the problem when the addition of an explicit GC.Collect() results in no memory problems. Neither Dispose() nor the presence of a finalizer result in the actual collection of objects. The former permits an object to suppress its finalization step (if any) and the latter allows for the cleanup of unmanaged resources of which none are exposed in an RCW.

Answer 1

Are you implementing IDisposable properly in the classes that instantiate your COM objects?

You need to implement IDisposable and dispose of your COM RCW's in the Dispose() method. Then all code that instantiates classes that implement IDisposable should call it either explicitly or by using a using() statement, like so:

var first = new DisposableObject();
...
first.Dispose();

and

using(var first = new DisposableObject())
{
   ...
}

IDisposable is the only way to get the CLR to dispose of these objects in a timely manner and to make sure you lose COM references.

Answer 2

Use a Finalizer or destructor to clean up the memory used by the COM objects.

http://msdn.microsoft.com/en-us/library/66x5fx1b.aspx

Alternatively, if you want the objects to clean up immediately, you can implement IDispose, and use a using statement in your code that instantiates the COM object.