Question

I am in the process of writing an application in C# which will open an Excel spreadsheet (2007, for now) via interop, do some magic, then close. The “magic” part is non-trivial, so this application will contain many references to many COM objects spawned by Excel.

I have written this kind of application before (too many times, in fact) but I’ve never found a comfortable, “good smell” approach to interacting with COM objects. The problem is partly that, despite significant study, I still don’t perfectly understand COM and partly that the interop wrappers hide much that probably shouldn’t be hidden. The fact that there are so many different, conflicting suggestions from the community only makes matters worse.

In case you can’t tell from the title, I’ve done my research. The title alludes to this post:
How do I properly clean up Excel interop objects?

First asked in 2008, the advice was really helpful and solid at the time (especially the “Never use 2 dots with com objects” bit) but now seems out of date. In March of 2010, the Visual Studio team posted a blog article warning fellow programmers that Marshal.ReleaseComObject [is] Considered Dangerous. The article referred to two articles, cbrumme’s WebLog > ReleaseComObject and The mapping between interface pointers and runtime callable wrappers (RCWs), suggesting that people have been using ReleaseComInterop incorrectly all along (cbrumme: “If you are a client application using a modest number of COM objects that are passed around freely in your managed code, you should not use ReleaseComObject”).

Does anyone have an example of a moderately complex application, preferably using multiple threads, that is able to successfully navigate between memory leaks (Excel continues running in the background after the application has closed) and InvalidComObjectExceptions? I’m looking for something which will allow a COM object to be used outside of the context in which it was created but can still be cleaned up once the application is finished with it: a hybrid of memory management strategies which can effectively straddle the managed/unmanaged divide.

A reference to an article or tutorial that discusses a correct approach to this problem would be a much appreciated alternative. My best Google-fu efforts have returned the apparently incorrect ReleaseComInterop approach.

---

UPDATE:
(This is not an answer)
I discovered this article not long after posting:
VSTO and COM Interop by Jake Ginnivan

I’ve been able to implement his strategy of wrapping COM objects in “AutoCleanup” classes via an extension method, and I’m pretty happy with the result. Though it does not provide a solution to allow COM objects to cross the boundaries of the context in which they were created and still makes use of the ReleaseComObject function, it does at least provide a neat and easy-to-read solution.

Here’s my implementation:

class AutoCleanup<T> : IDisposable {

    public T Resource {
        get;
        private set;
    }

    public AutoCleanup( T resource ) {
        this.Resource = resource;
    }

    ~AutoCleanup() {
        this.Dispose();
    }

    private bool _disposed = false;
    public void Dispose() {
        if ( !_disposed ) {
            _disposed = true;
            if ( this.Resource != null && 
                 Marshal.IsComObject( this.Resource ) ) {
                Marshal.FinalReleaseComObject( this.Resource );
            } else if ( this.Resource is IDisposable ) {
                ( (IDisposable) this.Resource ).Dispose();
            }
            this.Resource = null;
        }
    }

}

static class ExtensionMethods {

    public static AutoCleanup<T> WithComCleanup<T>( this T target ) {
        return new AutoCleanup<T>( target );
    }

}

Answer 1

did you now the NetOffice concept for COM proxy management?
NetOffice use wrapper classes for com proxies and the IDisposable pattern.
NetOffice keep the parent->child relationship for proxies. dispose a worksheet and all created childs from the instance(cells, styles), etc. was also disposed. you can also use a special event or static property to observe the count of open proxies in your application.

just take a look in this documentation snippet:
http://netoffice.codeplex.com/wikipage?title=Tec_Documentation_English_Management

you find some showstopper projects for com proxy management in the tutorials folder