This question is about dealing with unmanaged resources (COM interop) and making sure there won't be any resource leaks. I'd appreciate feedback on whether I seem to do things the right way.

Background:

Let's say I've got two classes:

• A class LimitedComResource which is a wrapper around a COM object (received via some API). There can only be a limited number of those COM objects, therefore my class implements the IDisposable interface which will be responsible for releasing a COM object when it's no longer needed.

• Objects of another type ManagedObject are temporarily created to perform some work on a LimitedComResource. They are not IDisposable.

To summarize the above in a diagram, my classes might look like this:

            +---------------+           +--------------------+
            | ManagedObject | <>------> | LimitedComResource |
            +---------------+           +--------------------+
                                                  |
                                                  o IDisposable

(I'll provide example code for these two classes in just a moment.)

Question:

Since my temporary ManagedObject objects are not disposable, I obviously have no control over how long they'll be around. However, in the meantime I might have Disposed the LimitedComObject that a ManagedObject is referring to. How can I make sure that a ManagedObject won't access a LimitedComResource that's no longer there?

            +---------------+           +--------------------+
            | managedObject | <>------> |   (dead object)    |
            +---------------+           +--------------------+

I've currently implemented this with a mix of weak references and a flag in LimitedResource which signals whether an object has already been disposed. Is there any better way?

Example code (what I've currently got):

LimitedComResource:

class LimitedComResource : IDisposable
{
    private readonly IUnknown comObject;  // <-- set in constructor

    ...

    void Dispose(bool notFromFinalizer)
    {
        if (!this.isDisposed)
        {
            Marshal.FinalReleaseComObject(comObject);
        }
        this.isDisposed = true;
    }

    internal bool isDisposed = false;
}

ManagedObject:

class ManagedObject
{
    private readonly WeakReference limitedComResource;  // <-- set in constructor

    ...

    public void DoSomeWork()
    {
        if (!limitedComResource.IsAlive())
        {
            throw new ObjectDisposedException();
            //        ^^^^^^^^^^^^^^^^^^^^^^^
            //  is there a more suitable exception class?
        }

        var ur = (LimitedComResource)limitedComResource.Target;
        if (ur.isDisposed)
        {
            throw new ObjectDisposedException();
        }

        ...  // <-- do something sensible here!
    }
}