Question

I have some COM component which I call from some c# dll.

I also have a winforms app that uses that .dll.

When I close the app I get this exception:

COM object that has been separated
from its underlying RCW cannot be
used.

The stack trace shows this exception comes from a destructor in the .dll. I implemented this destructor to call some cleanup method in the COM.

Why does this happen? How is it best to solve it?

Answer 1

The issue is described here:

Is it safe to call an RCW from a finalizer?

and here:

Release Excel Object In My Destructor

The trouble is that not only is the timing as to when these objects
are to be garbage collected uncertain, but the order in which the
finalizers are called is also nondeterministic. In this case, a
Runtime Callable Wrapper also has a finalizer, which calls
Marshal.FinalReleaseComObject on itself, which has the result of
decrementing the reference count on the COM side of the fence so that
this COM object can be released. But since the order in which the
finalizers are called is uncertain, it is very possible that the
finalizers for the COM objects that your object references will fire
before the finalizer for your object. So the code within your
finalizer could work sometimes, but, most of the time, one or more of
the Runtime Callable Wrappers that your object references will have
already had their finalizers called and the underlying COM object will
have been released before your finalizer gets to execute its code.