I was doing some research on how to use a certain component we were given as part of a project. It is a COM object written with ATL; I referenced it via interop and everything seemed to work perfectly. Until we integrated it into the main codebase, that is. For some reason the same code would barf on any call made to a method/property of a legacy ADO Recordset object; an instance of Recordset is passed by reference to the COM object, which initializes it to certain values. For some reason any call to the Recordset instance after the COM method call would result in a NullReferenceException being thrown by the framework.

Oddly enough, tests on the research codebase (on my own machine) now proved to generate the exact same error; considering I had written and tested the code merely two days before, I was disinclined to believe the code to be at fault. Something told me to look into the interop assemblies - we sign every COM/ActiveX import in the project with our own keyfile using a script which runs tlbimp/aximp - and reverting to the VS-generated interop assemblies did indeed resolve the issue. I couldn't find any solution using Google (a Groups search provided a similar issue with no solution offered). Finally I stumbled upon the following quote in this article:

But Primary Interop Assemblies have another important use. Interop Assemblies often need modifications, such as the ones shown in Chapter 7 to be completely usable in managed code. When you have a PIA with such customizations registered on your computer, you can benefit from these customizations simply by referencing the type library for the COM component you wish to use inside Visual Studio .NET. For example, the PIA for Microsoft ActiveX Data Objects (ADO), which ships with Visual Studio .NET, contains some customizations to handle object lifetime issues. If you created your own Interop Assembly for ADO using TLBIMP.EXE, you would not benefit from these customizations.

Since the ADO COM object was automatically imported along with our proprietary objects this got me wondering what sort of "custom optimizations" I might be missing out on. A quick look in the knowledgebase article on the ADO PIA didn't prove very effective (short of a vague statement about "the ADO PIA helps to avoid certain problems with ADO and .NET COM interoperability") but I decided to try it out anyway; I removed the preexisting reference from the project, added "adodb" from the .NET tab in the Add References dialogue (you could look it up manually in the GAC, but why would you?), fired it up - problem solved.

As an anecdote, referencing an external library with tlbimp's /reference command line parameter (particularly the ADODB PIA from the GAC) did not stop it from generating the imported library anyway. Just go ahead and delete it.