Question

For some time now, I’ve observed an intermittent COM problem in my VSIX package for Visual Studio 2010. Attempting to subscribe to one of the IDE’s COM-based event sinks randomly throws the following error:

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

A repro case boils down to this code (which must be used in VSIX, obviously):

using System;
using EnvDTE;
using EnvDTE80;

class Test
{
    private readonly Events _events;
    private readonly Events2 _events2;
    private readonly BuildEvents _buildEvents;
    private readonly ProjectItemsEvents _projectItemsEvents;

    public Test(IServiceProvider provider)
    {
        var dte = (DTE)provider.GetService(typeof(DTE));
        var dte2 = (DTE2)dte;

        // Store all references in fields as a GC precaution.
        _events = dte.Events;
        _events2 = (Events2)dte2.Events;
        _buildEvents = _events.BuildEvents;
        _projectItemsEvents = _events2.ProjectItemsEvents;

        // Proceed to subscribe to event sinks.
        _buildEvents.OnBuildBegin += BuildBeginHandler; // BOOM!
        _projectItemsEvents.ItemAdded += ItemAddedHandler;
    }

    private void ItemAddedHandler(ProjectItem projectItem) { }

    private void BuildBeginHandler(vsBuildScope scope, vsBuildAction action) { }
}

I’ve learned about a possible cause from numerous descriptions of similar problems that can be found on the net. It’s basically a side effect of the way Runtime Callable Wrappers and GC interact during COM interop. Here’s a similar problem complete with explanation.

I’m fine with that explanation, especially because it suggests an easy workaround – storing the event sink reference in a field in order to prevent it from being prematurely GC’ed. Indeed, many people seem to have solved their problem this way.

What bothers me is that it doesn’t work in my case. I’m really stumped as to why. As you can plainly see, I already store all object references in fields as a precaution. Yet the error still occurs. I tried being even more explicit using GC.KeepAlive() calls at the end of the ctor, but to no avail. Is there anything else left to do?

Without a solution, my VSIX randomly fails to load, leaving the user with a single option: to restart Visual Studio and hope it doesn’t happen the next time.

Answer 1

Well, I gave up and simply did the only thing that crossed my mind. I figured that since this is obviously a race condition I can’t affect in a predictable manner, I might as well reenter the race if I lose.

So I moved the subscription lines into a while loop that try..catch-es them and retries after a bit of Thread.Sleep(). The loop exits either when both subscriptions succeed or when I’ve been continuously losing the race for more than 2 seconds.

The kicker is, I haven’t lost the race once since I’ve implemented the change. A true Heisenbug, if I ever saw one.

Anyway, I’m going to stick with this until a proper solution occurs to me or the bug reappears.