We had some interesting bugs recently in VSTO 2.0 that basically involve letting the product sit for a while--like over lunch break--without doing anything.  Testers and devs hate this kind of a bug: Do X, do Y, wait 5-15 minutes, do Z. But if you are doing anything across appdomains--for example, you have an object that derives from MarshalByRefObject that you are passing from one appdomain to another--you should do the same.  Let your product sit over your lunch break and make sure it still works when you come back.

Turned out that this bug was related to our remoting leases expiring.  Thomas Quinn, a fantastic architect at large on the VSTO 2.0 team, fixed this bug and I quote from his checkin:

“These bugs both had to do with Remoting "leases" expiring. As you know, in .NET there is no refcounting. In the same AppDomain, object references are tracked by the garbage collector and they are marked for deletion when there are no more active references. When you pass an object across an AppDomain boundary, though, GC can't keep track of it any more -- GC is done on a per domain basis. When you pass an object by reference across the domain boundary (in order to do this it must derive from MarshalByRefObject) the Remoting infrastructure takes control. The lifetime is then controlled by a "lease." As long as there is an active lease on the object it stays alive. Each cross domain call renews the lease. By default, the lease is 5 minutes -- meaning that if you don't talk to an object passed to you across the domain boundary for 5 minutes or more it will get cleaned up, and subsequent calls to the proxy you are holding will throw an remoting exception. Anytime you derive an object from MarshalByRefObject you must take the lifetime of the object into account. There are several ways to override the default behavior, involving custom lease implementations or implementing sponsors for the objects that will keep them alive. If interested, you can read more about it here: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpconlifetimeleases.asp

The override used in this fix is the simplest one -- we give the MBRO objects an infinite lease (meaning they stay alive as long as the AppDomain is alive). This is done by simply overriding MarshalByRefObject.InitializeLifetimeService to return null. In this case, this is exactly what we want, since the objects are part of the runtime AppDomain manager that lives with the customization in its domain.

I changed all of the classes in the [VSTO 2.0 runtime] directory that derive from MBRO. These changes fixed the bugs in question. It is possible that there are other classes that derive from MBRO more indirectly and may exhibit the same problem through some other code path. Beware of MBRO.“

Of course giving your objects infinite lease may be the wrong fix given your application.  But that goes without saying right :)