In a previous post I discussed some issues I’ve been having with the ASP.NET Development Server (aka VS Host or Cassini).

I have more information direct from Microsoft on my issue. It turns out that it is “by design”, and a sad thing this is… VS Host is designed such that the thread on which your code runs can (and does) go between AppDomains.

Objects placed on the Thread object, such as the CurrentPrincipal, must be serializable and the assembly must be available to all AppDomains; even the primary AppDomain that isn’t running as part of your web site!

And this is the root of my problem. I create a custom IPrincipal object in an assembly (dll). I put it in the Bin directory and then use it – which of course means it ends up on the Thread object. Cassini then runs my code in an AppDomain for my web site and all is well until it switches out into another AppDomain that isn’t running my web site (but rather is just running Cassini itself). Boom!

Why boom? Well, that custom IPrincipal object on the thread is still on the thread. When the thread switches to the other AppDomain, objects directly attached to the thread (like CurrentPrincipal) are automatically serialized, the byte stream transferred to the new AppDomain, and deserialized into the new AppDomain. This means that the new AppDomain must have access to the assembly containing the custom IPrincipal class – but of course it doesn’t, because it isn’t running as part of the web site and thus doesn’t have access to the Bin directory.

What’s the answer? Either don’t use Cassini (which has been my answer), or install the assembly with the custom IPrincipal into the GAC. Technically the latter answer is the “right” one, but that has the ugly side-effect of preventing rapid application development. All of a sudden you can’t just change a bit of code and press F5 to test; instead you must build your code, update the GAC and then you can test. Nasty…

As an aside, this is exactly the same issue you’ll run into when using nunit to test code that uses a custom IPrincipal on the thread. Unlike nunit however, you can’t predict when Cassini will switch you to another AppDomain so you can’t work around the issue by clearing the CurrentPrincipal like you can with nunit (or at least I haven’t found the magic point at which to do it…).

What’s really scary is that it was implied that this could happen under IIS as well – but that flies in the face of years of experiential evidence to the contrary. I guess the safe thing to do is to treat IIS like Cassini, and put shared assemblies in the GAC. But I’m not sure I’m ready to advocate that yet, because that means complicating installs a whole lot, and I’ve never encountered this threading issue under IIS so I don’t think it is a real issue.