So to set the scene I was slow time migrating an existing ASP.NET MVC 1.0 application to v 2.0 (for no other reason than I wanted to catch up with the new stuff as I haven’t been using MVC in recent projects). After using the automated migration toolwritten by Eilon Lipton ’of off’ the ASP.NET team, and sorting some minor issue,s I found that a few tests that were failing with ArgumentNullException on the ModelValidator controllerContext parameter when attempting to UpdateModel. I knew controllerContext was null because I wasn’t setting it, the problem was why was it required?

Model validation is one of the new features in v 2.0, stepping through the MVC source code (peeling through the layers of abstraction) showed that the default model binder abstract base ModelValidator requires the ControllerContext on construction. For some background I found this interesting dissection of the MVC model validation from an architectural perspective.

Clearly there are many potential solutions to this issue to get the tests green. In this case I took the somewhat simple pragmatic approach of just setting the ControllerContext on the Controller.