I was attempting to write my first controller test using Castle.MonoRail.TestSupport class but was getting a System.TypeLoadException when I ran the unit test. So with some wasted hours trying to figure out what I was doing wrong I finally stumbled across a Castle forum topic that said the problem was caused by a “conflict of the trunk TestSupport’s assembly with the one in the GAC”. Removing the Castle.MonoRail.TestSupport dll from the GAC fixed it more me.