Embarrassment strikes twice here on the Typemock forums! In my manual updating of Typemock references to the latest version, for one project I forgot to also add a reference to TypeMock.dll, hence the errors for ObjectState not found. It's a great relief to know ObjectState is still with us! Perhaps the probability of future users experiencing a fate similar to mine could be reduced by merging TypeMock.dll into Typemock.ArrangeActAssert.dll, thereby making Isolator a one-DLL product instead of a two-DLL product.