As stated here custom field types (and any dlls they have a reference to?) must be deployed to the global assembly cache. Why, and what problems may arise by deploying to the bin directory? I’m trying to secure my code by writing CAS-policies and since my field controls uses a lot of help methods shared by my other code this seems like a show stopper.
So far the only problem I’ve found with a bin deployment is that when the feature is deactivated and then reactivated with STSADM I get an error message stating that my field type is not installed properly. Using the –force switch or activating through the GUI seems to be working just fine.
I forgot to put one of the interfaces my field type implemented in the GAC, which caused the problem I described above.
Conclusion: Custom field types can reference dll:s in the bin directory, as long as they are not required to create the object.