将自定义字段类型部署到bin目录

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. custom field types (and any dlls