You should not use the AllowPartiallyTrustedCallersAttribute (APTCA) assemblies when developing in Microsoft® Visual Studio®. When working in Visual Studio, you should continue to run the installer that accompanies the release assemblies (non-APTCA) as you always have. The installer places the release assemblies into your development machines Global Assembly Cache (GAC), ensuring that they run with full trust. The enhanced design-time experience provided by our designers in Visual Studio currently depend on having full trust.
Medium trust provides a security sandbox intended to aid systems administrators securely manage the resources of a Web server that can potentially be shared by multiple users. When you are designing your application on your own development machine, there is no analagous benefit to be gained b running Visual Studio under partial trust.