There have been some security changes in the .NET 4.0 Framework, so when moving a project to .NET 4.0 you may get the following exception: TypeLoadException – Derived Types must either match the security accessibility of the base type or be less accessible. There are two approaches to fixing this problem: 1. If you are you using the SecurityTransparentAttribute in your project’s AssemblyInfo.cs/vb file you need to remove this attribute since it’s now obsolete in the .NET 4 Framework. 2. If this isn’t the case, try adding the following attribute to your project’s AssemblyInfo.cs/vb file to get past the exception: … Continue reading Why do I get a TypeLoadException when moving my project to .NET 4?