Why do I get TypeLoadException when moving to .NET 4?

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 /vbfile 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:

   [assembly: System.Security.SecurityRules(System.Security.SecurityRuleSet.Level1)]

   For more information, check out this MSDN topic: http://msdn.microsoft.com/en-us/library/dd233102(VS.100).aspx