If you are upgrading an application from .NET 3.5 to .NET 4.0 and all of a sudden you get this error message:
System.Security.SecurityException: Request failed.
[SecurityException: Request failed.]System.RuntimeMethodHandle.PerformSecurityCheck(Object obj, RuntimeMethodHandleInternal …
Then you might wanna check if you are using an xslt files as a TransformFile in an XmlDataSource.
Currently, I haven’t found any other solution that re-implementing those features without using xslt.
Hope this helps anyone who is struggling with this issue.