确定用户是否在.NET 4.0应用程序的AD组中

I am trying to determine if a user is a member of an Active Directory (AD) group for an internal ASP.NET 4.0 application. The code below throws an "Attempted to access an unloaded appdomain" exception error on the last line (return statement) in the case when the user is not a member of the AD group.I am trying to determine if a user is a member