This exception signals that an attempt to invoke AppDomain.Unload( ) failed. This indicates that you either tried to unload the default application domain (AppDomain.CurrentDomain), the domain has a thread that cannot be stopped, or the domain has already been unloaded.

public class CannotUnloadAppDomainException : SystemException {
// Public Constructors
   public CannotUnloadAppDomainException( );
   public CannotUnloadAppDomainException(string message);
   public CannotUnloadAppDomainException(string message, Exception innerException);
// Protected Constructors
   protected CannotUnloadAppDomainException(System.Runtime.Serialization.SerializationInfo info, 
        System.Runtime.Serialization.StreamingContext context);
}

Hierarchy

Object Exception(System.Runtime.Serialization.ISerializable) SystemException CannotUnloadAppDomainException