I think I'm beginning to get the hang of this whole lambda thing. I think this will actually work! Well, it compiles at any rate:
List<RegistrationBatch> rbl = rbl.Where(x => x.Registrations.Where (y => y.RegistrationTypeUsed.MyClass == registrationType.RegClass.regular).Count() > 0).ToList();
It gives me all the registration batches which contain "regular" registrations. Yeah, I can do this in a heartbeat with a sql string, but that tends to net me unexpected behavior that doesn't show up until runtime, and won't let me use all my handy enums besides.
Wrong Wrong Wrong! comments in 3... 2... 1...