If I were interviewing someone who knew LINQ, I'd probably just ask them to explain it to me. They should be able to describe deferred execution, streaming, the IEnumerable/IEnumerator interfaces, foreach, iterator blocks, and expression trees (at least for extra points). (I'm thinking of the instance where they've claimed to know enough LINQ to make it a legitimate inquiry - admittedly, they could be "decent" developers and not "understand" LINQ yet.)
I've previously asked several of the questions listed above, as well as a few others:
- Difference between reference and value types
- Delegates
- Strings, immutability, character encodings
- Pass by reference vs pass by value
- IDisposable and finalizers
- Nullable types
- Floating point
- Generics