mongodb/mongo-csharp-driver — comment on commit b69dc19b15c11cd3fc0d47c0605ad6b32d9cc7e1
File: Driver/Linq/Translators/SelectQuery.cs line 293
By craiggwilson:

This is interesting because the single and first methods may actually have criteria in them. As in, this is legal: _collection.Where(x => x.Name == "Bob").First(x => x.LastName == "McBob"). This should be turned into a mongo query as follows: {Name: "Bob", LastName: "McBob"}.