class C
        {
            [BsonDictionaryOptions(DictionaryRepresentation.Document)]
            public IDictionary H { get; set; } // serialized as { H : { x : 1, ... } }
        }

object someInt = 42;
var query = from c in _collection.AsQueryable<C>()
                   where c.H["x"] == someInt
                   select c;