IMongoClient _client = new MongoClient(@"mongodb://...");IMongoDatabase _database = _client.GetDatabase("...");IMongoCollection<Parent> _collection = _database.GetCollection<Parent>("q35135879");var ca = new Child { Value = "A" };var cb = new Child { Value = "B" };var cc = new Child { Value = "C" };var fdb = Builders<Parent>.Filter;var filterLinq = fdb.All (x=>x.Children, new[] {ca, cb, cc});var filterFieldDefinition = fdb.All("Children", new[] { ca, cb, cc });var found1 = _collection.Find(filterLinq).ToList();var found2 = _collection.Find(filterFieldDefinition ).ToList();CollectionAssert.AreEqual(found1, found2);