如何选择一个LINQ GROUPBY前N行 [英] How to select top N rows in a Linq GroupBy

问题描述

希望你能帮助我在这里。
我有红黄牌，我想获得各组TourOperators的前2行的列表

hope you can help me out here. I have a list of Bookings where I would like to get the top 2 rows in each group of TourOperators.

这里的数据样本：

List<Booking> list = new List<Booking>();
        list.Add(new Booking() { BookingNo = "31111111", DepDate = new DateTime(2011, 5, 1), TourOperator = "SPI" });
        list.Add(new Booking() { BookingNo = "32222222", DepDate = new DateTime(2011, 5, 2), TourOperator = "SPI" });
        list.Add(new Booking() { BookingNo = "33333333", DepDate = new DateTime(2011, 5, 3), TourOperator = "SPI" });
        list.Add(new Booking() { BookingNo = "34444444", DepDate = new DateTime(2011, 5, 4), TourOperator = "SPI" });
        list.Add(new Booking() { BookingNo = "35555555", DepDate = new DateTime(2011, 5, 5), TourOperator = "SPI" });
        list.Add(new Booking() { BookingNo = "36666666", DepDate = new DateTime(2011, 5, 6), TourOperator = "SPI" });
        list.Add(new Booking() { BookingNo = "11111111", DepDate = new DateTime(2011, 5, 1), TourOperator = "VIN" });
        list.Add(new Booking() { BookingNo = "12222222", DepDate = new DateTime(2011, 5, 2), TourOperator = "VIN" });
        list.Add(new Booking() { BookingNo = "13333333", DepDate = new DateTime(2011, 5, 3), TourOperator = "VIN" });
        list.Add(new Booking() { BookingNo = "14444444", DepDate = new DateTime(2011, 5, 4), TourOperator = "VIN" });
        list.Add(new Booking() { BookingNo = "15555555", DepDate = new DateTime(2011, 5, 5), TourOperator = "VIN" });
        list.Add(new Booking() { BookingNo = "16666666", DepDate = new DateTime(2011, 5, 6), TourOperator = "VIN" });
        list.Add(new Booking() { BookingNo = "17777777", DepDate = new DateTime(2011, 5, 7), TourOperator = "VIN" });

我有这个LINQ的，但它只是给了我每个组中的第一个购票：

I have this Linq, but it only gives me the first Booking in each group:

List<Booking> yetAnotherList = list.GroupBy(row => row.TourOperator)
            .Select(g => g.OrderBy(row => row.DepDate).First()).ToList();

我所期待的数据是这样的：

The data I was hoping for was something like this:

31111111，2011-05-01，SPI结果
32222222，2011-05-02，SPI结果
11111111，2011-05-01，VIN结果
12222222，2011-05-02，VIN

31111111, 2011-05-01, SPI
32222222, 2011-05-02, SPI
11111111, 2011-05-01, VIN
12222222, 2011-05-02, VIN

感谢。

推荐答案

替换第一（）与取（2），并使用的SelectMany（） ：

List<Booking> yetAnotherList = 
                 list.GroupBy(row => row.TourOperator)
                     .SelectMany(g => g.OrderBy(row => row.DepDate).Take(2)) 
                     .ToList();

更新：忘记了的SelectMany 第一次。要扁平化的结果（其中的SelectMany 一样），否则你得到的名单 IEnumerables 。

Update: Forgot the SelectMany the first time. You want to flatten the result (which SelectMany does), otherwise you get a list of IEnumerables.

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