BG100 BG100 - 3 months ago 9
C# Question

How do I order the elements in a group by linq query, and pick the first?

I have a set of data that contains a type, a date, and a value.

I want to group by the type, and for each set of values in each group I want to pick the one with the newest date.

Here is some code that works and gives the correct result, but I want to do it all in one linq query rather than in the iteration. Any ideas how I can achieve the same result as this with purely a linq query...?

using System;
using System.Linq;
using System.Collections.Generic;

public class Program {

public static void Main() {

var mydata = new List<Item> {
new Item { Type = "A", Date = DateTime.Parse("2016/08/11"), Value = 1 },
new Item { Type = "A", Date = DateTime.Parse("2016/08/12"), Value = 2 },
new Item { Type = "B", Date = DateTime.Parse("2016/08/20"), Value = 3 },
new Item { Type = "A", Date = DateTime.Parse("2016/08/09"), Value = 4 },
new Item { Type = "A", Date = DateTime.Parse("2016/08/08"), Value = 5 },
new Item { Type = "C", Date = DateTime.Parse("2016/08/17"), Value = 6 },
new Item { Type = "B", Date = DateTime.Parse("2016/08/30"), Value = 7 },
new Item { Type = "B", Date = DateTime.Parse("2016/08/18"), Value = 8 },
};

var data = mydata.GroupBy(_ => _.Type);

foreach (var thing in data) {

#region

// How can I remove this section and make it part of the group by query above... ?
var subset = thing.OrderByDescending(_ => _.Date);
var top = subset.First();

#endregion

Console.WriteLine($"{thing.Key} {top.Date.ToString("yyyy-MM-dd")} {top.Value}");

}
}

public class Item {

public string Type {get;set;}
public DateTime Date {get;set;}
public int Value {get;set;}

}
}

// Output:
// A 2016-08-12 2
// B 2016-08-30 7
// C 2016-08-17 6

Answer

Use select to get the FirstOrDefault (or First - because of the grouping you won't get a null) ordered descending:

var data = mydata.GroupBy(item => item.Type)
                 .Select(group => group.OrderByDescending(x => x.Date)
                                       .FirstOrDetault())
                 .ToList();

Or SelectMany together with Take(1)

var data = mydata.GroupBy(item => item.Type)
                 .SelectMany(group => group.OrderByDescending(x => x.Date)
                                           .Take(1))
                 .ToList();

I recommend checking SO documentation about linq to learn more about playing with the different options