Linq选择具有唯一ID和最大日期的对象？

Question

I have a list of objects that look like this:

public class A
{
    public int Id {get; set;}
    public DateTime Date {get;set;}
    public int TypeId {get; set;}
    public int Version {get; set;}
}

My data looks like this:

 Id    Date     TypeId   Version
 1     10/3/18    1        1
 2     10/3/18    1        2
 3     10/4/18    1        1
 4     10/4/18    2        1

How can I make a linq query to return these 2 in a list where it gets the item with the greatest date where the version # is 1 and also uses the TypeId to return more items?

 Id    Date     TypeId   Version
 3     10/4/18    1        1
 4     10/4/18    2        1

This is what I have tried but my code only returns one item because of my FirstOrDefault function.

        var q = from n in A.All.Where(x => x.Version == 1)
                group n by n.TypeId into g
                select g.OrderByDescending(t => t.Date).FirstOrDefault();

Answer 1

You need to group by typeId and then in each group order elements by date. Then you can pick first element in each group. Try this code:

var input = new[]
{
    new A {Id = 1, Date = new DateTime(2018, 10, 3), TypeId = 1, Version = 1},
    new A {Id = 2, Date = new DateTime(2018, 10, 3), TypeId = 1, Version = 2},
    new A {Id = 3, Date = new DateTime(2018, 10, 4), TypeId = 1, Version = 1},
    new A {Id = 4, Date = new DateTime(2018, 10, 4), TypeId = 2, Version = 1},
};

var result = input.Where(a => a.Version == 1)
    .GroupBy(a => a.TypeId)
    .Select(g => g.OrderByDescending(x => x.Date).First())
    .ToArray();