c# - LINQ 通过 2 个属性和创建日期来区分

Question

我有一个包含 Person 模型的列表，如下所示:

public class Person
{
        [Key]
        public int Id { get; set; }      
        public int Rating1 { get; set; }
        public int Rating2 { get; set; }
        public DateTime CreateDate { get; set; }
}

Rating1 永远不等于Rating2。

问题:

如何获取 Rating1 或 Rating2 等于给定评级值的列表，以及该列表中最后创建的元素？

我通过示例解释我的观点。假设这里是列表:

Rating1    Rating2    CreateDate
4          8          2013-08-15 05:12:00
9          4          2013-08-15 07:12:00
8          4          2013-08-15 08:12:00
5          20         2013-08-15 09:12:00
20         4          2013-08-15 10:12:00
20         5          2013-08-15 11:12:00
4          9          2013-08-15 12:12:00

假设我将评级值“4”作为参数发送到此方法

public IEnumerable<Person> GetList1(int r) {}

并获取列表:

8          4          2013-08-15 08:12:00 //because there are many [4 and 8] couples, but this created last.
4          9          2013-08-15 12:12:00 //because there are many [4 and 9] couples, but this created last.
20         4          2013-08-15 10:12:00 //because there is one [4 and 20] couple, this also  created last.

如果我将评级值“20”作为参数发送给 GetList1() 方法，我想获取一个列表:

 20         5          2013-08-15 11:12:00 //because there are many [20 and 5] couples, but this created last.
 20         4          2013-08-15 10:12:00 //because there is one [20 and 4] couple, this also  created last.

Answer 1

应该是:

int r = 4; // your rating

var res = from p in lst
          where p.Rating1 == r || p.Rating2 == r
          group p by new { Min = Math.Min(p.Rating1, p.Rating2), Max = Math.Max(p.Rating1, p.Rating2) };

var res2 = res.Select(p => p.OrderByDescending(q => q.CreateDate).First());

我使用 Math.Min 和 Math.Max 将最低评分设为第一个，将最大评分设为第二个。因此 4, 8 相当于 8, 4

或者，(或多或少等效):

var res = from p in lst
    where p.Rating1 == r || p.Rating2 == r
    group p by new { Min = Math.Min(p.Rating1, p.Rating2), Max = Math.Max(p.Rating1, p.Rating2) } into q
    select q.OrderByDescending(s => s.CreateDate).First();

或纯 LINQ(但可读性较差)

var res = from p in lst
    where p.Rating1 == r || p.Rating2 == r
    group p by new { Min = Math.Min(p.Rating1, p.Rating2), Max = Math.Max(p.Rating1, p.Rating2) } into q
    select (from s in q orderby s.CreateDate descending select s).First();