从带有 Distinct/GroupBy 和排序的 IEnumerable 中选择 — 可能吗？

Question

假设你有这个：

class LogEntry
{
    int ID;
    int UserName;
    datetime TimeStamp;
    string Details;
}

你已经提取了一组这样的数据：

ID  Username   Timestamp   Details
1   foo        1/01/2010   Account created
2   zip        2/02/2010   Account created
3   bar        2/02/2010   Account created
4   sandwich   3/03/2010   Account created
5   bar        5/05/2010   Stole food
6   foo        5/05/2010   Can't find food
7   sandwich   8/08/2010   Donated food
8   sandwich   9/09/2010   Ate more food
9   foo        9/09/2010   Ate food
10  bar        11/11/2010  Can't find food

我想要做的是只为每个用户（即 GroupBy 用户名）选择最后一条记录（即按时间戳降序排序）。我可以理解 Distinct 和 GroupBy，但是将它们组合在一个语句中，该语句还返回非不同/分组字段/属性并按时间戳排序，这让我很头疼。

上面的例子应该是：

ID  Username   Timestamp   Details
2   zip        2/02/2010   Account created
8   sandwich   9/09/2010   Ate more food
9   foo        9/09/2010   Ate food
10  bar        11/11/2010  Can't find food

当我确信可以在单个 LINQ 语句中完成时，我不想“作弊”并采用冗长的方法。

Answer 1

希望我的 Linq-fu 是对的：=)

var results = sourceList
    .OrderByDescending(item => item.Timestamp)
    .GroupBy(item => item.Username)
    .Select(grp => grp.First())
    .ToArray();

此示例代码使用您的数据并按 ID 进行最终排序，给出的输出与您的示例完全相同：（如果您不介意粗略的格式！）

class Program
{
    static void Main(string[] args)
    {
        var sourceItems = new[] {
            new LogEntry {ID=1   ,UserName="foo      ", TimeStamp= new DateTime(2010 ,1,01),Details="Account created ",}    ,
            new LogEntry {ID=2   ,UserName="zip      ", TimeStamp= new DateTime(2010 ,2,02),Details="Account created ",}    ,
            new LogEntry {ID=3   ,UserName="bar      ", TimeStamp= new DateTime(2010 ,2,02),Details="Account created ",}    ,
            new LogEntry {ID=4   ,UserName="sandwich ", TimeStamp= new DateTime(2010 ,3,03),Details="Account created ",}    ,
            new LogEntry {ID=5   ,UserName="bar      ", TimeStamp= new DateTime(2010 ,5,05),Details="Stole food      ",}    ,
            new LogEntry {ID=6   ,UserName="foo      ", TimeStamp= new DateTime(2010 ,5,05),Details="Can't find food ",}    ,
            new LogEntry {ID=7   ,UserName="sandwich ", TimeStamp= new DateTime(2010 ,8,08),Details="Donated food    ",}    ,
            new LogEntry {ID=8   ,UserName="sandwich ", TimeStamp= new DateTime(2010 ,9,09),Details="Ate more food   ",}    ,
            new LogEntry {ID=9   ,UserName="foo      ", TimeStamp= new DateTime(2010 ,9,09),Details="Ate food        ",}    ,
            new LogEntry {ID=10  ,UserName="bar      ", TimeStamp= new DateTime(2010,11,11),Details="Can't find food ",}    ,
        };

        var results = sourceItems
            .OrderByDescending(item => item.TimeStamp)
            .GroupBy(item => item.UserName)
            .Select(grp => grp.First())
            .OrderBy(item=> item.ID)
            .ToArray();

        foreach (var item in results)
        {
            Console.WriteLine("{0} {1} {2} {3}",
                item.ID, item.UserName, item.TimeStamp, item.Details);
        }
        Console.ReadKey();
    }
}


public class LogEntry
{
    public int ID;
    public string UserName;
    public DateTime TimeStamp;
    public string Details;
}