c# - 将 EF LINQ 方法语法转换为查询语法

Question

您将如何使用 LINQ 查询语法编写此完全相同查询？

var q2 = list.GroupBy(x => x.GroupId)
             .Select(g => g
                 .OrderByDescending(x => x.Date)
                 .FirstOrDefault());

我认为这应该有效，但没有:

var q1 = from x in list
         group x by x.GroupId into g
         from y in g
         orderby y.Date descending
         select g.FirstOrDefault();

这里有一个测试程序，如果你想玩的话:

public class MyClass
{
    public int Id { get; set; }
    public string GroupId { get; set; }
    public DateTime Date { get; set; }

    public override bool Equals(object obj)
    {
        var item = obj as MyClass;
        return item == null ? false : item.Id == this.Id;
    }
}

static void Main(string[] args)
{
    var list = new List<MyClass> 
    {
        new MyClass { GroupId = "100", Date=DateTime.Parse("01/01/2000"), Id = 1},
        new MyClass { GroupId = "100", Date=DateTime.Parse("02/01/2000"), Id = 2},
        new MyClass { GroupId = "200", Date=DateTime.Parse("01/01/2000"), Id = 3},
        new MyClass { GroupId = "200", Date=DateTime.Parse("02/01/2000"), Id = 4},
        new MyClass { GroupId = "300", Date=DateTime.Parse("01/01/2000"), Id = 5},
        new MyClass { GroupId = "300", Date=DateTime.Parse("02/01/2000"), Id = 6},
    };

    var q1 = from x in list
                group x by x.GroupId into g
                from y in g
                orderby y.Date descending
                select g.FirstOrDefault();

    var q2 = list.GroupBy(x => x.GroupId)
                    .Select(g => g
                        .OrderByDescending(x => x.Date)
                        .FirstOrDefault());

    Debug.Assert(q1.SequenceEqual(q2));
}

Answer 1

Reflector 给我以下信息:

var q2 = from x in list
         group x by x.GroupId into g
         select (
             from x in g
             orderby x.Date descending
             select x).FirstOrDefault();

这看起来是正确的。

(正如 Lazy 所说，不需要 FirstOrDefault 代替 First，因为组不会它不是空的，但它不会伤害。)