lambda 表达式中的 GroupBy

Question

from x in myCollection
    group x by x.Id into y
    select new { 
       Id = y.Key, 
       Quantity = y.Sum(x => x.Quantity)
    };

你会如何将上面的内容写成 lambda 表达式？我陷入了 group into 部分。

Answer 1

查询延续（select...into 和group...into，但不是 join...into）相当于仅拆分查询表达式。因此，我喜欢将您的示例视为：

var tmp = from x in myCollection
          group x by x.Id;
var result = from y in tmp
             select new { 
               Id = y.Key, 
               Quantity = y.Sum(x => x.Quantity)
             };

将它们更改为点表示法：

var tmp = myCollection.GroupBy(x => x.Id);
var result = tmp.Select(y => new { 
               Id = y.Key, 
               Quantity = y.Sum(x => x.Quantity)
             });

然后您可以将它们组合回来：

var tmp = myCollection.GroupBy(x => x.Id)
                      .Select(y => new { 
                                Id = y.Key, 
                                Quantity = y.Sum(x => x.Quantity)
                              });

一旦您弄清楚 C# 编译器对查询表达式的作用，其余的就相对简单了:)

Answer 2

myCollection
    .GroupBy(x => x.Id)
    .Select(x => 
        new 
        { 
          Id = x.Key, 
          Quantity = x.Sum(y => x.Quantity
        });

Answer 3

myCollection.GroupBy(x => x.Id)
            .Select(y => new {
                                 Id = y.Key,
                                 Quantity = y.Sum(x => x.Quantity)
                             });

Answer 4

        var mostFrequent =
            lstIn.Where(i => !string.IsNullOrEmpty(i))
                 .GroupBy(s => s)
                 .OrderByDescending(g => g.Count())
                 .Select(s => s.Key)
                 .FirstOrDefault();

Answer 5

因此，对于这里的大多数答案，每个人似乎都在处理从组的计数中获取一个简单的 Id 对象，以及 Key 本身，即 group.Key。

虽然这可能是它的主要用途。并没有真正满足我的需求。

对于我自己的情况，我基本上想按某些对象属性进行分组，然后从该组中获取特定对象。这是示例代码。

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

public class Program
{
    public static void Main()
    {
        Console.WriteLine("Hello World");
        var response = new List<ResponseClass>();
            var listOfStudents = new List<Student>();
            // Insert some objects into listOfStudents object.
            listOfStudents.GroupBy(g => g.Class).ToList()
                .ForEach(g => response.Add(g.OrderByDescending(s => s.CreatedOn).Select(a =>
                new ResponseClass
                {
                    SName = a.StudentName,
                    SAge = a.Age,
                    SClass = a.Class,
                    SCreatedOn = a.CreatedOn,
                    RandomProperty = Guid.NewGuid().ToString()
                })
                .First()));

                Console.WriteLine("This compiles and should work just fine");
    }
    class Student
    {
        public string StudentName { get; set; }
        public int Age { get; set; }
        public string Class { get; set; }
        public DateTime CreatedOn { get; set; }
    }

    class ResponseClass
    {
        public string SName { get; set; }
        public int SAge { get; set; }
        public string SClass { get; set; }
        public DateTime SCreatedOn { get; set; }
        public string RandomProperty { get; set; }
    }
}

如果你更愿意使用 foreach 循环（我更喜欢 lambda，因为我发现它更容易阅读），但如果你这样做，你可以这样做。

foreach (IGrouping<string, Student> groupedStudents in listOfStudents.GroupBy(g => g.Class))
            {
                response.Add(groupedStudents.OrderByDescending(x => x.CreatedOn).Select(a =>
                new ResponseClass
                {
                    SName = a.StudentName,
                    SAge = a.Age,
                    SClass = a.Class,
                    SCreatedOn = a.CreatedOn,
                    RandomProperty = Guid.NewGuid().ToString()
                }).First());
            }

希望这对某人有帮助。 :)