在 LINQ 中分组

绝对 - 你基本上想要：

var results = from p in persons
              group p.car by p.PersonId into g
              select new { PersonId = g.Key, Cars = g.ToList() };

或作为非查询表达式：

var results = persons.GroupBy(
    p => p.PersonId, 
    p => p.car,
    (key, g) => new { PersonId = key, Cars = g.ToList() });

基本上，组的内容（当被视为 IEnumerable<T> 时）是给定键存在的投影（在本例中为 p.car）中的任何值的序列。

有关 GroupBy 工作原理的更多信息，请参阅我的 Edulinq post on the topic。

（我在上面将 PersonID 重命名为 PersonId，以跟随 .NET naming conventions。）

或者，您可以使用 Lookup：

var carsByPersonId = persons.ToLookup(p => p.PersonId, p => p.car);

然后，您可以很容易地为每个人获得汽车：

// This will be an empty sequence for any personId not in the lookup
var carsForPerson = carsByPersonId[personId];

---

var results = from p in persons
              group p by p.PersonID into g
              select new { PersonID = g.Key,
                           /**/car = g.Select(g=>g.car).FirstOrDefault()/**/}

---

你也可以试试这个：

var results= persons.GroupBy(n => new { n.PersonId, n.car})
                .Select(g => new {
                               g.Key.PersonId,
                               g.Key.car)}).ToList();

---

var results = from p in persons
              group p by p.PersonID into g
              select new { PersonID = g.Key, Cars = g.Select(m => m.car) };

---

尝试

persons.GroupBy(x => x.PersonId).Select(x => x)

或者

检查是否有人在您的列表中重复尝试

persons.GroupBy(x => x.PersonId).Where(x => x.Count() > 1).Any(x => x)

---

我已经使用查询语法和方法语法创建了一个工作代码示例。我希望它可以帮助其他人:)

You can also run the code on .Net Fiddle here:

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

class Person
{ 
    public int PersonId; 
    public string car  ; 
}

class Result
{ 
   public int PersonId;
   public List<string> Cars; 
}

public class Program
{
    public static void Main()
    {
        List<Person> persons = new List<Person>()
        {
            new Person { PersonId = 1, car = "Ferrari" },
            new Person { PersonId = 1, car = "BMW" },
            new Person { PersonId = 2, car = "Audi"}
        };

        //With Query Syntax

        List<Result> results1 = (
            from p in persons
            group p by p.PersonId into g
            select new Result()
                {
                    PersonId = g.Key, 
                    Cars = g.Select(c => c.car).ToList()
                }
            ).ToList();

        foreach (Result item in results1)
        {
            Console.WriteLine(item.PersonId);
            foreach(string car in item.Cars)
            {
                Console.WriteLine(car);
            }
        }

        Console.WriteLine("-----------");

        //Method Syntax

        List<Result> results2 = persons
            .GroupBy(p => p.PersonId, 
                     (k, c) => new Result()
                             {
                                 PersonId = k,
                                 Cars = c.Select(cs => cs.car).ToList()
                             }
                    ).ToList();

        foreach (Result item in results2)
        {
            Console.WriteLine(item.PersonId);
            foreach(string car in item.Cars)
            {
                Console.WriteLine(car);
            }
        }
    }
}

结果如下：

1
Ferrari
BMW
2
Audi
-----------
1
Ferrari
BMW
2
Audi

---

首先，设置您的关键字段。然后包括您的其他字段：

var results = 
    persons
    .GroupBy(n => n.PersonId)
    .Select(r => new Result {PersonID = r.Key, Cars = r.ToList() })
    .ToList()

---

尝试这个 ：

var results= persons.GroupBy(n => n.PersonId)
            .Select(g => new {
                           PersonId=g.Key,
                           Cars=g.Select(p=>p.car).ToList())}).ToList();

但在性能方面，以下做法在内存使用方面更好，更优化（当我们的数组包含更多项目，如数百万时）：

var carDic=new Dictionary<int,List<string>>();
for(int i=0;i<persons.length;i++)
{
   var person=persons[i];
   if(carDic.ContainsKey(person.PersonId))
   {
        carDic[person.PersonId].Add(person.car);
   }
   else
   {
        carDic[person.PersonId]=new List<string>(){person.car};
   }
}
//returns the list of cars for PersonId 1
var carList=carDic[1];

---

以下示例使用 GroupBy 方法返回按 PersonID 分组的对象。

var results = persons.GroupBy(x => x.PersonID)
              .Select(x => (PersonID: x.Key, Cars: x.Select(p => p.car).ToList())
              ).ToList();

或者

 var results = persons.GroupBy(
               person => person.PersonID,
               (key, groupPerson) => (PersonID: key, Cars: groupPerson.Select(x => x.car).ToList()));

或者

 var results = from person in persons
               group person by person.PersonID into groupPerson
               select (PersonID: groupPerson.Key, Cars: groupPerson.Select(x => x.car).ToList());

或者您可以使用 ToLookup，基本上 ToLookup 使用 EqualityComparer<TKey>。默认比较键并在使用 group by 和字典时执行您应该手动执行的操作。我认为这是在内存中执行的

 ILookup<int, string> results = persons.ToLookup(
            person => person.PersonID,
            person => person.car);

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另一种方法是选择不同的 PersonId 并使用 persons 加入组：

var result = 
    from id in persons.Select(x => x.PersonId).Distinct()
    join p2 in persons on id equals p2.PersonId into gr // apply group join here
    select new 
    {
        PersonId = id,
        Cars = gr.Select(x => x.Car).ToList(),
    };

或者与流畅的 API 语法相同：

var result = persons.Select(x => x.PersonId).Distinct()
    .GroupJoin(persons, id => id, p => p.PersonId, (id, gr) => new
    {
        PersonId = id,
        Cars = gr.Select(x => x.Car).ToList(),
    });

GroupJoin 在第一个列表（在我们的例子中是 PersonId 的列表）中生成一个条目列表，每个条目在第二个列表（persons 的列表）中都有一组连接的条目。

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