下面的示例使用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();

Or

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

Or

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

或者你可以使用ToLookup，基本上ToLookup使用EqualityComparer<TKey>。默认比较键，并执行在使用group by和to dictionary时应该手动执行的操作。 我认为它是在内存中执行的

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

试试这个:

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，下面的做法在内存使用方面更好，更优化(当我们的数组包含更多的项，如数百万):

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];

try

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

or

检查是否有人在你的列表中重复出现

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

我已经用查询语法和方法语法创建了一个工作代码示例。我希望这能帮助到其他人:)

你也可以在。net Fiddle上运行代码:

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

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) };