在LINQ分组

Question

我们假设我们有一个类似的课程

class Person { 
    internal int PersonID; 
    internal string car  ; 
}

现在我有一个这个类的列表: List<Person> persons;

现在,此列表可以具有多个具有相同PersonID的实例,例如:

persons[0] = new Person { PersonID = 1, car = "Ferrari" }; 
persons[1] = new Person { PersonID = 1, car = "BMW"     }; 
persons[2] = new Person { PersonID = 2, car = "Audi"    }; 

有没有办法可以分组personID并获得他拥有的所有汽车的清单？

例如,预期结果将是

class Result { 
   int PersonID;
   List<string> cars; 
}

分组后,我会得到:

results[0].PersonID = 1; 
List<string> cars = results[0].cars; 

result[1].PersonID = 2; 
List<string> cars = result[1].cars;

从我到目前为止所做的:

var results = from p in persons
              group p by p.PersonID into g
              select new { PersonID = g.Key, // this is where I am not sure what to do

有人可以指点我正确的方向吗？

Answer 1

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() };

或者作为非查询表达式:

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(当作为视图查看时IEnumerable<T>)是p.car给定键存在的投影(在这种情况下)中的任何值的序列.

有关如何GroupBy工作的更多信息,请参阅我关于该主题的Edulinq帖子.

(我已经改名PersonID,以PersonId在上面,遵循.NET命名约定.)

或者,您可以使用Lookup:

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

然后,您可以非常轻松地为每个人获取汽车:

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

Answer 2

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()/**/}

Answer 3

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

Answer 4

你也可以尝试这个.

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

Answer 5

尝试

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

要么

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)

Answer 6

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()

Answer 7

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:)

你也可以在.Net小提琴上运行代码:

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

Answer 8

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();

但从性能角度来看，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（例如数百万））：

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];