提问人：test123 提问时间：9/7/2011 最后编辑：ianktest123 更新时间：1/4/2023 访问量：1847491
在 LINQ 中分组依据
Group by in LINQ
问：
假设我们有一个这样的类：
class Person {
internal int PersonID;
internal string car;
}
我有一个这样的列表：List<Person> persons;
此列表可以包含多个具有相同 s 的实例，例如：PersonID
persons[0] = new Person { PersonID = 1, car = "Ferrari" };
persons[1] = new Person { PersonID = 1, car = "BMW" };
persons[2] = new Person { PersonID = 2, car = "Audi" };
有没有办法我可以按分组并获取他拥有的所有汽车的列表？PersonID
例如，预期结果为
class Result {
int PersonID;
List<string> cars;
}
所以在分组后，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事情来看：
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
有人可以给我指出正确的方向吗？
答：
2057赞
Jon Skeet
9/7/2011
#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（当被视为一个时）是给定键的投影（在本例中）中存在的任何值的序列。IEnumerable<T>p.car
有关工作原理的更多信息，请参阅我关于该主题的 Edulinq 帖子。GroupBy
（我已在上面重命名为 .NET 命名约定，该约定在“大写复合词和常用术语”一节中特别指出了这一点。PersonIDPersonId
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得汽车：
// This will be an empty sequence for any personId not in the lookup
var carsForPerson = carsByPersonId[personId];
65赞
Tallat
2/6/2014
#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()/**/}
46赞
Yogendra Paudyal
8/18/2014
#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) };
54赞
shuvo sarker
12/2/2015
#4
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();
5赞
akardon
5/26/2016
#5
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();
但从性能上讲，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（如数百万个）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];
40赞
Code First
8/16/2016
#6
尝试
persons.GroupBy(x => x.PersonId).Select(x => x)
或
要检查是否有人在您的列表中重复，请尝试
persons.GroupBy(x => x.PersonId).Where(x => x.Count() > 1).Any(x => x)
15赞
Mehmet Recep Yildiz
12/1/2017
#7
我创建了一个具有查询语法和方法语法的工作代码示例。我希望它能帮助其他人:)
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
14赞
user3474287
5/7/2019
#8
首先，设置您的密钥字段。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()
2赞
Dmitry Stepanov
6/5/2019
#9
执行此操作的另一种方法是选择不同并组连接：PersonIdpersons
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 在第一个列表中生成一个条目列表（在我们的例子中为 列表），每个条目在第二个列表（列表）中都有一组联接的条目。PersonIdpersons
4赞
Reza Jenabi
12/15/2019
#10
下面的示例使用 GroupBy 方法返回按 分组的对象。PersonID
var results = persons.GroupBy(x => x.PersonID)
.Select(x => (PersonID: x.Key, Cars: x.Select(p => p.car).ToList())
).ToList();
或
var results = persons.GroupBy(
person => person.PersonID,
(key, groupPerson) => (PersonID: key, Cars: groupPerson.Select(x => x.car).ToList()));
或
var results = from person in persons
group person by person.PersonID into groupPerson
select (PersonID: groupPerson.Key, Cars: groupPerson.Select(x => x.car).ToList());
或者也可以使用，基本上使用。默认情况下用于比较键，并执行在使用“分组依据”和“字典”时应手动执行的操作。
我认为它在内存中被执行了ToLookupToLookupEqualityComparer<TKey>
ILookup<int, string> results = persons.ToLookup(
person => person.PersonID,
person => person.car);
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