在LINQ中分组

假设我们有这样一个类:

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

我有一个这个类的列表:list <Person> persons;

这个列表可以有多个具有相同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"    };

有没有办法我可以通过PersonID分组，并获得他所有的汽车列表?

例如，预期的结果将是

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

分组之后，我得到:

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

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

从我目前所做的来看:

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

有人能告诉我正确的方向吗?

当前回答

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

2014-08-18 10:26:51

其他回答

首先，设置关键字段。然后写上你的其他领域:

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

2019-05-06 19:06:21

try

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

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)

2016-08-15 22:24:45

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();

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

 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);

2019-12-15 07:54:14

另一种方法可以选择不同的PersonId和组与人连接:

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(在本例中是PersonId列表)中生成一个条目列表，每个条目在第二个列表(人员列表)中生成一组已连接的条目。

2019-06-05 08:23:33

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();

2015-12-02 06:15:34

在LINQ中分组

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