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,
                           /**/car = g.Select(g=>g.car).FirstOrDefault()/**/}

2014-02-05 17:25:18

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

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

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()/**/}

2014-02-05 17:25:18

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];

2016-05-26 05:51:49

我想给出一个在c#中使用GroupBy的复杂查询的例子

    var records= (from x in db.Table
                        select new
                        {
                            x.Id,
                            x.ProgramId,
                            ProgramName = x.Program.Name,
                            Version= x.Program.Version,
                            x.Date,
                            x.Maks
                        });
    // I take two date(openDate and closeDate)
    var try= records
        .Where(x => x.Date>= openDate && x.Date<= closeDate)
        .GroupBy(x => new
        {
            x.ProgramId,
            MonthYear=x.Date.Value.Month + "-" + x.Date.Value.Year,
                
        })
        .Select(x => new
        {                  
           ProgramName = x.Select(y => y.ProgramName).FirstOrDefault(),
           Version= x.Select(y => y.Version).FirstOrDefault(),
           x.Key.MonthYear,
           x.Maks.Max()
          })
        .ToList();
    datagridview1.DataSource = try;

2022-12-01 14:19:59

在LINQ中分组

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