在LINQ中分组

假设我们有一个类

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

现在我有这个类的列表: List<Person> persons;

现在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" }; 

有没有一种方法可以通过personID进行分组并获得他拥有的所有赛车的列表?

例如,预期的结果是

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

所以分组后,我会得到:

 results[0].PersonID = 1; List<string> cars = results[0].cars; result[1].PersonID = 2; List<string> cars = result[1].cars; 

从我迄今所做的事情来看:

 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 

有人能指点我的方向吗?

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() }; 

或者作为一个非查询expression式:

 var results = persons.GroupBy( p => p.PersonId, p => p.car, (key, g) => new { PersonId = key, Cars = g.ToList() }); 

基本上,该组的内容(当作为IEnumerable<T>查看时)是给定键的投影中的任何值的序列(在这种情况下是p.car )。

有关GroupBy如何工作的更多信息,请参阅我的Edulinq主题 。

(我已经将PersonID重命名为PersonId ,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]; 
 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()/**/} 
 var results = from p in persons group p by p.PersonID into g select new { PersonID = g.Key, Cars = g.Select(m => m.car) }; 

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(); 

尝试

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

要么

检查是否有人在你的名单上重复尝试

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

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(); 

但是在性能方面,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目,比如数百万):

 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];