Group by in LINQ
假设我们有一个像
1 2 3 4 | class Person { internal int PersonID; internal string car ; } |
现在我有一张这个班的单子:
现在,此列表可以有多个具有相同个人ID的实例,例如:
1 2 3 |
有没有办法让我按
例如,预期结果是
1 2 3 4 | class Result { int PersonID; List<string> cars; } |
所以分组后,我会得到:
1 2 3 4 5 | results[0].PersonID = 1; List<string> cars = results[0].cars; result[1].PersonID = 2; List<string> cars = result[1].cars; |
从我迄今为止所做的工作来看:
1 2 3 | 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 |
有人能给我指个方向吗?
当然-你基本上想要:
1 2 3 | var results = from p in persons group p.car by p.PersonId into g select new { PersonId = g.Key, Cars = g.ToList() }; |
或作为非查询表达式:
1 2 3 4 | 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(当视图为
有关
(为了遵循.NET命名约定,我在上面将
或者,您可以使用
1 | var carsByPersonId = persons.ToLookup(p => p.PersonId, p => p.car); |
然后你可以很容易地为每个人得到汽车:
1 2 | // This will be an empty sequence for any personId not in the lookup var carsForPerson = carsByPersonId[personId]; |
1 2 3 4 | 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()/**/} |
1 2 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) }; |
你也可以试试这个。
1 2 3 4 |
尝试
1 | persons.GroupBy(x => x.PersonId).Select(x => x) |
或
要检查列表中是否有人重复,请尝试
1 | persons.GroupBy(x => x.PersonId).Where(x => x.Count() > 1).Any(x => x) |
我用查询语法和方法语法创建了一个工作代码示例。我希望它能帮助其他人:)
您也可以在.NET上运行代码:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 | 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 2 3 4 5 6 7 8 9 10 11 | 1 Ferrari BMW 2 Audi ----------- 1 Ferrari BMW 2 Audi |
试试这个:
1 2 3 4 | var results= persons.GroupBy(n => n.PersonId) .Select(g => new { PersonId=g.Key, Cars=g.Select(p=>p.car).ToList())}).ToList(); |
但是从性能上讲,下面的实践在内存使用方面更好、更优化(当我们的数组包含更多像数百万这样的项目时):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | 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]; |
1 | var results = persons.GroupBy(n => n.PersonId).Select(r => new Result {PersonID = r.Key, Cars = r.ToList() }).ToList() |