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?
当然，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IEnumerable<T>时)是给定键的投影(在本例中为p.car)中任何值的序列。
有关GroupBy如何工作的更多信息，请参阅我在Edulinq上关于该主题的帖子。
(我在上面将PersonID重命名为PersonID，以遵循. net命名约定，在“大写复合词和常用术语”一节中特别指出了这一点。)
或者，你也可以使用Lookup:
var carsByPersonId = persons.ToLookup(p => p.PersonId, p => p.car);
然后你可以很容易地得到每个人的车:
// This will be an empty sequence for any personId not in the lookup
var carsForPerson = carsByPersonId[personId];
我已经用查询语法和方法语法创建了一个工作代码示例。我希望这能帮助到其他人:)
你也可以在。net Fiddle上运行代码:
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
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。
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();
Or
var results = persons.GroupBy(
person => person.PersonID,
(key, groupPerson) => (PersonID: key, Cars: groupPerson.Select(x => x.car).ToList()));
Or
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);
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;
