Let's suppose if we have a class like:
class Person {
internal int PersonID;
internal string car;
}
I have a list of this class: List<Person> persons;
And this list can have multiple instances with same PersonID
s, for example:
persons[0] = new Person { PersonID = 1, car = "Ferrari" };
persons[1] = new Person { PersonID = 1, car = "BMW" };
persons[2] = new Person { PersonID = 2, car = "Audi" };
Is there a way I can group by PersonID
and get the list of all the cars he has?
For example, the expected result would be
class Result {
int PersonID;
List<string> cars;
}
So after grouping, I would get:
results[0].PersonID = 1;
List<string> cars = results[0].cars;
result[1].PersonID = 2;
List<string> cars = result[1].cars;
From what I have done so far:
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
Could someone please point me in the right direction?
try
persons.GroupBy(x => x.PersonId).Select(x => x)
or
to check if any person is repeating in your list try
persons.GroupBy(x => x.PersonId).Where(x => x.Count() > 1).Any(x => x)
You can also Try this:
var results= persons.GroupBy(n => new { n.PersonId, n.car})
.Select(g => new {
g.Key.PersonId,
g.Key.car)}).ToList();
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()/**/}
Try this :
var results= persons.GroupBy(n => n.PersonId)
.Select(g => new {
PersonId=g.Key,
Cars=g.Select(p=>p.car).ToList())}).ToList();
But performance-wise the following practice is better and more optimized in memory usage (when our array contains much more items like millions):
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];
An alternative way to do this could be select distinct PersonId
and group join with persons
:
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(),
};
Or the same with fluent API syntax:
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 produces a list of entries in the first list ( list of PersonId
in our case), each with a group of joined entries in the second list (list of persons
).
The following example uses the GroupBy method to return objects that are grouped by PersonID
.
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());
Or you can use ToLookup
, Basically ToLookup
uses EqualityComparer<TKey>
.Default to compare keys and do what you should do manually when using group by and to dictionary.
i think it's excuted inmemory
ILookup<int, string> results = persons.ToLookup(
person => person.PersonID,
person => person.car);
I have created a working code sample with Query Syntax and Method Syntax. I hope it helps the others :)
You can also run the code on .Net Fiddle here:
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);
}
}
}
}
Here is the result:
1 Ferrari BMW 2 Audi ----------- 1 Ferrari BMW 2 Audi
var results = from p in persons
group p by p.PersonID into g
select new { PersonID = g.Key, Cars = g.Select(m => m.car) };
First, set your key field. Then include your other fields:
var results =
persons
.GroupBy(n => n.PersonId)
.Select(r => new Result {PersonID = r.Key, Cars = r.ToList() })
.ToList()
Source: Stackoverflow.com