Group by in LINQ

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?

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];

var results = from p in persons
group p by p.PersonID into g
select new { PersonID = g.Key, Cars = g.Select(m => m.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:][1]

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:
<pre>
1
Ferrari
BMW
2
Audi
-----------
1
Ferrari
BMW
2
Audi

</pre>

[1]:

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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)