Group by in LINQ

Question

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 PersonIDs, 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?

Braxy

There is another example including Count and Sum here #3414580 – Sestertium

Answer 1

Absolutely - you basically want:

var results = from p in persons
              group p.car by p.PersonId into g
              select new { PersonId = g.Key, Cars = g.ToList() };

Or as a non-query expression:

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

Basically the contents of the group (when viewed as an IEnumerable<T>) is a sequence of whatever values were in the projection (p.car in this case) present for the given key.

For more on how GroupBy works, see my Edulinq post on the topic.

(I've renamed PersonID to PersonId in the above, to follow .NET naming conventions, which specifically call this out in the "Capitalizing Compound Words and Common Terms" section.)

Alternatively, you could use a Lookup:

var carsByPersonId = persons.ToLookup(p => p.PersonId, p => p.car);

You can then get the cars for each person very easily:

// This will be an empty sequence for any personId not in the lookup
var carsForPerson = carsByPersonId[personId];

Tranquilizer

@jon Skeet what if i want to add another property like name – Dozier

@Mohammad: Then you include that in the anonymous type. – Tranquilizer

@Dozier here's a good explanation of group by, it also includes an example of grouping by a composite key: How to: Group Query Results (C# Programming Guide) – Rodomontade

@Mohammad you can do something like .GroupBy(p => new {p.Id, p.Name}, p => p, (key, g) => new { PersonId = key.Id, PersonName = key.Name, PersonCount = g.Count()}) and you will get all the people that occur with an Id, Name, and a number of occurrences for each person. – Scottyscotus

instead of var you should put the actual type of results, to be even more clear about what this does/creates. – Arnold

@JonSkeet: What if you want to group by PersonId, but you will still need to access the other properties of Person? – Collectivize

@JonathanWood: Then you'd just change p.car to p in the example in my code, and you'd end up with groups of Person elements. – Tranquilizer

This is actually not usable as you cannot result in further queries. – Lalittah

@user6694745: It may not be usable for your specific use case, but it's perfectly usable for very many use cases. (Additionally, you can use ILookup<,> in further queries, although it's rarely beneficial to do that instead of using GroupBy in my experience.) – Tranquilizer

@user6694745: I'd need more details about what exactly you mean by that to comment further. If you believe you have a use case that isn't covered by this, please ask a new question - but for future comments, please bear in mind that there's a big difference between "this doesn't cover my specific use case" and "this is actually not usable". – Tranquilizer

hate to do it to @JonSkeet but the naming conventions for dot net actually states that for 2 letter acronyms there is an exception to the rule, and then both should be Capitalized. So it should actually be PersonID and not PersonId as stated in the documentation that he linked to. – Killebrew

@GerriePretorius: "Id" isn't an acronym. It's an abbreviation but it isn't formed of the first letters of multiple words. (I'll try to find examples from the framework - although I suspect I'll be able to find multiple examples of both...) – Tranquilizer

@GerriePretorius: Examples of both: ApplicationOptions.ClientId; ClientIDMode – Tranquilizer

@GerriePretorius: Turns out there's a whole question about this: stackoverflow.com/questions/596062 – Tranquilizer

@GerriePretorius: Ah - it is actually clear; see the section on "Capitalizing Compound Words and Common Terms" which says to use Id for Pascal case, id for Camel case, and not ID. – Tranquilizer

Answer 2

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()/**/}

Scotsman

Answer 3

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();

Gibbet

It is wrong returns the same list does not group by – Hygeia

It's work,i think something is missing that's why it's doesn't work in your code. – Gibbet

Answer 4

var results = from p in persons
              group p by p.PersonID into g
              select new { PersonID = g.Key, Cars = g.Select(m => m.car) };

Cuba

Answer 5

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)

Hamo

You can simplify by: persons.GroupBy(x => x.PersonId).Any(x => x.Count() > 1); – Cruikshank

Answer 6

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

Dada

Please, explain what your code does for sake. This is just a code answer which is nearly a wrong answer. – Romeu

Answer 7

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()

Astrid

Answer 8

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];

Huffish

g.Key.PersonId? g.SelectMany?? You clearly didn't try this. – Parthia

you're write I edited some codes codes in it and didn't test it. My main point was the second part. But anyway thanks for your consideration. It was too late to edit that code when I realized it's wrong. so g.Key replaces g.Key.PersonId, and Select rather than SelectMany ! so messy sorry :))) – Huffish

@akazemis: I was actually trying to create (to use terms equivalent to OP's domain) SortedDictionary <PersonIdInt, SortedDictionary <CarNameString, CarInfoClass>>. The closest I could get using LINQ was IEnumerable <IGrouping <PersonIdInt, Dictionary <CarNameString, PersonIdCarNameXrefClass>>>. I ended using your for loop method which, btw, was 2x faster. Also, I would use: a) foreach vs. for and b) TryGetValue vs. ContainsKey (both for DRY principle - in code & runtime). – Cuculiform

Answer 9

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);

Canzona

Answer 10

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).

Jar