Group by in LINQ

Let's suppose that we have a class like:

class Person { 
    internal int PersonID; 
    internal string car; 
}

I have a list of this class: List persons;

And this list can have multiple instances with the 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 cars; 
}

So after grouping, I would get:

results[0].PersonID = 1; 
List cars = results[0].cars; 

result[1].PersonID = 2; 
List 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?

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) 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];

Stack Overflow Q#7325278, score: 2115