Group by in LINQ
Let's suppose if we have a class like
class Person {
internal int PersonID;
internal string car ;
}
Now I have a list of this class: List<Person> persons;
Now 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, 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?
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 view 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.)
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];
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()/**/}
var results = from p in persons
group p by p.PersonID into g
select new { PersonID = g.Key, Cars = g.Select(m => m.car) };
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