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

Answer 1

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.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 results = persons.ToLookup(
            person => person.PersonID,
            person => person.car);