Group by in LINQ

The Solution to Group by in LINQ is

Absolutely - you basically want:

var results = from p in persons
              group 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 =>,
    (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 ( 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 =>;

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];

~ Answered on 2011-09-06 19:46:13

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