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 lt Person gt  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 lt string gt  cars       So after grouping  I would get    results 0  PersonID   1   List lt string gt  cars   results 0  cars    result 1  PersonID   2   List lt string gt  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

User · Answer

First  set your key field  Then include your other fields   var results        persons      GroupBy n   gt  n PersonId       Select r   gt  new Result  PersonID   r Key  Cars   r ToList           ToList

User · Answer

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   gt  p PersonId       p   gt  p car       key  g    gt  new   PersonId   key  Cars   g ToList         Basically the contents of the group  when viewed as an IEnumerable lt T gt   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   gt  p PersonId  p   gt  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

User · Answer

var results   from p in persons               group p by p PersonID into g               select new   PersonID   g Key                                 car   g Select g  gt g car  FirstOrDefault

User · Answer

Try this    var results  persons GroupBy n   gt  n PersonId               Select g   gt  new                              PersonId g Key                             Cars g Select p  gt 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 lt int List lt string gt  gt     for int i 0 i lt persons length i         var person persons i      if carDic ContainsKey person PersonId                carDic person PersonId  Add person car           else              carDic person PersonId  new List lt string gt    person car            returns the list of cars for PersonId 1 var carList carDic 1

User · Answer

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 lt string gt  Cars      public class Program       public static void Main                 List lt Person gt  persons   new List lt Person gt                          new Person   PersonId   1  car    Ferrari                 new Person   PersonId   1  car    BMW                 new Person   PersonId   2  car    Audi                         With Query Syntax          List lt Result gt  results1                 from p in persons             group p by p PersonId into g             select new Result                                         PersonId   g Key                       Cars   g Select c   gt  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 lt Result gt  results2   persons              GroupBy p   gt  p PersonId                         k  c    gt  new Result                                                                   PersonId   k                                   Cars   c Select cs   gt  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

User · Answer

var results   from p in persons               group p by p PersonID into g               select new   PersonID   g Key  Cars   g Select m   gt  m car

User · Answer

An alternative way to do this could be select distinct PersonId and group join with persons   var result        from id in persons Select x   gt  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   gt  x Car  ToList             Or the same with fluent API syntax   var result   persons Select x   gt  x PersonId  Distinct        GroupJoin persons  id   gt  id  p   gt  p PersonId   id  gr    gt  new               PersonId   id          Cars   gr Select x   gt  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

User · Answer

You can also Try this   var results  persons GroupBy n   gt  new   n PersonId  n car                    Select g   gt  new                                  g Key PersonId                                 g Key car    ToList

User · Answer

try  persons GroupBy x   gt  x PersonId  Select x   gt  x    or   to check if any person is repeating in your list try  persons GroupBy x   gt  x PersonId  Where x   gt  x Count    gt  1  Any x   gt  x

User · Answer

The following example uses the GroupBy method to return objects that are grouped by PersonID   var results   persons GroupBy x   gt  x PersonID                 Select x   gt   PersonID  x Key  Cars  x Select p   gt  p car  ToList                    ToList      Or   var results   persons GroupBy                 person   gt  person PersonID                  key  groupPerson    gt   PersonID  key  Cars  groupPerson Select x   gt  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   gt  x car  ToList       Or you can use ToLookup  Basically ToLookup uses EqualityComparer lt TKey gt  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 lt int  string gt  results   persons ToLookup              person   gt  person PersonID              person   gt  person car

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