Convert list to dictionary using linq and not worrying about duplicates

Question

I have a list of Person objects  I want to convert to a Dictionary where the key is the first and last name  concatenated  and the value is the Person object   The issue is that I have some duplicated people  so this blows up if I use this code   private Dictionary lt string  Person gt   people   new Dictionary lt string  Person gt       people   personList ToDictionary      e   gt  e FirstandLastName      StringComparer OrdinalIgnoreCase     I know it sounds weird but I don t really care about duplicates names for now  If there are multiple names I just want to grab one  Is there anyway I can write this code above so it just takes one of the names and doesn t blow up on duplicates

User · Answer

In case we want all the Person  instead of only one Person  in the returning dictionary  we could   var  people   personList  GroupBy p   gt  p FirstandLastName   ToDictionary g   gt  g Key  g   gt  g Select x  gt x

User · Answer

This should work with lambda expression   personList Distinct   ToDictionary i   gt  i FirstandLastName  i   gt  i

User · Answer

You can create an extension method similar to ToDictionary   with the difference being that it allows duplicates  Something like       public static Dictionary lt TKey  TElement gt  SafeToDictionary lt TSource  TKey  TElement gt           this IEnumerable lt TSource gt  source           Func lt TSource  TKey gt  keySelector           Func lt TSource  TElement gt  elementSelector           IEqualityComparer lt TKey gt  comparer   null                var dictionary   new Dictionary lt TKey  TElement gt  comparer            if  source    null                        return dictionary                     foreach  TSource element in source                        dictionary keySelector element     elementSelector element                      return dictionary           In this case  if there are duplicates  then the last value wins

User · Answer

Here s the obvious  non linq solution  foreach var person in personList      if  myDictionary ContainsKey person FirstAndLastName       myDictionary Add person FirstAndLastName  person      If you don t mind always getting the last one added  you can avoid the double lookup like this  foreach var person in personList        myDictionary person FirstAndLastName    person

User · Answer

DataTable DT   new DataTable            DT Columns Add  first   typeof string            DT Columns Add  second   typeof string             DT Rows Add  ss    test1            DT Rows Add  sss    test2            DT Rows Add  sys    test3            DT Rows Add  ss    test4            DT Rows Add  ss    test5            DT Rows Add  sts    test6             var dr   DT AsEnumerable   GroupBy S   gt  S Field lt string gt   first    Select S   gt  S First                 Select S   gt  new KeyValuePair lt string  string gt  S Field lt string gt   first    S Field lt string gt   second                 ToDictionary S   gt  S Key  T   gt  T Value            foreach  var item in dr                        Console WriteLine item Key    -    item Value

User · Answer

You can also use the ToLookup LINQ function  which you then can use almost interchangeably with a Dictionary    people   personList      ToLookup e   gt  e FirstandLastName  StringComparer OrdinalIgnoreCase    people ToDictionary kl   gt  kl Key  kl   gt  kl First        Potentially unnecessary   This will essentially do the GroupBy in LukeH s answer  but will give the hashing that a Dictionary provides  So  you probably don t need to convert it to a Dictionary  but just use the LINQ First function whenever you need to access the value for the key

User · Answer

A Linq-solution using Distinct   and and no grouping is   var  people   personList      Select item   gt  new   Key   item Key  FirstAndLastName   item FirstAndLastName         Distinct        ToDictionary item   gt  item Key  item   gt  item FirstFirstAndLastName  StringComparer OrdinalIgnoreCase     I don t know if it is nicer than LukeH s solution but it works as well

User · Answer

LINQ solution      Use the first value in group var  people   personList      GroupBy p   gt  p FirstandLastName  StringComparer OrdinalIgnoreCase       ToDictionary g   gt  g Key  g   gt  g First    StringComparer OrdinalIgnoreCase       Use the last value in group var  people   personList      GroupBy p   gt  p FirstandLastName  StringComparer OrdinalIgnoreCase       ToDictionary g   gt  g Key  g   gt  g Last    StringComparer OrdinalIgnoreCase     If you prefer a non-LINQ solution then you could do something like this      Use the first value in list var  people   new Dictionary lt string  Person gt  StringComparer OrdinalIgnoreCase   foreach  var p in personList        if    people ContainsKey p FirstandLastName            people p FirstandLastName    p        Use the last value in list var  people   new Dictionary lt string  Person gt  StringComparer OrdinalIgnoreCase   foreach  var p in personList         people p FirstandLastName    p

User · Answer

The issue with most of the other answers is that they use Distinct  GroupBy or ToLookup  which creates an extra Dictionary under the hood  Equally ToUpper creates extra string  This is what I did  which is an almost an exact copy of Microsoft s code except for one change       public static Dictionary lt TKey  TSource gt  ToDictionaryIgnoreDup lt TSource  TKey gt           this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector  IEqualityComparer lt TKey gt  comparer   null    gt          source ToDictionaryIgnoreDup keySelector  i   gt  i  comparer        public static Dictionary lt TKey  TElement gt  ToDictionaryIgnoreDup lt TSource  TKey  TElement gt           this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector  Func lt TSource  TElement gt  elementSelector  IEqualityComparer lt TKey gt  comparer   null                if  keySelector    null              throw new ArgumentNullException nameof keySelector            if  elementSelector    null              throw new ArgumentNullException nameof elementSelector            var d   new Dictionary lt TKey  TElement gt  comparer    EqualityComparer lt TKey gt  Default           foreach  var element in source              d keySelector element     elementSelector element           return d          Because a set on the indexer causes it to add the key  it will not throw  and will also do only one key lookup  You can also give it an IEqualityComparer  for example StringComparer OrdinalIgnoreCase

User · Answer

Starting from Carra s solution you can also write it as   foreach var person in personList Where el   gt   myDictionary ContainsKey el FirstAndLastName          myDictionary Add person FirstAndLastName  person

User · Answer

To handle eliminating duplicates  implement an IEqualityComparer lt Person gt  that can be used in the Distinct   method  and then getting your dictionary will be easy  Given   class PersonComparer   IEqualityComparer lt Person gt        public bool Equals Person x  Person y                return x FirstAndLastName Equals y FirstAndLastName  StringComparison OrdinalIgnoreCase              public int GetHashCode Person obj                return obj FirstAndLastName ToUpper   GetHashCode             class Person       public string FirstAndLastName   get  set        Get your dictionary   List lt Person gt  people   new List lt Person gt          new Person     FirstAndLastName    Bob Sanders         new Person     FirstAndLastName    Bob Sanders         new Person     FirstAndLastName    Jane Thomas        Dictionary lt string  Person gt  dictionary       people Distinct new PersonComparer    ToDictionary p   gt  p FirstAndLastName  p   gt  p

[c#] Convert list to dictionary using linq and not worrying about duplicates

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