Sorting a Dictionary in place with respect to keys

Question

I have a dictionary in C  like  Dictionary lt Person  int gt    and I want to sort that dictionary in place with respect to keys  a field in class Person   How can I do it  Every available help on the internet is that of lists with no particular example of in place sorting of Dictionary  Any help would be highly appreciated

User · Accepted Answer

You can t sort a Dictionary lt TKey  TValue gt  - it s inherently unordered   Or rather  the order in which entries are retrieved is implementation-specific  You shouldn t rely on it working the same way between versions  as ordering isn t part of its designed functionality    You can use SortedList lt TKey  TValue gt  or SortedDictionary lt TKey  TValue gt   both of which sort by the key  in a configurable way  if you pass an IEqualityComparer lt T gt  into the constructor  - might those be of use to you   Pay little attention to the word  list  in the name SortedList - it s still a dictionary in that it maps keys to values  It s implemented using a list internally  effectively - so instead of looking up by hash code  it does a binary search  SortedDictionary is similarly based on binary searches  but via a tree instead of a list

User · Answer

Due to this answers high search placing I thought the LINQ OrderBy solution is worth showing   class Person       public Person string firstname  string lastname                FirstName   firstname          LastName   lastname            public string FirstName   get  set        public string LastName   get  set       static void Main string   args        Dictionary lt Person  int gt  People   new Dictionary lt Person  int gt          People Add new Person  John    Doe    1       People Add new Person  Mary    Poe    2       People Add new Person  Richard    Roe    3       People Add new Person  Anne    Roe    4       People Add new Person  Mark    Moe    5       People Add new Person  Larry    Loe    6       People Add new Person  Jane    Doe    7        foreach  KeyValuePair lt Person  int gt  person in People OrderBy i   gt  i Key LastName                 Debug WriteLine person Key LastName          person Key FirstName     - Id      person Value ToString               Output   Doe  John - Id  1 Doe  Jane - Id  7 Loe  Larry - Id  6 Moe  Mark - Id  5 Poe  Mary - Id  2 Roe  Richard - Id  3 Roe  Anne - Id  4   In this example it would make sense to also use ThenBy for first names   foreach  KeyValuePair lt Person  int gt  person in People OrderBy i   gt  i Key LastName  ThenBy i   gt  i Key FirstName     Then the output is   Doe  Jane - Id  7 Doe  John - Id  1 Loe  Larry - Id  6 Moe  Mark - Id  5 Poe  Mary - Id  2 Roe  Anne - Id  4 Roe  Richard - Id  3   LINQ also has the OrderByDescending and ThenByDescending for those that need it

User · Answer

Try using SortedDictionary

User · Answer

Take a look at SortedDictionary  there s even a constructor overload so you can pass in your own IComparable for the comparisons

User · Answer

While Dictionary is implemented as a hash table  SortedDictionary is implemented as a Red-Black Tree   If you don t take advantage of the order in your algorithm and only need to sort the data before output  using SortedDictionary would have negative impact on performance   You can  sort  the dictionary like this   Dictionary lt string  int gt  dictionary   new Dictionary lt string  int gt        algorithm return new SortedDictionary lt string  int gt  dictionary

User · Answer

By design  dictionaries are not sortable  If you need this capability in a dictionary  look at SortedDictionary instead

User · Answer

The correct answer is already stated  just use SortedDictionary    However  if by chance you have some need to retain your collection as Dictionary  it is possible to access the Dictionary keys in an ordered way  by  for example  ordering the keys in a List  then using this list to access the Dictionary  An example     Dictionary lt string  int gt  dupcheck   new Dictionary lt string  int gt          some code that fills in  dupcheck   then     if  dupcheck Count  gt  0      Console WriteLine   ndupcheck  count   0   n----   dupcheck Count     var keys sorted   dupcheck Keys ToList        keys sorted Sort      foreach  var k in keys sorted        Console WriteLine   0     1    k  dupcheck k            Don t forget using System Linq  for this

[c#] Sorting a Dictionary in place with respect to keys

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