What s the difference between SortedList and SortedDictionary

Question

Is there any real practical difference between a SortedList lt TKey TValue gt  and a SortedDictionary lt TKey TValue gt    Are there any circumstances where you would specifically use one and not the other

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Here is a tabular view if it helps    From a performance perspective   ------------------ --------- ---------- -------- ---------- ---------- ---------    Collection         Indexed   Keyed      Value    Addition    Removal   Memory                         lookup    lookup     lookup                                    ------------------ --------- ---------- -------- ---------- ---------- ---------    SortedList         O 1       O log n    O n      O n        O n        Lesser      SortedDictionary   O n       O log n    O n      O log n    O log n    Greater    ------------------ --------- ---------- -------- ---------- ---------- ---------       Insertion is O log n  for data that are already in sort order  so that each      element is added to the end of the list  If a resize is required  that element     takes O n  time  but inserting n elements is still amortized O n log n       list     Available through enumeration  e g  Enumerable ElementAt   From an implementation perspective   ------------ --------------- ---------- ------------ ------------ ------------------    Underlying   Lookup          Ordering   Contiguous   Data         Exposes Key  amp         structure    strategy                   storage      access       Value collection    ------------ --------------- ---------- ------------ ------------ ------------------    2 arrays     Binary search   Sorted     Yes          Key  Index   Yes                  BST          Binary search   Sorted     No           Key          Yes                 ------------ --------------- ---------- ------------ ------------ ------------------   To roughly paraphrase  if you require raw performance SortedDictionary could be a better choice  If you require lesser memory overhead and indexed retrieval SortedList fits better  See this question for more on when to use which  You can read more here  here  here  here and here

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This is visual representation of how performances compare to each other

User · Answer

Index access  mentioned here  is the practical difference   If you need to access the successor or predecessor  you need SortedList  SortedDictionary cannot do that so you are fairly limited with how you can use the sorting  first   foreach

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I cracked open Reflector to have a look at this as there seems to be a bit of confusion about SortedList  It is in fact not a binary search tree  it is a sorted  by key  array of key-value pairs  There is also a TKey   keys variable which is sorted in sync with the key-value pairs and used to binary search   Here is some source  targeting  NET 4 5  to backup my claims   Private members     Fields private const int  defaultCapacity   4  private int  size   NonSerialized  private object  syncRoot  private IComparer lt TKey gt  comparer  private static TKey   emptyKeys  private static TValue   emptyValues  private KeyList lt TKey  TValue gt  keyList  private TKey   keys  private const int MaxArrayLength   0x7fefffff  private ValueList lt TKey  TValue gt  valueList  private TValue   values  private int version    SortedList ctor IDictionary  IComparer    public SortedList IDictionary lt TKey  TValue gt  dictionary  IComparer lt TKey gt  comparer    this  dictionary    null    dictionary Count   0  comparer        if  dictionary    null                ThrowHelper ThrowArgumentNullException ExceptionArgument dictionary             dictionary Keys CopyTo this keys  0       dictionary Values CopyTo this values  0       Array Sort lt TKey  TValue gt  this keys  this values  comparer       this  size   dictionary Count      SortedList Add TKey  TValue    void  public void Add TKey key  TValue value        if  key    null                ThrowHelper ThrowArgumentNullException ExceptionArgument key             int num   Array BinarySearch lt TKey gt  this keys  0  this  size  key  this comparer       if  num  gt   0                ThrowHelper ThrowArgumentException ExceptionResource Argument AddingDuplicate             this Insert  num  key  value       SortedList RemoveAt int    void  public void RemoveAt int index        if   index  lt  0      index  gt   this  size                 ThrowHelper ThrowArgumentOutOfRangeException ExceptionArgument index  ExceptionResource ArgumentOutOfRange Index             this  size--      if  index  lt  this  size                Array Copy this keys  index   1  this keys  index  this  size - index           Array Copy this values  index   1  this values  index  this  size - index             this keys this  size    default TKey       this values this  size    default TValue       this version

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Check out the MSDN page for SortedList   From Remarks section      The SortedList lt  Of  lt  TKey  TValue gt   gt   generic class is a binary search tree with O log n  retrieval  where n is the number of elements in the dictionary  In this  it is similar to the SortedDictionary lt  Of  lt  TKey  TValue gt   gt   generic class  The two classes have similar object models  and both have O log n  retrieval  Where the two classes differ is in memory use and speed of insertion and removal          SortedList lt  Of  lt  TKey  TValue gt   gt   uses less memory than SortedDictionary lt  Of  lt  TKey  TValue gt   gt      SortedDictionary lt  Of  lt  TKey  TValue gt   gt   has faster insertion and removal operations for unsorted data  O log n  as opposed to O n  for SortedList lt  Of  lt  TKey  TValue gt   gt      If the list is populated all at once from sorted data  SortedList lt  Of  lt  TKey  TValue gt   gt   is faster than SortedDictionary lt  Of  lt  TKey  TValue gt   gt

User · Answer

Yes - their performance characteristics differ significantly  It would probably be better to call them SortedList and SortedTree as that reflects the implementation more closely   Look at the MSDN docs for each of them  SortedList  SortedDictionary  for details of the performance for different operations in different situtations  Here s a nice summary  from the SortedDictionary docs       The SortedDictionary lt TKey  TValue gt  generic   class is a binary search tree with   O log n  retrieval  where n is the   number of elements in the dictionary    In this  it is similar to the   SortedList lt TKey  TValue gt  generic   class  The two classes have similar   object models  and both have O log n    retrieval  Where the two classes   differ is in memory use and speed of   insertion and removal          SortedList lt TKey  TValue gt  uses less   memory than SortedDictionary lt TKey    TValue gt     SortedDictionary lt TKey  TValue gt  has   faster insertion and removal   operations for unsorted data  O log n    as opposed to O n  for   SortedList lt TKey  TValue gt     If the list is populated all at once   from sorted data  SortedList lt TKey    TValue gt  is faster than   SortedDictionary lt TKey  TValue gt         SortedList actually maintains a sorted array  rather than using a tree  It still uses binary search to find elements

User · Answer

Enough is said already on the topic  however to keep it simple  here s my take   Sorted dictionary should be used when-   More inserts and delete operations are required  Data in un-ordered  Key access is enough and index access is not required  Memory is not a bottleneck    On the other side  Sorted List should be used when-   More lookups and less inserts and delete operations are required  Data is already sorted  if not all  most   Index access is required  Memory is an overhead    Hope this helps

[c#] What's the difference between SortedList and SortedDictionary?

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