What is the best way to iterate over a dictionary

Question

I ve seen a few different ways to iterate over a dictionary in C    Is there a standard way

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As already pointed out on this answer  KeyValuePair lt TKey  TValue gt  implements a Deconstruct method starting on  NET Core 2 0   NET Standard 2 1 and  NET Framework 5 0  preview    With this  it s possible to iterate through a dictionary in a KeyValuePair agnostic way   var dictionary   new Dictionary lt int  string gt              foreach  var  key  value  in dictionary

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Depends on whether you re after the keys or the values     From the MSDN Dictionary TKey  TValue  Class description      When you use foreach to enumerate dictionary elements     the elements are retrieved as KeyValuePair objects  Console WriteLine    foreach  KeyValuePair lt string  string gt  kvp in openWith         Console WriteLine  Key    0   Value    1             kvp Key  kvp Value         To get the values alone  use the Values property  Dictionary lt string  string gt  ValueCollection valueColl       openWith Values      The elements of the ValueCollection are strongly typed    with the type that was specified for dictionary values  Console WriteLine    foreach  string s in valueColl         Console WriteLine  Value    0    s         To get the keys alone  use the Keys property  Dictionary lt string  string gt  KeyCollection keyColl       openWith Keys      The elements of the KeyCollection are strongly typed    with the type that was specified for dictionary keys  Console WriteLine    foreach  string s in keyColl         Console WriteLine  Key    0    s

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I would say foreach is the standard way  though it obviously depends on what you re looking for  foreach var kvp in my dictionary              Is that what you re looking for

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Using C  7  add this extension method to any project of your solution   public static class IDictionaryExtensions       public static IEnumerable lt  TKey  TValue  gt  Tuples lt TKey  TValue gt           this IDictionary lt TKey  TValue gt  dict                foreach  KeyValuePair lt TKey  TValue gt  kvp in dict              yield return  kvp Key  kvp Value              And use this simple syntax  foreach  var id  value  in dict Tuples             your code using  id  and  value       Or this one  if you prefer  foreach   string id  object value  in dict Tuples             your code using  id  and  value       In place of the traditional  foreach  KeyValuePair lt string  object gt  kvp in dict        string id   kvp Key      object value   kvp Value          your code using  id  and  value       The extension method transforms the KeyValuePair of your IDictionary lt TKey  TValue gt  into a strongly typed tuple  allowing you to use this new comfortable syntax   It converts -just- the required dictionary entries to tuples  so it does NOT converts the whole dictionary to tuples  so there are no performance concerns related to that   There is a only minor cost calling the extension method for creating a tuple in comparison with using the KeyValuePair directly  which should NOT be an issue if you are assigning the KeyValuePair s properties Key and Value to new loop variables anyway   In practice  this new syntax suits very well for most cases  except for low-level ultra-high performance scenarios  where you still have the option to simply not use it on that specific spot   Check this out  MSDN Blog - New features in C  7

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In some cases you may need a counter that may be provided by for-loop implementation  For that  LINQ provides ElementAt which enables the following   for  int index   0  index  lt  dictionary Count  index        var item   dictionary ElementAt index     var itemKey   item Key    var itemValue   item Value

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The best answer is of course  Don  t use exact a dictionary if you plan to iterate over it- as Vikas Gupta mentioned already in the discussion under the question  But that discussion as this whole thread still lacks surprisingly good alternatives  One is  SortedList lt string  string gt  x   new SortedList lt string  string gt      x Add  quot key1 quot    quot value1 quot    x Add  quot key2 quot    quot value2 quot    x  quot key3 quot      quot value3 quot   foreach  KeyValuePair lt string  string gt  kvPair in x               Console WriteLine   quot  kvPair Key    kvPair Value  quot     Why a lot of people consider it a code smell of iterating over a dictionary  e g  by foreach KeyValuePair lt   gt    It is quite surprising  but there is a little cited  but basic principle of Clean Coding   quot Express intent  quot  Robert C  Martin writes in  quot Clean Code quot    quot Choosing names that reveal intent quot   Obviously this is too weak   quot Express  reveal  intent with every coding decision quot  would be better  My wording  I am lacking a good first source but I am sure there are    A related principle is  quot Principle of least surprise quot    Principle of Least Astonishment   Why this is related to iterating over a dictionary  Choosing a dictionary expresses the intent of choosing a data structure which was made for finding data by key  Nowadays there are so much alternatives in  NET  if you want to iterate through key value pairs that you don  t need this crutch  Moreover  If you iterate over something  you have to reveal something about how the items are  to be  ordered and expected to be ordered  AFAIK  Dictionary has no specification about ordering  implementation-specific conventions only   What are the alternatives  TLDR  SortedList  If your collection is not getting too large  a simple solution would be to use SortedList lt   gt  which gives you also full indexing of key value pairs  Microsoft has a long article about mentioning and explaining fitting collections  Keyed collection To mention the most important  KeyedCollection lt   gt  and SortedDictionary lt   gt    SortedDictionary lt   gt  is a bit faster than SortedList for oly inserting if it gets large  but lacks indexing and is needed only if O log n  for inserting is preferenced over other operations  If you really need O 1  for inserting and accept slower iterating in exchange  you have to stay with simple Dictionary lt   gt   Obviously there is no data structure which is the fastest for every possible operation   Additionally there is ImmutableSortedDictionary lt   gt   And if one data structure is not exactly what you need  then derivate from Dictionary lt   gt  or even from the new ConcurrentDictionary lt   gt  and add explicit iteration sorting functions

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Just wanted to add my 2 cent  as the most answers relate to foreach-loop  Please  take a look at the following code   Dictionary lt String  Double gt  myProductPrices   new Dictionary lt String  Double gt        Add some entries to the dictionary  myProductPrices ToList   ForEach kvP   gt         kvP Value    1 15      Console Writeline String Format  Product   0   has a new price   1      kvp Key  kvP Value          Altought this adds a additional call of   ToList     there might be a slight performance-improvement  as pointed out here foreach vs someList Foreach        espacially when working with large Dictionaries and running in parallel is no option   won t have an effect at all   Also  please note that you wont be able to assign values to the  Value  property inside a foreach-loop  On the other hand  you will be able to manipulate the  Key  as well  possibly getting you into trouble at runtime   When you just want to  read  Keys and Values  you might also use IEnumerable Select     var newProductPrices   myProductPrices Select kvp   gt  new   Name   kvp Key  Price   kvp Value   1 15

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I appreciate this question has already had a lot of responses but I wanted to throw in a little research   Iterating over a dictionary can be rather slow when compared with iterating over something like an array  In my tests an iteration over an array took 0 015003 seconds whereas an iteration over a dictionary  with the same number of elements  took 0 0365073 seconds that s 2 4 times as long  Although I have seen much bigger differences  For comparison a List was somewhere in between at 0 00215043 seconds   However  that is like comparing apples and oranges  My point is that iterating over dictionaries is slow   Dictionaries are optimised for lookups  so with that in mind I ve created two methods  One simply does a foreach  the other iterates the keys then looks up   public static string Normal Dictionary lt string  string gt  dictionary        string value      int count   0      foreach  var kvp in dictionary                value   kvp Value          count               return  Normal       This one loads the keys and iterates over them instead  I did also try pulling the keys into a string   but the difference was negligible   public static string Keys Dictionary lt string  string gt  dictionary        string value      int count   0      foreach  var key in dictionary Keys                value   dictionary key           count               return  Keys       With this example the normal foreach test took 0 0310062 and the keys version took 0 2205441  Loading all the keys and iterating over all the lookups is clearly a LOT slower   For a final test I ve performed my iteration ten times to see if there are any benefits to using the keys here  by this point I was just curious    Here s the RunTest method if that helps you visualise what s going on   private static string RunTest lt T gt  T dictionary  Func lt T  string gt  function                    DateTime start   DateTime Now      string name   null      for  int i   0  i  lt  10  i                  name   function dictionary             DateTime end   DateTime Now      var duration   end Subtract start       return string Format   0  took  1  seconds   name  duration TotalSeconds       Here the normal foreach run took 0 2820564 seconds  around ten times longer than a single iteration took - as you d expect   The iteration over the keys took 2 2249449 seconds   Edited To Add  Reading some of the other answers made me question what would happen if I used Dictionary instead of Dictionary  In this example the array took 0 0120024 seconds  the list 0 0185037 seconds and the dictionary 0 0465093 seconds  It s reasonable to expect that the data type makes a difference on how much slower the dictionary is   What are my Conclusions    Avoid iterating over a dictionary if you can  they are substantially slower than iterating over an array with the same data in it  If you do choose to iterate over a dictionary don t try to be too clever  although slower you could do a lot worse than using the standard foreach method

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C  7 0 introduced Deconstructors and if you are using  NET Core 2 0  Application  the struct KeyValuePair lt  gt  already include a Deconstruct   for you  So you can do   var dic   new Dictionary lt int  string gt        1   One       2   Two       3   Three       foreach  var  key  value  in dic        Console WriteLine   Item   key      value         Or foreach  var     value  in dic        Console WriteLine   Item  NO ID     value         Or foreach   int key  string value  in dic        Console WriteLine   Item   key      value

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If you want to use for loop  you can do this   var keyList new List lt string gt  dictionary Keys   for  int i   0  i  lt  keyList Count  i         var key  keyList i      var value   dictionary key

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The standard way to iterate over a Dictionary  according to official documentation on MSDN is   foreach  DictionaryEntry entry in myDictionary           Read entry Key and entry Value here

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I found this method in the documentation for the DictionaryBase class on MSDN   foreach  DictionaryEntry de in myDictionary           Do some stuff with de Value or de Key    This was the only one I was able to get functioning correctly in a class that inherited from the DictionaryBase

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in addition to the highest ranking posts where there is a discussion between using  foreach KeyValuePair lt string  string gt  entry in myDictionary           do something with entry Value or entry Key     or   foreach var entry in myDictionary           do something with entry Value or entry Key     most complete is the following because you can see the dictionary type from the initialization  kvp is KeyValuePair  var myDictionary   new Dictionary lt string  string gt  x    fill dictionary with x  foreach var kvp in myDictionary   iterate over dictionary          do something with kvp Value or kvp Key

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I know this is a very old question  but I created some extension methods that might be useful       public static void ForEach lt T  U gt  this Dictionary lt T  U gt  d  Action lt KeyValuePair lt T  U gt  gt  a                foreach  KeyValuePair lt T  U gt  p in d    a p                public static void ForEach lt T  U gt  this Dictionary lt T  U gt  KeyCollection k  Action lt T gt  a                foreach  T t in k    a t                public static void ForEach lt T  U gt  this Dictionary lt T  U gt  ValueCollection v  Action lt U gt  a                foreach  U u in v    a u             This way I can write code like this   myDictionary ForEach pair   gt  Console Write   key   pair Key   value   pair Value      myDictionary Keys ForEach key   gt  Console Write key     myDictionary Values ForEach value   gt  Console Write value

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You suggested below to iterate  Dictionary lt string object gt  myDictionary   new Dictionary lt string object gt       Populate your dictionary here  foreach  KeyValuePair lt string object gt  kvp in myDictionary          Do some interesting things      FYI  foreach doesn t work if the value are of type object

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foreach is fastest and if you only iterate over     Values  it is also faster

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foreach KeyValuePair lt string  string gt  entry in myDictionary           do something with entry Value or entry Key

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I will take the advantage of  NET 4 0  and provide an updated answer to the originally accepted one   foreach var entry in MyDic           do something with entry Value or entry Key

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var dictionary   new Dictionary lt string  int gt           Key   12       var aggregateObjectCollection   dictionary Select      entry   gt  new AggregateObject entry Key  entry Value

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With  NET Framework 4 7 one can use decomposition  var fruits   new Dictionary lt string  int gt         foreach  var  fruit  number  in fruits        Console WriteLine fruit          number       To make this code work on lower C  versions  add System ValueTuple NuGet package and write somewhere  public static class MyExtensions       public static void Deconstruct lt T1  T2 gt  this KeyValuePair lt T1  T2 gt  tuple          out T1 key  out T2 value                key   tuple Key          value   tuple Value

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Simplest form to iterate a dictionary   foreach var item in myDictionary         Console WriteLine item Key       Console WriteLine item Value

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As of C  7  you can deconstruct objects into variables  I believe this to be the best way to iterate over a dictionary   Example   Create an extension method on KeyValuePair lt TKey  TVal gt  that deconstructs it   public static void Deconstruct lt TKey  TVal gt  this KeyValuePair lt TKey  TVal gt  pair  out TKey key  out TVal value       key   pair Key     value   pair Value      Iterate over any Dictionary lt TKey  TVal gt  in the following manner     Dictionary can be of any types  just using  int  and  string  as examples  Dictionary lt int  string gt  dict   new Dictionary lt int  string gt         Deconstructor gets called here  foreach  var  key  value  in dict       Console WriteLine    key     value

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There are plenty of options  My personal favorite is by KeyValuePair  Dictionary lt string  object gt  myDictionary   new Dictionary lt string  object gt        Populate your dictionary here  foreach  KeyValuePair lt string object gt  kvp in myDictionary            Do some interesting things     You can also use the Keys and Values Collections

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Dictionary lt  TKey  TValue   It is a generic collection class in c  and it stores the data in the key value format Key must be unique and it can not be null whereas value can be duplicate and null As each item in the dictionary is treated as KeyValuePair lt  TKey  TValue   structure representing a key and its value  and hence we should take the element type  KeyValuePair lt  TKey  TValue  during the iteration of element Below is the example   Dictionary lt int  string gt  dict   new Dictionary lt int  string gt     dict Add 1  One    dict Add 2  Two    dict Add 3  Three     foreach  KeyValuePair lt int  string gt  item in dict        Console WriteLine  Key   0   Value   1    item Key  item Value

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Sometimes if you only needs the values to be enumerated  use the dictionary s value collection   foreach var value in dictionary Values           do something with entry Value only     Reported by this post which states it is the fastest method  http   alexpinsker blogspot hk 2010 02 c-fastest-way-to-iterate-over html

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If say  you want to iterate over the values collection by default  I believe you can implement IEnumerable lt    Where T is the type of the values object in the dictionary  and  this  is a Dictionary    public new IEnumerator lt T gt  GetEnumerator        return this Values GetEnumerator

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I wrote an extension to loop over a dictionary    public static class DictionaryExtension       public static void ForEach lt T1  T2 gt  this Dictionary lt T1  T2 gt  dictionary  Action lt T1  T2 gt  action            foreach KeyValuePair lt T1  T2 gt  keyValue in dictionary                action keyValue Key  keyValue Value                       Then you can call  myDictionary ForEach  x y    gt  Console WriteLine x     -     y

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Generally  asking for  the best way  without a specific context is like asking  what is the best color   One the one hand  there are many colors and there s no best color   It depends on the need and often on taste  too   On the other hand  there are many ways to iterate over a Dictionary in C  and there s no best way  It depends on the need and often on taste  too   Most straightforward way  foreach  var kvp in items           key is kvp Key     doStuff kvp Value      If you need only the value  allows to call it item  more readable than kvp Value    foreach  var item in items Values        doStuff item      If you need a specific sort order  Generally  beginners are surprised about order of enumeration of a Dictionary   LINQ provides a concise syntax that allows to specify order  and many other things   e g    foreach  var kvp in items OrderBy kvp   gt  kvp Key            key is kvp Key     doStuff kvp Value      Again you might only need the value  LINQ also provides a concise solution to     iterate directly on the value  allows to call it item  more readable than kvp Value  but sorted by the keys   Here it is   foreach  var item in items OrderBy kvp   gt  kvp Key  Select kvp   gt  kvp Value         doStuff item      There are many more real-world use case you can do from these examples  If you don t need a specific order  just stick to the  most straightforward way   see above

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If say  you want to iterate over the values collection by default  I believe you can implement IEnumerable lt    Where T is the type of the values object in the dictionary  and  this  is a Dictionary    public new IEnumerator lt T gt  GetEnumerator        return this Values GetEnumerator

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You can also try this on big dictionaries for multithreaded processing   dictionary  AsParallel    ForAll pair   gt             Process pair Key and pair Value here

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If you are trying to use a generic Dictionary in C  like you would use an associative array in another language   foreach var item in myDictionary      foo item Key     bar item Value       Or  if you only need to iterate over the collection of keys  use  foreach var item in myDictionary Keys      foo item       And lastly  if you re only interested in the values   foreach var item in myDictionary Values      foo item        Take note that the var keyword is an optional C  3 0 and above feature  you could also use the exact type of your keys values here

[c#] What is the best way to iterate over a dictionary?

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