Duplicate keys in NET dictionaries

Question

Are there any dictionary classes in the  NET base class library which allow duplicate keys to be used   The only solution I ve found is to create  for example  a class like   Dictionary lt string  List lt object gt  gt    But this is quite irritating to actually use   In Java  I believe a MultiMap accomplishes this  but cannot find an analog in  NET

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In answer to the original question   Something like Dictionary lt string  List lt object gt  gt  is implemented in a class called MultiMap in The Code Project   You could find more info to the below link    http   www codeproject com KB cs MultiKeyDictionary aspx

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If you re using  NET 3 5  use the Lookup class   EDIT  You generally create a Lookup using Enumerable ToLookup  This does assume that you don t need to change it afterwards - but I typically find that s good enough   If that doesn t work for you  I don t think there s anything in the framework which will help - and using the dictionary is as good as it gets

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This is a tow way Concurrent dictionary I think this will help you   public class HashMapDictionary lt T1  T2 gt    System Collections IEnumerable       private System Collections Concurrent ConcurrentDictionary lt T1  List lt T2 gt  gt   keyValue   new System Collections Concurrent ConcurrentDictionary lt T1  List lt T2 gt  gt         private System Collections Concurrent ConcurrentDictionary lt T2  List lt T1 gt  gt   valueKey   new System Collections Concurrent ConcurrentDictionary lt T2  List lt T1 gt  gt          public ICollection lt T1 gt  Keys               get                       return  keyValue Keys                       public ICollection lt T2 gt  Values               get                       return  valueKey Keys                       public int Count               get                       return  keyValue Count                       public bool IsReadOnly               get                       return false                       public List lt T2 gt  this T1 index                get   return  keyValue index             set    keyValue index    value               public List lt T1 gt  this T2 index                get   return  valueKey index             set    valueKey index    value               public void Add T1 key  T2 value                lock  this                        if    keyValue TryGetValue key  out List lt T2 gt  result                    keyValue TryAdd key  new List lt T2 gt      value                 else if   result Contains value                   result Add value                if    valueKey TryGetValue value  out List lt T1 gt  result2                    valueKey TryAdd value  new List lt T1 gt      key                 else if   result2 Contains key                   result2 Add key                        public bool TryGetValues T1 key  out List lt T2 gt  value                return  keyValue TryGetValue key  out value              public bool TryGetKeys T2 value  out List lt T1 gt  key                return  valueKey TryGetValue value  out key              public bool ContainsKey T1 key                return  keyValue ContainsKey key              public bool ContainsValue T2 value                return  valueKey ContainsKey value              public void Remove T1 key                lock  this                        if   keyValue TryRemove key  out List lt T2 gt  values                                 foreach  var item in values                                        var remove2    valueKey TryRemove item  out List lt T1 gt  keys                                                        public void Remove T2 value                lock  this                        if   valueKey TryRemove value  out List lt T1 gt  keys                                 foreach  var item in keys                                        var remove2    keyValue TryRemove item  out List lt T2 gt  values                                                        public void Clear                  keyValue Clear             valueKey Clear               IEnumerator IEnumerable GetEnumerator                 return  keyValue GetEnumerator              examples   public class TestA       public int MyProperty   get  set       public class TestB       public int MyProperty   get  set                   HashMapDictionary lt TestA  TestB gt  hashMapDictionary   new HashMapDictionary lt TestA  TestB gt                  var a   new TestA     MyProperty   9999                var b   new TestB     MyProperty   60                var b2   new TestB     MyProperty   5                hashMapDictionary Add a  b               hashMapDictionary Add a  b2               hashMapDictionary TryGetValues a  out List lt TestB gt  result               foreach  var item in result                                  do something

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When using the List lt KeyValuePair lt string  object gt  gt  option  you could use LINQ to do the search    List lt KeyValuePair lt string  object gt  gt  myList   new List lt KeyValuePair lt string  object gt  gt       fill it here var q   from a in myList Where a Key Equals  somevalue   Select a Value if q Count    gt  0     you ve got your value

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Also this is possible   Dictionary lt string  string   gt  previousAnswers   null    This way  we can have unique keys  Hope this works for you

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I stumbled across this post in search of the same answer  and found none  so I rigged up a bare-bones example solution using a list of dictionaries  overriding the    operator to add a new dictionary to the list when all others have a given key set   and return a list of values  get   It s ugly and inefficient  it ONLY gets sets by key  and it always returns a list  but it works    class DKD           List lt Dictionary lt string  string gt  gt  dictionaries          public DKD                dictionaries   new List lt Dictionary lt string  string gt  gt              public object this string key                get                  string temp                  List lt string gt  valueList   new List lt string gt                     for  int i   0  i  lt  dictionaries Count  i                         dictionaries i  TryGetValue key  out temp                       if  temp    key                           valueList Add temp                     return valueList               set                  for  int i   0  i  lt  dictionaries Count  i                         if  dictionaries i  ContainsKey key                            continue                       else                          dictionaries i  Add key  string  value                           return                    dictionaries Add new Dictionary lt string  string gt                      dictionaries Last   key    string value

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You can create your own dictionary wrapper  something like this one  as a bonus it supports null value as a key         lt summary gt      Dictionary which supports duplicates and null entries      lt  summary gt       lt typeparam name  TKey  gt Type of key lt  typeparam gt       lt typeparam name  TValue  gt Type of items lt  typeparam gt  public class OpenDictionary lt TKey  TValue gt        private readonly Lazy lt List lt TValue gt  gt   nullStorage   new Lazy lt List lt TValue gt  gt                gt  new List lt TValue gt           private readonly Dictionary lt TKey  List lt TValue gt  gt   innerDictionary           new Dictionary lt TKey  List lt TValue gt  gt               lt summary gt          Get all entries          lt  summary gt      public IEnumerable lt TValue gt  Values   gt           innerDictionary Values              SelectMany x   gt  x               Concat  nullStorage Value             lt summary gt          Add an item          lt  summary gt      public OpenDictionary lt TKey  TValue gt  Add TKey key  TValue item                if  ReferenceEquals key  null                nullStorage Value Add item           else                       if    innerDictionary ContainsKey key                    innerDictionary Add key  new List lt TValue gt                    innerDictionary key  Add item                      return this                  lt summary gt          Remove an entry by key          lt  summary gt      public OpenDictionary lt TKey  TValue gt  RemoveEntryByKey TKey key  TValue entry                if  ReferenceEquals key  null                         int targetIdx    nullStorage Value FindIndex x   gt  x Equals entry                if  targetIdx  lt  0                  return this                nullStorage Value RemoveAt targetIdx                     else                       if    innerDictionary ContainsKey key                   return this               List lt TValue gt  targetChain    innerDictionary key               if  targetChain Count    0                  return this               int targetIdx   targetChain FindIndex x   gt  x Equals entry                if  targetIdx  lt  0                  return this               targetChain RemoveAt targetIdx                      return this                  lt summary gt          Remove all entries by key          lt  summary gt      public OpenDictionary lt TKey  TValue gt  RemoveAllEntriesByKey TKey key                if  ReferenceEquals key  null                         if   nullStorage IsValueCreated                   nullStorage Value Clear                             else                       if   innerDictionary ContainsKey key                    innerDictionary key  Clear                       return this                  lt summary gt          Try get entries by key          lt  summary gt      public bool TryGetEntries TKey key  out IReadOnlyList lt TValue gt  entries                entries   null           if  ReferenceEquals key  null                         if   nullStorage IsValueCreated                                entries    nullStorage Value                  return true                            else return false                    else                       if   innerDictionary ContainsKey key                                 entries    innerDictionary key                   return true                            else return false                      The sample of usage   var dictionary   new OpenDictionary lt string  int gt     dictionary Add  1   1       The next line won t throw an exception   dictionary Add  1   2    dictionary TryGetEntries  1   out List lt int gt  result       result is   1  2    dictionary Add null  42   dictionary Add null  24   dictionary TryGetEntries null  out List lt int gt  result       result is   42  24

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Since the new C   I belive it s from 7 0   you can also do something like this    var duplicatedDictionaryExample   new List lt  string Key  string Value  gt                     and you are using it as a standard List  but with two values named whatever you want  foreach var entry in duplicatedDictionaryExample            do something with the values     entry Key      entry Value

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If you are using     NET 4  then you can use Tuple Class      declaration var list   new List lt Tuple lt string  List lt object gt  gt  gt         to add an item to the list var item   Tuple lt string  List lt object gt  gt   key   new List lt object gt    list Add item       to iterate foreach var i in list        Console WriteLine i Item1 ToString

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Do you mean congruent and not an actual duplicate  Otherwise a hashtable wouldn t be able to work   Congruent means that two separate keys can hash to the equivalent value  but the keys aren t equal   For example  say your hashtable s hash function was just hashval   key mod 3  Both 1 and 4 map to 1  but are different values  This is where your idea of a list comes into play   When you need to lookup 1  that value is hashed to 1  the list is traversed until the Key    1 is found   If you allowed for duplicate keys to be inserted  you wouldn t be able to differentiate which keys map to which values

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Duplicate keys break the entire contract of the Dictionary  In a dictionary each key is unique and mapped to a single value  If you want to link an object to an arbitrary number of additional objects  the best bet might be something akin to a DataSet  in common parlance a table   Put your keys in one column and your values in the other  This is significantly slower than a dictionary  but that s your tradeoff for losing the ability to hash the key objects

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The List class actually works quite well for key value collections containing duplicates where you would like to iterate over the collection   Example   List lt KeyValuePair lt string  string gt  gt  list   new List lt KeyValuePair lt string  string gt  gt         add some values to the collection here  for  int i   0   i  lt  list Count   i          Print list i  Key  list i  Value

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The way I use is just a  Dictionary lt string  List lt string gt  gt   This way you have a single key holding a list of strings   Example   List lt string gt  value   new List lt string gt     if  dictionary Contains key          value   dictionary key     value Add newValue

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Very important note regarding use of Lookup   You can create an instance of a Lookup TKey  TElement  by calling ToLookup on an object that implements IEnumerable T   There is no public constructor to create a new instance of a Lookup TKey  TElement   Additionally  Lookup TKey  TElement  objects are immutable  that is  you cannot add or remove elements or keys from a Lookup TKey  TElement  object after it has been created    from MSDN   I d think this would be a show stopper for most uses

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I just came across the PowerCollections library which includes  among other things  a class called MultiDictionary   This neatly wraps this type of functionality

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i use this simple class   public class ListMap lt T V gt    List lt KeyValuePair lt T  V gt  gt        public void Add T key  V value            Add new KeyValuePair lt T  V gt  key  value               public List lt V gt  Get T key            return FindAll p   gt  p Key Equals key   ConvertAll p  gt  p Value             usage   var fruits   new ListMap lt int  string gt     fruits Add 1   apple    fruits Add 1   orange    var c   fruits Get 1  Count    c   2

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The NameValueCollection supports multiple string values under one key  which is also a string   but it is the only example I am aware of   I tend to create constructs similar to the one in your example when I run into situations where I need that sort of functionality

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I think something like List lt KeyValuePair lt object  object gt  gt  would do the Job

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If you are using strings as both the keys and the values  you can use System Collections Specialized NameValueCollection  which will return an array of string values via the GetValues string key  method

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I changed  Hector Correa  s answer into an extension with generic types and also added a custom TryGetValue to it     public static class ListWithDuplicateExtensions         public static void Add lt TKey  TValue gt  this List lt KeyValuePair lt TKey  TValue gt  gt  collection  TKey key  TValue value              var element   new KeyValuePair lt TKey  TValue gt  key  value         collection Add element              public static int TryGetValue lt TKey  TValue gt  this List lt KeyValuePair lt TKey  TValue gt  gt  collection  TKey key  out IEnumerable lt TValue gt  values              values   collection Where pair   gt  pair Key Equals key   Select pair   gt  pair Value         return values Count

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You can add same keys with different case like   key1 Key1 KEY1 KeY1 kEy1 keY1  I know is dummy answer  but worked for me

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It s easy enough to  roll your own  version of a dictionary that allows  duplicate key  entries  Here is a rough simple implementation  You might want to consider adding support for basically most  if not all  on IDictionary lt T gt    public class MultiMap lt TKey TValue gt        private readonly Dictionary lt TKey IList lt TValue gt  gt  storage       public MultiMap                 storage   new Dictionary lt TKey IList lt TValue gt  gt                public void Add TKey key  TValue value                if   storage ContainsKey key   storage Add key  new List lt TValue gt              storage key  Add value              public IEnumerable lt TKey gt  Keys               get   return storage Keys               public bool ContainsKey TKey key                return storage ContainsKey key              public IList lt TValue gt  this TKey key                get                       if   storage ContainsKey key                   throw new KeyNotFoundException                      string Format                           The given key  0  was not found in the collection    key                return storage key                       A quick example on how to use it   const string key    supported encodings   var map   new MultiMap lt string Encoding gt     map Add key  Encoding ASCII   map Add key  Encoding UTF8   map Add key  Encoding Unicode    foreach  var existingKey in map Keys        var values   map existingKey       Console WriteLine string Join      values

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Here is one way of doing this with List lt  KeyValuePair lt  string  string      public class ListWithDuplicates   List lt KeyValuePair lt string  string gt  gt        public void Add string key  string value                var element   new KeyValuePair lt string  string gt  key  value           this Add element            var list   new ListWithDuplicates    list Add  k1    v1    list Add  k1    v2    list Add  k1    v3     foreach var item in list        string x   string format   0   1      item Key  item Value       Outputs k1 v1  k1 v2  k1 v3

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U can define a method to building a Compound string key every where u want to using dictionary u must using this method to build your key for example   private string keyBuilder int key1  int key2        return string Format   0   1    key1  key2       for using   myDict ContainsKey keyBuilder key1  key2

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