Getting multiple keys of specified value of a generic Dictionary

Question

It s easy to get the value of a key from a  NET generic Dictionary   Dictionary lt int  string gt  greek   new Dictionary lt int  string gt     greek Add 1   Alpha    greek Add 2   Beta    string secondGreek   greek 2       Beta   But trying to get the keys given a value is not as straightforward because there could be multiple keys   int   betaKeys   greek WhatDoIPutHere  Beta        expecting single 2

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Maybe the easiest way to do it  without Linq  can be to loop over the pairs   int betaKey   foreach  KeyValuePair lt int  string gt  pair in lookup        if  pair Value    value                betaKey   pair Key     Found         break          betaKey   -1     Not found   If you had Linq  it could have done easily this way   int betaKey   greek SingleOrDefault x   gt  x Value     Beta   Key

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A dictionary doesn t keep an hash of the values  only the keys  so any search over it using a value is going to take at least linear time   Your best bet is to simply iterate over the elements in the dictionary and keep track of the matching keys or switch to a different data structure  perhaps maintain two dictionary mapping key- value and value- List of keys   If you do the latter you will trade storage for look up speed   It wouldn t take much to turn  Cybis example into such a data structure

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A dictionary doesn t keep an hash of the values  only the keys  so any search over it using a value is going to take at least linear time   Your best bet is to simply iterate over the elements in the dictionary and keep track of the matching keys or switch to a different data structure  perhaps maintain two dictionary mapping key- value and value- List of keys   If you do the latter you will trade storage for look up speed   It wouldn t take much to turn  Cybis example into such a data structure

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revised  okay to have some kind of find you would need something other than dictionary  since if you think about it dictionary are one way keys   that is  the values might not be unique  that said it looks like you re using c 3 0 so you might not have to resort to looping and could use something like   var key    from k in yourDictionary where string Compare k Value   yourValue   true      0 select k Key  FirstOrDefault

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Then layman s solution A function similar to the one below could be written to make such a dictionary      public Dictionary lt TValue  TKey gt  Invert Dictionary lt TKey  TValue gt  dict        Dictionary lt TValue  TKey gt  ret   new Dictionary lt TValue  TKey gt         foreach  var kvp in dict   ret kvp value    kvp key   return ret

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Dictionary lt string  string gt  dic   new Dictionary lt string  string gt     dic  A      Ahmed   dic  B      Boys    foreach  string mk in dic Keys        if dic mk      Ahmed                 Console WriteLine  The key that contains   Ahmed   is     mk

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A dictionary doesn t keep an hash of the values  only the keys  so any search over it using a value is going to take at least linear time   Your best bet is to simply iterate over the elements in the dictionary and keep track of the matching keys or switch to a different data structure  perhaps maintain two dictionary mapping key- value and value- List of keys   If you do the latter you will trade storage for look up speed   It wouldn t take much to turn  Cybis example into such a data structure

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revised  okay to have some kind of find you would need something other than dictionary  since if you think about it dictionary are one way keys   that is  the values might not be unique  that said it looks like you re using c 3 0 so you might not have to resort to looping and could use something like   var key    from k in yourDictionary where string Compare k Value   yourValue   true      0 select k Key  FirstOrDefault

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Okay  here s the multiple bidirectional version   using System  using System Collections Generic  using System Text   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  IList lt TSecond gt  gt  firstToSecond   new Dictionary lt TFirst  IList lt TSecond gt  gt         IDictionary lt TSecond  IList lt TFirst gt  gt  secondToFirst   new Dictionary lt TSecond  IList lt TFirst gt  gt          private static IList lt TFirst gt  EmptyFirstList   new TFirst 0       private static IList lt TSecond gt  EmptySecondList   new TSecond 0        public void Add TFirst first  TSecond second                IList lt TFirst gt  firsts          IList lt TSecond gt  seconds          if   firstToSecond TryGetValue first  out seconds                         seconds   new List lt TSecond gt                 firstToSecond first    seconds                    if   secondToFirst TryGetValue second  out firsts                         firsts   new List lt TFirst gt                 secondToFirst second    firsts                    seconds Add second           firsts Add first                 Note potential ambiguity using indexers  e g  mapping from int to int         Hence the methods as well        public IList lt TSecond gt  this TFirst first                get   return GetByFirst first                public IList lt TFirst gt  this TSecond second                get   return GetBySecond second                public IList lt TSecond gt  GetByFirst TFirst first                IList lt TSecond gt  list          if   firstToSecond TryGetValue first  out list                         return EmptySecondList                    return new List lt TSecond gt  list      Create a copy for sanity            public IList lt TFirst gt  GetBySecond TSecond second                IList lt TFirst gt  list          if   secondToFirst TryGetValue second  out list                         return EmptyFirstList                    return new List lt TFirst gt  list      Create a copy for sanity          class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            greek Add 5   Beta            ShowEntries greek   Alpha            ShowEntries greek   Beta            ShowEntries greek   Gamma               static void ShowEntries BiDictionary lt int  string gt  dict  string key                IList lt int gt  values   dict key           StringBuilder builder   new StringBuilder            foreach  int value in values                        if  builder Length    0                                builder Append                                  builder Append value                     Console WriteLine   0     1     key  builder

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revised  okay to have some kind of find you would need something other than dictionary  since if you think about it dictionary are one way keys   that is  the values might not be unique  that said it looks like you re using c 3 0 so you might not have to resort to looping and could use something like   var key    from k in yourDictionary where string Compare k Value   yourValue   true      0 select k Key  FirstOrDefault

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Use LINQ to do a reverse Dictionary lt K  V gt  lookup  But keep in mind that the values in your Dictionary lt K  V gt  values may not be distinct   Demonstration   using System  using System Collections Generic  using System Linq   class ReverseDictionaryLookupDemo       static void Main                 var dict   new Dictionary lt int  string gt             dict Add 4   Four            dict Add 5   Five            dict Add 1   One            dict Add 11   One       duplicate          dict Add 3   Three            dict Add 2   Two            dict Add 44   Four       duplicate           Console WriteLine   n   Enumerating Distinct Values               foreach  string value in dict Values Distinct                          string valueString                   String Join       GetKeysFromValue dict  value                 Console WriteLine   0    gt    1     value  valueString                        static List lt int gt  GetKeysFromValue Dictionary lt int  string gt  dict  string value                   Use LINQ to do a reverse dictionary lookup             Returns a  List lt T gt   to account for the possibility            of duplicate values          return              from item in dict              where item Value Equals value               select item Key  ToList              Expected Output      Enumerating Distinct Values    Four   gt   4  44  Five   gt   5  One   gt   1  11  Three   gt   3  Two   gt   2

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Dictionaries aren t really meant to work like this  because while uniqueness of keys is guaranteed  uniqueness of values isn t  So e g  if you had  var greek   new Dictionary lt int  string gt      1   Alpha       2   Alpha         What would you expect to get for greek WhatDoIPutHere  Alpha     Therefore you can t expect something like this to be rolled into the framework  You d need your own method for your own unique uses---do you want to return an array  or IEnumerable lt T gt    Do you want to throw an exception if there are multiple keys with the given value  What about if there are none   Personally I d go for an enumerable  like so   IEnumerable lt TKey gt  KeysFromValue lt TKey  TValue gt  this Dictionary lt TKey  TValue gt  dict  TValue val        if  dict    null                throw new ArgumentNullException  dict              return dict Keys Where k   gt  dict k     val      var keys   greek KeysFromValue  Beta    int exceptionIfNotExactlyOne   greek KeysFromValue  Beta   Single

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Can t you create a subclass of Dictionary which has that functionality         public class MyDict  lt TKey  TValue  gt   Dictionary  lt TKey  TValue  gt               private Dictionary  lt TValue  TKey  gt  keys           public TValue this TKey key                        get                               return base key                             set                                 base key    value                   keys value    key                                   public MyDict                          keys   new Dictionary  lt TValue  TKey  gt                       public TKey GetKeyFromValue TValue value                        return  keys value                      EDIT  Sorry  didn t get code right first time

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Dictionary class is not optimized for this case  but if you really wanted to do it  in C  2 0   you can do   public List lt TKey gt  GetKeysFromValue lt TKey  TVal gt  Dictionary lt TKey  TVal gt  dict  TVal val       List lt TKey gt  ks   new List lt TKey gt        foreach TKey k in dict Keys             if  dict k     val    ks Add k             return ks      I prefer the LINQ solution for elegance  but this is the 2 0 way

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Dictionary class is not optimized for this case  but if you really wanted to do it  in C  2 0   you can do   public List lt TKey gt  GetKeysFromValue lt TKey  TVal gt  Dictionary lt TKey  TVal gt  dict  TVal val       List lt TKey gt  ks   new List lt TKey gt        foreach TKey k in dict Keys             if  dict k     val    ks Add k             return ks      I prefer the LINQ solution for elegance  but this is the 2 0 way

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Then layman s solution A function similar to the one below could be written to make such a dictionary      public Dictionary lt TValue  TKey gt  Invert Dictionary lt TKey  TValue gt  dict        Dictionary lt TValue  TKey gt  ret   new Dictionary lt TValue  TKey gt         foreach  var kvp in dict   ret kvp value    kvp key   return ret

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As a twist of the accepted answer  https   stackoverflow com a 255638 986160  assuming that the keys will be associated with signle values in the dictionary  Similar to  https   stackoverflow com a 255630 986160  but a bit more elegant  The novelty is in that the consuming class can be used as an enumeration alternative  but for strings too  and that the dictionary implements IEnumerable   using System  using System Collections Generic  using System Linq  using System Text  using System Collections   namespace MyApp Dictionaries        class BiDictionary lt TFirst  TSecond gt    IEnumerable               IDictionary lt TFirst  TSecond gt  firstToSecond   new Dictionary lt TFirst  TSecond gt             IDictionary lt TSecond  TFirst gt  secondToFirst   new Dictionary lt TSecond  TFirst gt              public void Add TFirst first  TSecond second                        firstToSecond Add first  second               secondToFirst Add second  first                      public TSecond this TFirst first                        get   return GetByFirst first                        public TFirst this TSecond second                        get   return GetBySecond second                        public TSecond GetByFirst TFirst first                        return firstToSecond first                      public TFirst GetBySecond TSecond second                        return secondToFirst second                      public IEnumerator GetEnumerator                         return GetFirstEnumerator                       public IEnumerator GetFirstEnumerator                         return firstToSecond GetEnumerator                       public IEnumerator GetSecondEnumerator                         return secondToFirst GetEnumerator                        And as a consuming class you could have   using System  using System Collections Generic  using System Linq  using System Text   namespace MyApp Dictionaries       class Greek                public static readonly string Alpha    Alpha           public static readonly string Beta    Beta           public static readonly string Gamma    Gamma           public static readonly string Delta    Delta             private static readonly BiDictionary lt int  string gt  Dictionary   new BiDictionary lt int  string gt               static Greek                 Dictionary Add 1  Alpha               Dictionary Add 2  Beta               Dictionary Add 3  Gamma               Dictionary Add 4  Delta                      public static string getById int id               return Dictionary GetByFirst id                      public static int getByValue string value                        return Dictionary GetBySecond value

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Many of these answers are now outdated  here is a modern C  approach  using LINQ Since values aren t necessarily unique  you may get multiple results  You can return an IEnumerable lt KeyValuePair lt int  string gt  gt   var betaKeys   greek Where x   gt  x Value     quot beta quot     To transform this into an IEnumerable lt int gt  type  just use  Select    var betaKeys   greek Where x   gt  x Value     quot beta quot   Select x   gt  x Key

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Okay  here s the multiple bidirectional version   using System  using System Collections Generic  using System Text   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  IList lt TSecond gt  gt  firstToSecond   new Dictionary lt TFirst  IList lt TSecond gt  gt         IDictionary lt TSecond  IList lt TFirst gt  gt  secondToFirst   new Dictionary lt TSecond  IList lt TFirst gt  gt          private static IList lt TFirst gt  EmptyFirstList   new TFirst 0       private static IList lt TSecond gt  EmptySecondList   new TSecond 0        public void Add TFirst first  TSecond second                IList lt TFirst gt  firsts          IList lt TSecond gt  seconds          if   firstToSecond TryGetValue first  out seconds                         seconds   new List lt TSecond gt                 firstToSecond first    seconds                    if   secondToFirst TryGetValue second  out firsts                         firsts   new List lt TFirst gt                 secondToFirst second    firsts                    seconds Add second           firsts Add first                 Note potential ambiguity using indexers  e g  mapping from int to int         Hence the methods as well        public IList lt TSecond gt  this TFirst first                get   return GetByFirst first                public IList lt TFirst gt  this TSecond second                get   return GetBySecond second                public IList lt TSecond gt  GetByFirst TFirst first                IList lt TSecond gt  list          if   firstToSecond TryGetValue first  out list                         return EmptySecondList                    return new List lt TSecond gt  list      Create a copy for sanity            public IList lt TFirst gt  GetBySecond TSecond second                IList lt TFirst gt  list          if   secondToFirst TryGetValue second  out list                         return EmptyFirstList                    return new List lt TFirst gt  list      Create a copy for sanity          class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            greek Add 5   Beta            ShowEntries greek   Alpha            ShowEntries greek   Beta            ShowEntries greek   Gamma               static void ShowEntries BiDictionary lt int  string gt  dict  string key                IList lt int gt  values   dict key           StringBuilder builder   new StringBuilder            foreach  int value in values                        if  builder Length    0                                builder Append                                  builder Append value                     Console WriteLine   0     1     key  builder

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Can t you create a subclass of Dictionary which has that functionality         public class MyDict  lt TKey  TValue  gt   Dictionary  lt TKey  TValue  gt               private Dictionary  lt TValue  TKey  gt  keys           public TValue this TKey key                        get                               return base key                             set                                 base key    value                   keys value    key                                   public MyDict                          keys   new Dictionary  lt TValue  TKey  gt                       public TKey GetKeyFromValue TValue value                        return  keys value                      EDIT  Sorry  didn t get code right first time

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The  simple  bidirectional dictionary solution proposed here is complex and may be be difficult to understand  maintain or extend  Also the original question asked for  the key for a value   but clearly there could be multiple keys  I ve since edited the question   The whole approach is rather suspicious    Software changes  Writing code that is easy to maintain should be given priority other  clever  complex workarounds  The way to get keys back from values in a dictionary is to loop  A dictionary isn t designed to be bidirectional

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Can t you create a subclass of Dictionary which has that functionality         public class MyDict  lt TKey  TValue  gt   Dictionary  lt TKey  TValue  gt               private Dictionary  lt TValue  TKey  gt  keys           public TValue this TKey key                        get                               return base key                             set                                 base key    value                   keys value    key                                   public MyDict                          keys   new Dictionary  lt TValue  TKey  gt                       public TKey GetKeyFromValue TValue value                        return  keys value                      EDIT  Sorry  didn t get code right first time

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Dictionary class is not optimized for this case  but if you really wanted to do it  in C  2 0   you can do   public List lt TKey gt  GetKeysFromValue lt TKey  TVal gt  Dictionary lt TKey  TVal gt  dict  TVal val       List lt TKey gt  ks   new List lt TKey gt        foreach TKey k in dict Keys             if  dict k     val    ks Add k             return ks      I prefer the LINQ solution for elegance  but this is the 2 0 way

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Maybe the easiest way to do it  without Linq  can be to loop over the pairs   int betaKey   foreach  KeyValuePair lt int  string gt  pair in lookup        if  pair Value    value                betaKey   pair Key     Found         break          betaKey   -1     Not found   If you had Linq  it could have done easily this way   int betaKey   greek SingleOrDefault x   gt  x Value     Beta   Key

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Okay  here s the multiple bidirectional version   using System  using System Collections Generic  using System Text   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  IList lt TSecond gt  gt  firstToSecond   new Dictionary lt TFirst  IList lt TSecond gt  gt         IDictionary lt TSecond  IList lt TFirst gt  gt  secondToFirst   new Dictionary lt TSecond  IList lt TFirst gt  gt          private static IList lt TFirst gt  EmptyFirstList   new TFirst 0       private static IList lt TSecond gt  EmptySecondList   new TSecond 0        public void Add TFirst first  TSecond second                IList lt TFirst gt  firsts          IList lt TSecond gt  seconds          if   firstToSecond TryGetValue first  out seconds                         seconds   new List lt TSecond gt                 firstToSecond first    seconds                    if   secondToFirst TryGetValue second  out firsts                         firsts   new List lt TFirst gt                 secondToFirst second    firsts                    seconds Add second           firsts Add first                 Note potential ambiguity using indexers  e g  mapping from int to int         Hence the methods as well        public IList lt TSecond gt  this TFirst first                get   return GetByFirst first                public IList lt TFirst gt  this TSecond second                get   return GetBySecond second                public IList lt TSecond gt  GetByFirst TFirst first                IList lt TSecond gt  list          if   firstToSecond TryGetValue first  out list                         return EmptySecondList                    return new List lt TSecond gt  list      Create a copy for sanity            public IList lt TFirst gt  GetBySecond TSecond second                IList lt TFirst gt  list          if   secondToFirst TryGetValue second  out list                         return EmptyFirstList                    return new List lt TFirst gt  list      Create a copy for sanity          class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            greek Add 5   Beta            ShowEntries greek   Alpha            ShowEntries greek   Beta            ShowEntries greek   Gamma               static void ShowEntries BiDictionary lt int  string gt  dict  string key                IList lt int gt  values   dict key           StringBuilder builder   new StringBuilder            foreach  int value in values                        if  builder Length    0                                builder Append                                  builder Append value                     Console WriteLine   0     1     key  builder

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Use LINQ to do a reverse Dictionary lt K  V gt  lookup  But keep in mind that the values in your Dictionary lt K  V gt  values may not be distinct   Demonstration   using System  using System Collections Generic  using System Linq   class ReverseDictionaryLookupDemo       static void Main                 var dict   new Dictionary lt int  string gt             dict Add 4   Four            dict Add 5   Five            dict Add 1   One            dict Add 11   One       duplicate          dict Add 3   Three            dict Add 2   Two            dict Add 44   Four       duplicate           Console WriteLine   n   Enumerating Distinct Values               foreach  string value in dict Values Distinct                          string valueString                   String Join       GetKeysFromValue dict  value                 Console WriteLine   0    gt    1     value  valueString                        static List lt int gt  GetKeysFromValue Dictionary lt int  string gt  dict  string value                   Use LINQ to do a reverse dictionary lookup             Returns a  List lt T gt   to account for the possibility            of duplicate values          return              from item in dict              where item Value Equals value               select item Key  ToList              Expected Output      Enumerating Distinct Values    Four   gt   4  44  Five   gt   5  One   gt   1  11  Three   gt   3  Two   gt   2

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As everyone else has said  there s no mapping within a dictionary from value to key   I ve just noticed you wanted to map to from value to multiple keys - I m leaving this solution here for the single value version  but I ll then add another answer for a multi-entry bidirectional map   The normal approach to take here is to have two dictionaries - one mapping one way and one the other  Encapsulate them in a separate class  and work out what you want to do when you have duplicate key or value  e g  throw an exception  overwrite the existing entry  or ignore the new entry   Personally I d probably go for throwing an exception - it makes the success behaviour easier to define  Something like this   using System  using System Collections Generic   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  TSecond gt  firstToSecond   new Dictionary lt TFirst  TSecond gt         IDictionary lt TSecond  TFirst gt  secondToFirst   new Dictionary lt TSecond  TFirst gt          public void Add TFirst first  TSecond second                if  firstToSecond ContainsKey first                 secondToFirst ContainsKey second                         throw new ArgumentException  Duplicate first or second                      firstToSecond Add first  second           secondToFirst Add second  first              public bool TryGetByFirst TFirst first  out TSecond second                return firstToSecond TryGetValue first  out second              public bool TryGetBySecond TSecond second  out TFirst first                return secondToFirst TryGetValue second  out first            class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            int x          greek TryGetBySecond  Beta   out x           Console WriteLine x

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A dictionary doesn t keep an hash of the values  only the keys  so any search over it using a value is going to take at least linear time   Your best bet is to simply iterate over the elements in the dictionary and keep track of the matching keys or switch to a different data structure  perhaps maintain two dictionary mapping key- value and value- List of keys   If you do the latter you will trade storage for look up speed   It wouldn t take much to turn  Cybis example into such a data structure

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As I wanted a full fledged BiDirectional Dictionary  and not only a Map   I added the missing functions to make it an IDictionary compatible class  This is based on the version with unique Key-Value Pairs  Here s the file if desired  Most work was the XMLDoc through    using System  using System Collections  using System Collections Generic  using System Linq  using System Text  using System Threading Tasks   namespace Common            lt summary gt Represents a bidirectional collection of keys and values  lt  summary gt           lt typeparam name  TFirst  gt The type of the keys in the dictionary lt  typeparam gt           lt typeparam name  TSecond  gt The type of the values in the dictionary lt  typeparam gt       System Runtime InteropServices ComVisible false        System Diagnostics DebuggerDisplay  Count    Count            System Diagnostics DebuggerTypeProxy typeof System Collections Generic Mscorlib DictionaryDebugView lt   gt            System Reflection DefaultMember  Item        public class BiDictionary lt TFirst  TSecond gt    Dictionary lt TFirst  TSecond gt                IDictionary lt TSecond  TFirst gt   ValueKey   new Dictionary lt TSecond  TFirst gt                  lt summary gt  PropertyAccessor for Iterator over KeyValue-Relation  lt  summary gt          public IDictionary lt TFirst  TSecond gt  KeyValue   gt  this               lt summary gt  PropertyAccessor for Iterator over ValueKey-Relation  lt  summary gt          public IDictionary lt TSecond  TFirst gt  ValueKey   gt   ValueKey            region Implemented members               lt Summary gt Gets or sets the value associated with the specified key  lt  Summary gt               lt param name  key  gt The key of the value to get or set  lt  param gt               lt Returns gt The value associated with the specified key  If the specified key is not found                   a get operation throws a  lt see cref  KeyNotFoundException   gt   and                  a set operation creates a new element with the specified key  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  key   gt  is null  lt  exception gt               lt exception cref  T System Collections Generic KeyNotFoundException  gt              The property is retrieved and  lt paramref name  key   gt  does not exist in the collection  lt  exception gt               lt exception cref  T System ArgumentException  gt  An element with the same key already             exists in the  lt see cref  ValueKey   gt   lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  exception gt          public new TSecond this TFirst key                        get   return base key                 set    ValueKey Remove base key    base key    value   ValueKey Add value  key                             lt Summary gt Gets or sets the key associated with the specified value  lt  Summary gt               lt param name  val  gt The value of the key to get or set  lt  param gt               lt Returns gt The key associated with the specified value  If the specified value is not found                   a get operation throws a  lt see cref  KeyNotFoundException   gt   and                  a set operation creates a new element with the specified value  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  val   gt  is null  lt  exception gt               lt exception cref  T System Collections Generic KeyNotFoundException  gt              The property is retrieved and  lt paramref name  val   gt  does not exist in the collection  lt  exception gt               lt exception cref  T System ArgumentException  gt  An element with the same value already             exists in the  lt see cref  KeyValue   gt   lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  exception gt          public TFirst this TSecond val                        get   return  ValueKey val                 set   base Remove  ValueKey val     ValueKey val    value  base Add value  val                             lt Summary gt Adds the specified key and value to the dictionary  lt  Summary gt               lt param name  key  gt The key of the element to add  lt  param gt               lt param name  value  gt The value of the element to add  lt  param gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  key   gt  or  lt paramref name  value   gt  is null  lt  exception gt               lt exception cref  T System ArgumentException  gt An element with the same key or value already exists in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  exception gt          public new void Add TFirst key  TSecond value                base Add key  value                ValueKey Add value  key                           lt Summary gt Removes all keys and values from the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Summary gt          public new void Clear     base Clear     ValueKey Clear                    lt Summary gt Determines whether the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt  contains the specified                  KeyValuePair  lt  Summary gt               lt param name  item  gt The KeyValuePair to locate in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  param gt               lt Returns gt true if the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt  contains an element with                  the specified key which links to the specified value  otherwise  false  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  item   gt  is null  lt  exception gt          public bool Contains KeyValuePair lt TFirst  TSecond gt  item    gt  base ContainsKey item Key   amp   ValueKey ContainsKey item Value                 lt Summary gt Removes the specified KeyValuePair from the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Summary gt               lt param name  item  gt The KeyValuePair to remove  lt  param gt               lt Returns gt true if the KeyValuePair is successfully found and removed  otherwise  false  This                  method returns false if  lt paramref name  item   gt  is not found in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  item   gt  is null  lt  exception gt          public bool Remove KeyValuePair lt TFirst  TSecond gt  item    gt  base Remove item Key   amp   ValueKey Remove item Value                 lt Summary gt Removes the value with the specified key from the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Summary gt               lt param name  key  gt The key of the element to remove  lt  param gt               lt Returns gt true if the element is successfully found and removed  otherwise  false  This                  method returns false if  lt paramref name  key   gt  is not found in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  key   gt  is null  lt  exception gt          public new bool Remove TFirst key    gt   ValueKey Remove base key    amp  base Remove key                 lt Summary gt Gets the key associated with the specified value  lt  Summary gt               lt param name  value  gt The value of the key to get  lt  param gt               lt param name  key  gt When this method returns  contains the key associated with the specified value                   if the value is found  otherwise  the default value for the type of the key parameter                   This parameter is passed uninitialized  lt  param gt               lt Returns gt true if  lt see cref  ValueKey   gt  contains an element with the specified value                    otherwise  false  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  value   gt  is null  lt  exception gt          public bool TryGetValue TSecond value  out TFirst key    gt   ValueKey TryGetValue value  out key            endregion

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As everyone else has said  there s no mapping within a dictionary from value to key   I ve just noticed you wanted to map to from value to multiple keys - I m leaving this solution here for the single value version  but I ll then add another answer for a multi-entry bidirectional map   The normal approach to take here is to have two dictionaries - one mapping one way and one the other  Encapsulate them in a separate class  and work out what you want to do when you have duplicate key or value  e g  throw an exception  overwrite the existing entry  or ignore the new entry   Personally I d probably go for throwing an exception - it makes the success behaviour easier to define  Something like this   using System  using System Collections Generic   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  TSecond gt  firstToSecond   new Dictionary lt TFirst  TSecond gt         IDictionary lt TSecond  TFirst gt  secondToFirst   new Dictionary lt TSecond  TFirst gt          public void Add TFirst first  TSecond second                if  firstToSecond ContainsKey first                 secondToFirst ContainsKey second                         throw new ArgumentException  Duplicate first or second                      firstToSecond Add first  second           secondToFirst Add second  first              public bool TryGetByFirst TFirst first  out TSecond second                return firstToSecond TryGetValue first  out second              public bool TryGetBySecond TSecond second  out TFirst first                return secondToFirst TryGetValue second  out first            class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            int x          greek TryGetBySecond  Beta   out x           Console WriteLine x

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As everyone else has said  there s no mapping within a dictionary from value to key   I ve just noticed you wanted to map to from value to multiple keys - I m leaving this solution here for the single value version  but I ll then add another answer for a multi-entry bidirectional map   The normal approach to take here is to have two dictionaries - one mapping one way and one the other  Encapsulate them in a separate class  and work out what you want to do when you have duplicate key or value  e g  throw an exception  overwrite the existing entry  or ignore the new entry   Personally I d probably go for throwing an exception - it makes the success behaviour easier to define  Something like this   using System  using System Collections Generic   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  TSecond gt  firstToSecond   new Dictionary lt TFirst  TSecond gt         IDictionary lt TSecond  TFirst gt  secondToFirst   new Dictionary lt TSecond  TFirst gt          public void Add TFirst first  TSecond second                if  firstToSecond ContainsKey first                 secondToFirst ContainsKey second                         throw new ArgumentException  Duplicate first or second                      firstToSecond Add first  second           secondToFirst Add second  first              public bool TryGetByFirst TFirst first  out TSecond second                return firstToSecond TryGetValue first  out second              public bool TryGetBySecond TSecond second  out TFirst first                return secondToFirst TryGetValue second  out first            class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            int x          greek TryGetBySecond  Beta   out x           Console WriteLine x

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Dictionary lt string  string gt  dic   new Dictionary lt string  string gt     dic  A      Ahmed   dic  B      Boys    foreach  string mk in dic Keys        if dic mk      Ahmed                 Console WriteLine  The key that contains   Ahmed   is     mk

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Dictionaries aren t really meant to work like this  because while uniqueness of keys is guaranteed  uniqueness of values isn t  So e g  if you had  var greek   new Dictionary lt int  string gt      1   Alpha       2   Alpha         What would you expect to get for greek WhatDoIPutHere  Alpha     Therefore you can t expect something like this to be rolled into the framework  You d need your own method for your own unique uses---do you want to return an array  or IEnumerable lt T gt    Do you want to throw an exception if there are multiple keys with the given value  What about if there are none   Personally I d go for an enumerable  like so   IEnumerable lt TKey gt  KeysFromValue lt TKey  TValue gt  this Dictionary lt TKey  TValue gt  dict  TValue val        if  dict    null                throw new ArgumentNullException  dict              return dict Keys Where k   gt  dict k     val      var keys   greek KeysFromValue  Beta    int exceptionIfNotExactlyOne   greek KeysFromValue  Beta   Single

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Dictionary class is not optimized for this case  but if you really wanted to do it  in C  2 0   you can do   public List lt TKey gt  GetKeysFromValue lt TKey  TVal gt  Dictionary lt TKey  TVal gt  dict  TVal val       List lt TKey gt  ks   new List lt TKey gt        foreach TKey k in dict Keys             if  dict k     val    ks Add k             return ks      I prefer the LINQ solution for elegance  but this is the 2 0 way

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revised  okay to have some kind of find you would need something other than dictionary  since if you think about it dictionary are one way keys   that is  the values might not be unique  that said it looks like you re using c 3 0 so you might not have to resort to looping and could use something like   var key    from k in yourDictionary where string Compare k Value   yourValue   true      0 select k Key  FirstOrDefault

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Dictionaries aren t really meant to work like this  because while uniqueness of keys is guaranteed  uniqueness of values isn t  So e g  if you had  var greek   new Dictionary lt int  string gt      1   Alpha       2   Alpha         What would you expect to get for greek WhatDoIPutHere  Alpha     Therefore you can t expect something like this to be rolled into the framework  You d need your own method for your own unique uses---do you want to return an array  or IEnumerable lt T gt    Do you want to throw an exception if there are multiple keys with the given value  What about if there are none   Personally I d go for an enumerable  like so   IEnumerable lt TKey gt  KeysFromValue lt TKey  TValue gt  this Dictionary lt TKey  TValue gt  dict  TValue val        if  dict    null                throw new ArgumentNullException  dict              return dict Keys Where k   gt  dict k     val      var keys   greek KeysFromValue  Beta    int exceptionIfNotExactlyOne   greek KeysFromValue  Beta   Single

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The  simple  bidirectional dictionary solution proposed here is complex and may be be difficult to understand  maintain or extend  Also the original question asked for  the key for a value   but clearly there could be multiple keys  I ve since edited the question   The whole approach is rather suspicious    Software changes  Writing code that is easy to maintain should be given priority other  clever  complex workarounds  The way to get keys back from values in a dictionary is to loop  A dictionary isn t designed to be bidirectional

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As I wanted a full fledged BiDirectional Dictionary  and not only a Map   I added the missing functions to make it an IDictionary compatible class  This is based on the version with unique Key-Value Pairs  Here s the file if desired  Most work was the XMLDoc through    using System  using System Collections  using System Collections Generic  using System Linq  using System Text  using System Threading Tasks   namespace Common            lt summary gt Represents a bidirectional collection of keys and values  lt  summary gt           lt typeparam name  TFirst  gt The type of the keys in the dictionary lt  typeparam gt           lt typeparam name  TSecond  gt The type of the values in the dictionary lt  typeparam gt       System Runtime InteropServices ComVisible false        System Diagnostics DebuggerDisplay  Count    Count            System Diagnostics DebuggerTypeProxy typeof System Collections Generic Mscorlib DictionaryDebugView lt   gt            System Reflection DefaultMember  Item        public class BiDictionary lt TFirst  TSecond gt    Dictionary lt TFirst  TSecond gt                IDictionary lt TSecond  TFirst gt   ValueKey   new Dictionary lt TSecond  TFirst gt                  lt summary gt  PropertyAccessor for Iterator over KeyValue-Relation  lt  summary gt          public IDictionary lt TFirst  TSecond gt  KeyValue   gt  this               lt summary gt  PropertyAccessor for Iterator over ValueKey-Relation  lt  summary gt          public IDictionary lt TSecond  TFirst gt  ValueKey   gt   ValueKey            region Implemented members               lt Summary gt Gets or sets the value associated with the specified key  lt  Summary gt               lt param name  key  gt The key of the value to get or set  lt  param gt               lt Returns gt The value associated with the specified key  If the specified key is not found                   a get operation throws a  lt see cref  KeyNotFoundException   gt   and                  a set operation creates a new element with the specified key  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  key   gt  is null  lt  exception gt               lt exception cref  T System Collections Generic KeyNotFoundException  gt              The property is retrieved and  lt paramref name  key   gt  does not exist in the collection  lt  exception gt               lt exception cref  T System ArgumentException  gt  An element with the same key already             exists in the  lt see cref  ValueKey   gt   lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  exception gt          public new TSecond this TFirst key                        get   return base key                 set    ValueKey Remove base key    base key    value   ValueKey Add value  key                             lt Summary gt Gets or sets the key associated with the specified value  lt  Summary gt               lt param name  val  gt The value of the key to get or set  lt  param gt               lt Returns gt The key associated with the specified value  If the specified value is not found                   a get operation throws a  lt see cref  KeyNotFoundException   gt   and                  a set operation creates a new element with the specified value  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  val   gt  is null  lt  exception gt               lt exception cref  T System Collections Generic KeyNotFoundException  gt              The property is retrieved and  lt paramref name  val   gt  does not exist in the collection  lt  exception gt               lt exception cref  T System ArgumentException  gt  An element with the same value already             exists in the  lt see cref  KeyValue   gt   lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  exception gt          public TFirst this TSecond val                        get   return  ValueKey val                 set   base Remove  ValueKey val     ValueKey val    value  base Add value  val                             lt Summary gt Adds the specified key and value to the dictionary  lt  Summary gt               lt param name  key  gt The key of the element to add  lt  param gt               lt param name  value  gt The value of the element to add  lt  param gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  key   gt  or  lt paramref name  value   gt  is null  lt  exception gt               lt exception cref  T System ArgumentException  gt An element with the same key or value already exists in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  exception gt          public new void Add TFirst key  TSecond value                base Add key  value                ValueKey Add value  key                           lt Summary gt Removes all keys and values from the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Summary gt          public new void Clear     base Clear     ValueKey Clear                    lt Summary gt Determines whether the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt  contains the specified                  KeyValuePair  lt  Summary gt               lt param name  item  gt The KeyValuePair to locate in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  param gt               lt Returns gt true if the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt  contains an element with                  the specified key which links to the specified value  otherwise  false  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  item   gt  is null  lt  exception gt          public bool Contains KeyValuePair lt TFirst  TSecond gt  item    gt  base ContainsKey item Key   amp   ValueKey ContainsKey item Value                 lt Summary gt Removes the specified KeyValuePair from the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Summary gt               lt param name  item  gt The KeyValuePair to remove  lt  param gt               lt Returns gt true if the KeyValuePair is successfully found and removed  otherwise  false  This                  method returns false if  lt paramref name  item   gt  is not found in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  item   gt  is null  lt  exception gt          public bool Remove KeyValuePair lt TFirst  TSecond gt  item    gt  base Remove item Key   amp   ValueKey Remove item Value                 lt Summary gt Removes the value with the specified key from the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Summary gt               lt param name  key  gt The key of the element to remove  lt  param gt               lt Returns gt true if the element is successfully found and removed  otherwise  false  This                  method returns false if  lt paramref name  key   gt  is not found in the  lt see cref  Dictionary amp lt TFirst TSecond amp gt    gt   lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  key   gt  is null  lt  exception gt          public new bool Remove TFirst key    gt   ValueKey Remove base key    amp  base Remove key                 lt Summary gt Gets the key associated with the specified value  lt  Summary gt               lt param name  value  gt The value of the key to get  lt  param gt               lt param name  key  gt When this method returns  contains the key associated with the specified value                   if the value is found  otherwise  the default value for the type of the key parameter                   This parameter is passed uninitialized  lt  param gt               lt Returns gt true if  lt see cref  ValueKey   gt  contains an element with the specified value                    otherwise  false  lt  Returns gt               lt exception cref  T System ArgumentNullException  gt  lt paramref name  value   gt  is null  lt  exception gt          public bool TryGetValue TSecond value  out TFirst key    gt   ValueKey TryGetValue value  out key            endregion

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Many of these answers are now outdated  here is a modern C  approach  using LINQ Since values aren t necessarily unique  you may get multiple results  You can return an IEnumerable lt KeyValuePair lt int  string gt  gt   var betaKeys   greek Where x   gt  x Value     quot beta quot     To transform this into an IEnumerable lt int gt  type  just use  Select    var betaKeys   greek Where x   gt  x Value     quot beta quot   Select x   gt  x Key

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Can t you create a subclass of Dictionary which has that functionality         public class MyDict  lt TKey  TValue  gt   Dictionary  lt TKey  TValue  gt               private Dictionary  lt TValue  TKey  gt  keys           public TValue this TKey key                        get                               return base key                             set                                 base key    value                   keys value    key                                   public MyDict                          keys   new Dictionary  lt TValue  TKey  gt                       public TKey GetKeyFromValue TValue value                        return  keys value                      EDIT  Sorry  didn t get code right first time

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Maybe the easiest way to do it  without Linq  can be to loop over the pairs   int betaKey   foreach  KeyValuePair lt int  string gt  pair in lookup        if  pair Value    value                betaKey   pair Key     Found         break          betaKey   -1     Not found   If you had Linq  it could have done easily this way   int betaKey   greek SingleOrDefault x   gt  x Value     Beta   Key

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As everyone else has said  there s no mapping within a dictionary from value to key   I ve just noticed you wanted to map to from value to multiple keys - I m leaving this solution here for the single value version  but I ll then add another answer for a multi-entry bidirectional map   The normal approach to take here is to have two dictionaries - one mapping one way and one the other  Encapsulate them in a separate class  and work out what you want to do when you have duplicate key or value  e g  throw an exception  overwrite the existing entry  or ignore the new entry   Personally I d probably go for throwing an exception - it makes the success behaviour easier to define  Something like this   using System  using System Collections Generic   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  TSecond gt  firstToSecond   new Dictionary lt TFirst  TSecond gt         IDictionary lt TSecond  TFirst gt  secondToFirst   new Dictionary lt TSecond  TFirst gt          public void Add TFirst first  TSecond second                if  firstToSecond ContainsKey first                 secondToFirst ContainsKey second                         throw new ArgumentException  Duplicate first or second                      firstToSecond Add first  second           secondToFirst Add second  first              public bool TryGetByFirst TFirst first  out TSecond second                return firstToSecond TryGetValue first  out second              public bool TryGetBySecond TSecond second  out TFirst first                return secondToFirst TryGetValue second  out first            class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            int x          greek TryGetBySecond  Beta   out x           Console WriteLine x

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As a twist of the accepted answer  https   stackoverflow com a 255638 986160  assuming that the keys will be associated with signle values in the dictionary  Similar to  https   stackoverflow com a 255630 986160  but a bit more elegant  The novelty is in that the consuming class can be used as an enumeration alternative  but for strings too  and that the dictionary implements IEnumerable   using System  using System Collections Generic  using System Linq  using System Text  using System Collections   namespace MyApp Dictionaries        class BiDictionary lt TFirst  TSecond gt    IEnumerable               IDictionary lt TFirst  TSecond gt  firstToSecond   new Dictionary lt TFirst  TSecond gt             IDictionary lt TSecond  TFirst gt  secondToFirst   new Dictionary lt TSecond  TFirst gt              public void Add TFirst first  TSecond second                        firstToSecond Add first  second               secondToFirst Add second  first                      public TSecond this TFirst first                        get   return GetByFirst first                        public TFirst this TSecond second                        get   return GetBySecond second                        public TSecond GetByFirst TFirst first                        return firstToSecond first                      public TFirst GetBySecond TSecond second                        return secondToFirst second                      public IEnumerator GetEnumerator                         return GetFirstEnumerator                       public IEnumerator GetFirstEnumerator                         return firstToSecond GetEnumerator                       public IEnumerator GetSecondEnumerator                         return secondToFirst GetEnumerator                        And as a consuming class you could have   using System  using System Collections Generic  using System Linq  using System Text   namespace MyApp Dictionaries       class Greek                public static readonly string Alpha    Alpha           public static readonly string Beta    Beta           public static readonly string Gamma    Gamma           public static readonly string Delta    Delta             private static readonly BiDictionary lt int  string gt  Dictionary   new BiDictionary lt int  string gt               static Greek                 Dictionary Add 1  Alpha               Dictionary Add 2  Beta               Dictionary Add 3  Gamma               Dictionary Add 4  Delta                      public static string getById int id               return Dictionary GetByFirst id                      public static int getByValue string value                        return Dictionary GetBySecond value

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Okay  here s the multiple bidirectional version   using System  using System Collections Generic  using System Text   class BiDictionary lt TFirst  TSecond gt        IDictionary lt TFirst  IList lt TSecond gt  gt  firstToSecond   new Dictionary lt TFirst  IList lt TSecond gt  gt         IDictionary lt TSecond  IList lt TFirst gt  gt  secondToFirst   new Dictionary lt TSecond  IList lt TFirst gt  gt          private static IList lt TFirst gt  EmptyFirstList   new TFirst 0       private static IList lt TSecond gt  EmptySecondList   new TSecond 0        public void Add TFirst first  TSecond second                IList lt TFirst gt  firsts          IList lt TSecond gt  seconds          if   firstToSecond TryGetValue first  out seconds                         seconds   new List lt TSecond gt                 firstToSecond first    seconds                    if   secondToFirst TryGetValue second  out firsts                         firsts   new List lt TFirst gt                 secondToFirst second    firsts                    seconds Add second           firsts Add first                 Note potential ambiguity using indexers  e g  mapping from int to int         Hence the methods as well        public IList lt TSecond gt  this TFirst first                get   return GetByFirst first                public IList lt TFirst gt  this TSecond second                get   return GetBySecond second                public IList lt TSecond gt  GetByFirst TFirst first                IList lt TSecond gt  list          if   firstToSecond TryGetValue first  out list                         return EmptySecondList                    return new List lt TSecond gt  list      Create a copy for sanity            public IList lt TFirst gt  GetBySecond TSecond second                IList lt TFirst gt  list          if   secondToFirst TryGetValue second  out list                         return EmptyFirstList                    return new List lt TFirst gt  list      Create a copy for sanity          class Test       static void Main                 BiDictionary lt int  string gt  greek   new BiDictionary lt int  string gt             greek Add 1   Alpha            greek Add 2   Beta            greek Add 5   Beta            ShowEntries greek   Alpha            ShowEntries greek   Beta            ShowEntries greek   Gamma               static void ShowEntries BiDictionary lt int  string gt  dict  string key                IList lt int gt  values   dict key           StringBuilder builder   new StringBuilder            foreach  int value in values                        if  builder Length    0                                builder Append                                  builder Append value                     Console WriteLine   0     1     key  builder

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Maybe the easiest way to do it  without Linq  can be to loop over the pairs   int betaKey   foreach  KeyValuePair lt int  string gt  pair in lookup        if  pair Value    value                betaKey   pair Key     Found         break          betaKey   -1     Not found   If you had Linq  it could have done easily this way   int betaKey   greek SingleOrDefault x   gt  x Value     Beta   Key

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Dictionaries aren t really meant to work like this  because while uniqueness of keys is guaranteed  uniqueness of values isn t  So e g  if you had  var greek   new Dictionary lt int  string gt      1   Alpha       2   Alpha         What would you expect to get for greek WhatDoIPutHere  Alpha     Therefore you can t expect something like this to be rolled into the framework  You d need your own method for your own unique uses---do you want to return an array  or IEnumerable lt T gt    Do you want to throw an exception if there are multiple keys with the given value  What about if there are none   Personally I d go for an enumerable  like so   IEnumerable lt TKey gt  KeysFromValue lt TKey  TValue gt  this Dictionary lt TKey  TValue gt  dict  TValue val        if  dict    null                throw new ArgumentNullException  dict              return dict Keys Where k   gt  dict k     val      var keys   greek KeysFromValue  Beta    int exceptionIfNotExactlyOne   greek KeysFromValue  Beta   Single

[c#] Getting multiple keys of specified value of a generic Dictionary?

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