c dictionary How to add multiple values for single key

Question

I have created dictionary object   Dictionary lt string  List lt string gt  gt  dictionary       new Dictionary lt string List lt string gt  gt       I want to add string values to the list of string for a given single key  If the key doesn t already exists then I have to add a new key  List lt string gt  is not predefined  I mean I didn t create any list object and then supplied to dictionary Add  key  Listname   How to create dynamically this list object in dictionary Add  key  Listname  and then add strings to this list  If I have to add 100 keys then do I have to create 100 lists before executing dictionary Add instruction and also do I have to pedefine the contents of this lists    Thank you

User · Answer

Use NameValuedCollection    Good starting point is here  Straight from the link   System Collections Specialized NameValueCollection myCollection       new System Collections Specialized NameValueCollection       myCollection Add    Arcane        http   arcanecode com        myCollection Add    PWOP        http   dotnetrocks com        myCollection Add    PWOP        http   dnrtv com        myCollection Add    PWOP        http   www hanselminutes com        myCollection Add    TWIT        http   www twit tv        myCollection Add    TWIT        http   www twit tv SN

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Update  check for existence using TryGetValue to do only one lookup in the case where you have the list   List lt int gt  list   if   dictionary TryGetValue  foo   out list         list   new List lt int gt         dictionary Add  foo   list      list Add 2      Original  Check for existence and add once  then key into the dictionary to get the list and add to the list as normal   var dictionary   new Dictionary lt string  List lt int gt  gt      if   dictionary ContainsKey  foo        dictionary Add  foo   new List lt int gt       dictionary  foo   Add 42   dictionary  foo   AddRange oneHundredInts     Or List lt string gt  as in your case   As an aside  if you know how many items you are going to add to a dynamic collection such as List lt T gt   favour the constructor that takes the initial list capacity  new List lt int gt  100     This will grab the memory required to satisfy the specified capacity upfront  instead of grabbing small chunks every time it starts to fill up  You can do the same with dictionaries if you know you have 100 keys

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I was trying to add List to existing key in dictionary and reached the following solution   Dictionary lt string List lt string gt  gt  NewParent   new Dictionary lt string List lt string gt  gt     child   new List lt string gt      child Add  SomeData    NewParent  item1   AddRange child     It will not show any exception and won t replace previous values

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Though nearly the same as most of the other responses  I think this is the most efficient and concise way to implement it   Using TryGetValue is faster than using ContainsKey and reindexing into the dictionary as some other solutions have shown   void Add string key  string val        List lt string gt  list       if   dictionary TryGetValue someKey  out list                values   new List lt string gt            dictionary Add key  list              list Add val

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You could use my implementation of a multimap  which derives from a Dictionary lt K  List lt V gt  gt   It is not perfect  however it does a good job        lt summary gt      Represents a collection of keys and values      Multiple values can have the same key       lt  summary gt       lt typeparam name  TKey  gt Type of the keys  lt  typeparam gt       lt typeparam name  TValue  gt Type of the values  lt  typeparam gt  public class MultiMap lt TKey  TValue gt    Dictionary lt TKey  List lt TValue gt  gt         public MultiMap             base                    public MultiMap int capacity            base capacity                        lt summary gt          Adds an element with the specified key and value into the MultiMap            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      public void Add TKey key  TValue value                List lt TValue gt  valueList           if  TryGetValue key  out valueList                 valueList Add value             else               valueList   new List lt TValue gt                 valueList Add value               Add key  valueList                             lt summary gt          Removes first occurence of an element with a specified key and value           lt  summary gt           lt param name  key  gt The key of the element to remove  lt  param gt           lt param name  value  gt The value of the element to remove  lt  param gt           lt returns gt true if the an element is removed          false if the key or the value were not found  lt  returns gt      public bool Remove TKey key  TValue value                List lt TValue gt  valueList           if  TryGetValue key  out valueList                 if  valueList Remove value                     if  valueList Count    0                        Remove key                                     return true                                  return false                  lt summary gt          Removes all occurences of elements with a specified key and value           lt  summary gt           lt param name  key  gt The key of the elements to remove  lt  param gt           lt param name  value  gt The value of the elements to remove  lt  param gt           lt returns gt Number of elements removed  lt  returns gt      public int RemoveAll TKey key  TValue value                List lt TValue gt  valueList          int n   0           if  TryGetValue key  out valueList                 while  valueList Remove value                     n                              if  valueList Count    0                    Remove key                                   return n                  lt summary gt          Gets the total number of values contained in the MultiMap           lt  summary gt      public int CountAll               get                       int n   0               foreach  List lt TValue gt  valueList in Values                    n    valueList Count                            return n                            lt summary gt          Determines whether the MultiMap contains an element with a specific         key   value pair           lt  summary gt           lt param name  key  gt Key of the element to search for  lt  param gt           lt param name  value  gt Value of the element to search for  lt  param gt           lt returns gt true if the element was found  otherwise false  lt  returns gt      public bool Contains TKey key  TValue value                List lt TValue gt  valueList           if  TryGetValue key  out valueList                 return valueList Contains value                     return false                  lt summary gt          Determines whether the MultiMap contains an element with a specific value           lt  summary gt           lt param name  value  gt Value of the element to search for  lt  param gt           lt returns gt true if the element was found  otherwise false  lt  returns gt      public bool Contains TValue value                foreach  List lt TValue gt  valueList in Values                if  valueList Contains value                     return true                                  return false             Note that the Add method looks if a key is already present  If the key is new  a new list is created  the value is added to the list and the list is added to the dictionary  If the key was already present  the new value is added to the existing list

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If I understood what you want   dictionary Add  key   new List lt string gt         later      dictionary  key   Add  string to your list

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When you add a string  do it differently depending on whether the key exists already or not  To add the string value for the key key   List lt string gt  list  if  dictionary ContainsKey key       list   dictionary key     else     list   new List lt string gt       dictionary Add ley  list     list Add value

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There is a NuGet package Microsoft Experimental Collections that contains a class MultiValueDictionary which does exactly what you need    Here is a blog post of the creator of the package that describes it further   Here is another blog post if you re feeling curious   Example Usage   MultiDictionary lt string  int gt  myDictionary   new MultiDictionary lt string  int gt     myDictionary Add  key   1   myDictionary Add  key   2   myDictionary Add  key   3     myDictionary  key   now contains the values 1  2  and 3

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Here are many variations of the one answer    My is another one and it uses extension mechanism as comfortable way to execute  handy    public static void AddToList lt T  U gt  this IDictionary lt T  List lt U gt  gt  dict  T key  U elementToList         List lt U gt  list       bool exists   dict TryGetValue key  out list        if  exists                dict key  Add elementToList             else               dict key    new List lt U gt             dict key  Add elementToList              Then you use it as follows   Dictionary lt int  List lt string gt  gt  dict   new Dictionary lt int  List lt string gt  gt      dict AddToList 4   test1    dict AddToList 4   test2    dict AddToList 4   test3     dict AddToList 5   test4

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Instead of using a Dictionary  why not convert to an ILookup   var myData   new   new  a 1 b  frog    new  a 1 b  cat    new  a 2 b  giraffe     ILookup lt int string gt  lookup   myData ToLookup x   gt  x a  x   gt  x b   IEnumerable lt string gt  allOnes   lookup 1     enumerable of 2 items  frog and cat   An ILookup is an immutable data structure that allows multiple values per key  Probably not much use if you need to add items at different times  but if you have all your data up-front  this is definitely the way to go

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Dictionary lt string  List lt string gt  gt  dictionary   new Dictionary lt string List lt string gt  gt      foreach string key in keys        if  dictionary ContainsKey key               add         dictionary Add key  new List lt string gt                dictionary key  Add  theString        If the key doesn t exist  a new List is added  inside if   Else the key exists  so just add a new value to the List under that key

[c#] c# dictionary How to add multiple values for single key?

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