c dictionary one key many values

Question

I want to create a data store to allow me to store some data   The first idea was to create a dictionary where you have 1 key with many values  so a bit like a one to many relationship   I think the dictionary only has 1 key value   How else could I store this information

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Dictionary lt int  string   gt  dictionaty    new Dictionary lt int  string   gt                   1  new string    a   b   c                   2  new string    222   str

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A  NET dictionary does only have a 1-to-1 relationship for keys and values  But that doesn t mean that a value can t be another array list dictionary   I can t think of a reason to have a 1 to many relationship in a dictionary  but obviously there is one   If you have different types of data that you want to store to a key  then that sounds like the ideal time to create your own class  Then you have a 1 to 1  but you have the value class storing more that 1 piece of data

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Your dictionary s value type could be a List  or other class that holds multiple objects   Something like   Dictionary lt int  List lt string gt  gt     for a Dictionary that is keyed by ints and holds a List of strings   A main consideration in choosing the value type is what you ll be using the Dictionary for  if you ll have to do searching or other operations on the values  then maybe think about using a data structure that helps you do what you want -- like a HashSet

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As of  net3 5  instead of using a Dictionary lt IKey  List lt IValue gt  gt  you can use a Lookup from the Linq namespace      lookup Order by payment status  1 m      would need something like Dictionary lt Boolean  IEnumerable lt Order gt  gt  orderIdByIsPayed ILookup lt Boolean  Order gt  byPayment   orderList ToLookup o   gt  o IsPayed   IEnumerable lt Order gt  payedOrders   byPayment false     From msdn      A Lookup resembles a Dictionary  The   difference is that a Dictionary maps keys to single   values  whereas a Lookup maps keys to collections of   values       You can create an instance of a Lookup by calling   ToLookup on an object that implements IEnumerable    You may also want to read this answer to a related question  For more info  consult msdn   Full example   using System  using System Collections Generic  using System Linq   namespace LinqLookupSpike       class Program               static void Main String   args                           init              var orderList   new List lt Order gt                 orderList Add new Order 1  1  2010  true      orderId  customerId  year  isPayed              orderList Add new Order 2  2  2010  true                orderList Add new Order 3  1  2010  true                orderList Add new Order 4  2  2011  true                orderList Add new Order 5  2  2011  false                orderList Add new Order 6  1  2011  true                orderList Add new Order 7  3  2012  false                    lookup Order by its id  1 1  so usual dictionary is ok              Dictionary lt Int32  Order gt  orders   orderList ToDictionary o   gt  o OrderId  o   gt  o                   lookup Order by customer  1 n                  would need something like Dictionary lt Int32  IEnumerable lt Order gt  gt  orderIdByCustomer             ILookup lt Int32  Order gt  byCustomerId   orderList ToLookup o   gt  o CustomerId               foreach  var customerOrders in byCustomerId                                Console WriteLine  Customer  0  ordered    customerOrders Key                   foreach  var order in customerOrders                                        Console WriteLine      Order  0  is payed   1    order OrderId  order IsPayed                                                   the same using old fashioned Dictionary             Dictionary lt Int32  List lt Order gt  gt  orderIdByCustomer              orderIdByCustomer   byCustomerId ToDictionary g   gt  g Key  g   gt  g ToList                 foreach  var customerOrders in orderIdByCustomer                                Console WriteLine  Customer  0  ordered    customerOrders Key                   foreach  var order in customerOrders Value                                        Console WriteLine      Order  0  is payed   1    order OrderId  order IsPayed                                                   lookup Order by payment status  1 m                  would need something like Dictionary lt Boolean  IEnumerable lt Order gt  gt  orderIdByIsPayed             ILookup lt Boolean  Order gt  byPayment   orderList ToLookup o   gt  o IsPayed               IEnumerable lt Order gt  payedOrders   byPayment false               foreach  var payedOrder in payedOrders                                Console WriteLine  Order  0  from Customer  1  is not payed    payedOrder OrderId  payedOrder CustomerId                                    class Order                          key properties             public Int32 OrderId   get  private set                public Int32 CustomerId   get  private set                public Int32 Year   get  private set                public Boolean IsPayed   get  private set                    additional properties                private List lt OrderItem gt   items               public Order Int32 orderId  Int32 customerId  Int32 year  Boolean isPayed                                OrderId   orderId                  CustomerId   customerId                  Year   year                  IsPayed   isPayed                                    Remark on Immutability  By default  Lookups are kind of immutable and accessing the internals would involve reflection   If you need mutability and don t want to write your own wrapper  you could use MultiValueDictionary  formerly known as MultiDictionary  from corefxlab  formerly part ofMicrosoft Experimental Collections which isn t updated anymore

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You can have a dictionary with a collection  or any other type class  as a value  That way you have a single key and you store the values in your collection

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Microsoft just added an official prelease version of exactly what you re looking for  called a MultiDictionary  available through NuGet here  https   www nuget org packages Microsoft Experimental Collections   Info on usage and more details can be found through the official MSDN blog post here  http   blogs msdn com b dotnet archive 2014 06 20 would-you-like-a-multidictionary aspx  I m the developer for this package  so let me know either here or on MSDN if you have any questions about performance or anything   Hope that helps   Update  The MultiValueDictionary is now on the corefxlab repo  and you can get the NuGet package from this MyGet feed

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You can also use    List lt KeyValuePair lt string  string gt  gt  Mappings

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Use a dictionary of lists  or another type of collection   for example   var myDictionary   new Dictionary lt string  IList lt int gt  gt      myDictionary  My key     new List lt int gt   1  2  3  4  5

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You can use a list for the second generic type  For example a dictionary of strings keyed by a string   Dictionary lt string  List lt string gt  gt  myDict

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Use this   Dictionary lt TKey  Tuple lt TValue1  TValue2  TValue3      gt  gt

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Here s my approach to achieve this behavior   For a more comprehensive solution involving ILookup lt TKey  TElement gt   check out my other answer   public abstract class Lookup lt TKey  TElement gt    KeyedCollection lt TKey  ICollection lt TElement gt  gt      protected override TKey GetKeyForItem ICollection lt TElement gt  item    gt      item      Select b   gt  GetKeyForItem b        Distinct        SingleOrDefault       protected abstract TKey GetKeyForItem TElement item      public void Add TElement item          var key   GetKeyForItem item       if  Dictionary    null  amp  amp  Dictionary TryGetValue key  out var collection         collection Add item       else       Add new List lt TElement gt    item            public void Remove TElement item          var key   GetKeyForItem item       if  Dictionary    null  amp  amp  Dictionary TryGetValue key  out var collection               collection Remove item         if  collection Count    0          Remove key                 Usage   public class Item     public string Key   get      public string Value   get  set      public Item string key  string value   null    Key   key  Value   value       public class Lookup   Lookup lt string  Item gt      protected override string GetKeyForItem Item item    gt  item Key     static void Main string   args      var toRem   new Item  1    different      var single   new Item  2    single      var lookup   new Lookup           new Item  1    hello        new Item  1    hello2        new Item          new Item      helloo        toRem      single         lookup Remove toRem     lookup Remove single       Note  the key must be immutable  or remove and re-add upon key-change

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You can create a very simplistic multi-dictionary  which automates to process of inserting values like this   public class MultiDictionary lt TKey  TValue gt    Dictionary lt TKey  List lt TValue gt  gt        public void Add TKey key  TValue value                if  TryGetValue key  out List lt TValue gt  valueList                 valueList Add value             else               Add key  new List lt TValue gt    value                         This creates an overloaded version of the Add method  The original one allows you to insert a list of items for a key  if no entry for this entry exists yet  This version allows you to insert a single item in any case

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You could use a Dictionary lt TKey  List lt TValue gt  gt    That would allow each key to reference a list of values

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Take a look at MultiValueDictionary from Microsoft   Example Code    MultiValueDictionary lt string  string gt  Parameters   new MultiValueDictionary lt string  string gt      Parameters Add  Malik    Ali    Parameters Add  Malik    Hamza    Parameters Add  Malik    Danish       Parameters  Malik   now contains the values Ali  Hamza  and Danish

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