C# dictionary - one key, many values

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 one key with many values, so a bit like a one-to-many relationship.

I think the dictionary only has one key value.

How else could I store this information?

As of .NET 3.5+, instead of using a Dictionary<IKey, List<IValue>>, you can use a Lookup from the LINQ namespace:

// Lookup Order by payment status (1:m)
// would need something like Dictionary<Boolean, IEnumerable<Order>> orderIdByIsPayed
ILookup<Boolean, Order> byPayment = orderList.ToLookup(o => o.IsPayed);
IEnumerable<Order> 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 information, 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<Order>();
            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<Int32, Order> orders = orderList.ToDictionary(o => o.OrderId, o => o);

            // Lookup Order by customer (1:n)
            // would need something like Dictionary<Int32, IEnumerable<Order>> orderIdByCustomer
            ILookup<Int32, Order> byCustomerId = orderList.ToLookup(o => 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<Int32, List<Order>> orderIdByCustomer;
            orderIdByCustomer = byCustomerId.ToDictionary(g => g.Key, g => 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<Boolean, IEnumerable<Order>> orderIdByIsPayed
            ILookup<Boolean, Order> byPayment = orderList.ToLookup(o => o.IsPayed);
            IEnumerable<Order> 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<OrderItem> _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).

You can use a list for the second generic type. For example a dictionary of strings keyed by a string:

Dictionary<string, List<string>> myDict;

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.

Use this:

Dictionary<TKey, Tuple<TValue1, TValue2, TValue3, ...>>

Your dictionary's value type could be a List, or other class that holds multiple objects. Something like

Dictionary<int, List<string>>

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.

You could use a Dictionary<TKey, List<TValue>>.

That would allow each key to reference a list of values.

Use a dictionary of lists (or another type of collection), for example:

var myDictionary = new Dictionary<string, IList<int>>();

myDictionary["My key"] = new List<int> {1, 2, 3, 4, 5};

You can create a very simplistic multi-dictionary, which automates to process of inserting values like this:

public class MultiDictionary<TKey, TValue> : Dictionary<TKey, List<TValue>>
{
    public void Add(TKey key, TValue value)
    {
        if (TryGetValue(key, out List<TValue> valueList)) {
            valueList.Add(value);
        } else {
            Add(key, new List<TValue> { 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.

You can also base it on a Dictionary<TKey, HashSet<TValue>> instead, if you don't want to have duplicate values.

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.

A .NET dictionary does only have a one-to-one 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 one-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 one-to-one relationship, but you have the value class storing more that one piece of data.

You can also use;

 List<KeyValuePair<string, string>> Mappings;

Take a look at MultiValueDictionary from Microsoft.

Example Code:

MultiValueDictionary<string, string> Parameters = new MultiValueDictionary<string, string>();

Parameters.Add("Malik", "Ali");
Parameters.Add("Malik", "Hamza");
Parameters.Add("Malik", "Danish");

//Parameters["Malik"] now contains the values Ali, Hamza, and Danish

Here's my approach to achieve this behavior.

For a more comprehensive solution involving ILookup<TKey, TElement>, check out my other answer.

public abstract class Lookup<TKey, TElement> : KeyedCollection<TKey, ICollection<TElement>>
{
  protected override TKey GetKeyForItem(ICollection<TElement> item) =>
    item
    .Select(b => GetKeyForItem(b))
    .Distinct()
    .SingleOrDefault();

  protected abstract TKey GetKeyForItem(TElement item);

  public void Add(TElement item)
  {
    var key = GetKeyForItem(item);
    if (Dictionary != null && Dictionary.TryGetValue(key, out var collection))
      collection.Add(item);
    else
      Add(new List<TElement> { item });
  }

  public void Remove(TElement item)
  {
    var key = GetKeyForItem(item);
    if (Dictionary != null && 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<string, Item>
{
  protected override string GetKeyForItem(Item item) => 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).

You can declare an dictionary with <T,T[]> type, (when T = any type you want) When you initialize the values of dictionary items, declare an array each key.

For Example:

 `Dictionary<int, string[]> dictionaty  = new Dictionary<int, string[]>() {
                {1, new string[]{"a","b","c"} },
                {2, new string[]{"222","str"} }
            }; `

The proper solution is to have a Dictionary<TKey1, TKey2, TValue>, where 2 keys are needed to access a certain item. Solutions using Dictionary<TKey, List<TValue>> will create as many lists as there are unique values for TKey, which takes a lot of memory and slows down the performance. The other problem when having only 1 key is that it becomes difficult to remove one particular item.

Since I couldn't find such a class, I wrote one myself:

  public class SortedBucketCollectionClass<TKey1, TKey2, TValue>:
    IEnumerable<TValue>, ICollection<TValue>, 
    IReadOnlySortedBucketCollection<TKey1, TKey2, TValue>
    where TKey1 : notnull, IComparable<TKey1>
    where TKey2 : notnull, IComparable<TKey2>
    where TValue : class {...}

It supports access with only TKey1, which returns an enumerator over all items having TKey1 and access with TKey1, TKEy2, which returns a particular item. There are also enumerators over all stored items and one that enumerates all items with a certain range of TKey. This is convenient, when TKey1 is DateTime and one wants all items from a certain week, month or year.

I wrote a detailed article on CodeProject with code samples: SortedBucketCollection: A memory efficient SortedList accepting multiple items with the same key

You can get the source code on CodeProject or Github: StorageLib/StorageLib/SortedBucketCollection.cs