class Dictionary Dictionary ContainsKey Dictionary TryGetValue Generic Indexer List

`MultiMap`

A MultiMap has multiple values at a key. With it we add multiple values to a single key in a Dictionary. In C#, MultiMaps can be useful.

We see an example implementation of the MultiMap associative array in the C# language. We then use the class. This is not an ideal implementation—but it works.

Example

The base class library does not provide a MultiMap. This type is a Dictionary with values of type List. Here we show a MultiMap facade class.

Info The MultiMap here uses a string key and any type of value. We add objects at specific keys.

Note The MultiMap provides a simple Add method. This method is implemented with TryGetValue.

Next To continue, we have code add the value to the List. The value is always stored at its key, even with duplicates.

Also The MultiMap exposes Keys. You will want to enumerate over Keys in your code.

using System;
using System.Collections.Generic;

public class MultiMap<V>
{
    Dictionary<string, List<V>> _dictionary = new Dictionary<string, List<V>>();

    public void Add(string key, V value)
    {
        // Add a key.
        List<V> list;
        if (this._dictionary.TryGetValue(key, out list))
        {
            list.Add(value);
        }
        else
        {
            list = new List<V>();
            list.Add(value);
            this._dictionary[key] = list;
        }
    }

    public IEnumerable<string> Keys
    {
        get
        {
            // Get all keys.
            return this._dictionary.Keys;
        }
    }

    public List<V> this[string key]
    {
        get
        {
            // Get list at a key.
            List<V> list;
            if (!this._dictionary.TryGetValue(key, out list))
            {
                list = new List<V>();
                this._dictionary[key] = list;
            }
            return list;
        }
    }
}

class Program
{
    static void Main()
    {
        // Create first MultiMap.
        var multiMap = new MultiMap<bool>();
        multiMap.Add("key1", true);
        multiMap.Add("key1", false);
        multiMap.Add("key2", false);

        foreach (string key in multiMap.Keys)
        {
            foreach (bool value in multiMap[key])
            {
                Console.WriteLine("MULTIMAP: " + key + "=" + value);
            }
        }

        // Create second MultiMap.
        var multiMap2 = new MultiMap<string>();
        multiMap2.Add("animal", "cat");
        multiMap2.Add("animal", "dog");
        multiMap2.Add("human", "tom");
        multiMap2.Add("human", "tim");
        multiMap2.Add("mineral", "calcium");

        foreach (string key in multiMap2.Keys)
        {
            foreach (string value in multiMap2[key])
            {
                Console.WriteLine("MULTIMAP2: " + key + "=" + value);
            }
        }
    }
}MULTIMAP: key1=True
MULTIMAP: key1=False
MULTIMAP: key2=False
MULTIMAP2: animal=cat
MULTIMAP2: animal=dog
MULTIMAP2: human=tom
MULTIMAP2: human=tim
MULTIMAP2: mineral=calcium

Notes, indexer

The MultiMap provides an indexer. This provides natural and short syntax for accessing the MultiMap. It returns an empty List if nothing exists.

Notes, example programs

The Main() method shows how to use the MultiMap to store bool and string values. It is fairly simple to figure out, and can be extended in many ways.

We saw an implementation of the MultiMap class in the C# language. The facade pattern here simplifies Dictionary and wraps it with MultiMap features.