C# Cookbook

10.5. Creating a One-to-Many Map (MultiMap)

Problem

A Hashtable can map only a single key to a single value, but you need to map a key to one or more values. In addition, it may also be possible to map a key to null.

Solution

Use a Hashtable whose values are ArrayLists. This structure allows you to add multiple values (in the ArrayList) for each key of the Hashtable. The MultiMap class, which is used in practically the same manner as a Hashtable class, does this:

using System; using System.Collections; public class MultiMap { private Hashtable map = new Hashtable( ); public ArrayList this[object key] { get {return ((ArrayList)map[key]);} set {map[key] = value;} } public void Add(object key, object item) { AddSingleMap(key, item); } public void Clear( ) { map.Clear( ); } public int Count { get {return (map.Count);} } public bool ContainsKey (object key) { return (map.ContainsKey(key)); } public bool ContainsValue(object item) { if (item == null) { foreach (DictionaryEntry de in map) { if (((ArrayList)de.Value).Count == 0) { return (true); } } return (false); } else { foreach (DictionaryEntry de in map) { if (((ArrayList)de.Value).Contains(item)) { return (true); } } return (false); } } public IEnumerator GetEnumerator( ) { return (map.GetEnumerator( )); } public void Remove(object key) { RemoveSingleMap(key); } public object Clone( ) { MultiMap clone = new MultiMap( ); foreach (DictionaryEntry de in map) { clone[de.Key] = (ArrayList)((ArrayList)de.Value).Clone( ); } return (clone); ...