multimap in .NET

Question

I need an equivalent to c++'s std::multimap<K, V, Comp, Alloc> in C-sharp. Does it exist in the standard library?

Answer 1

Because it's almost christmas :)

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//////////////////////////////////////////////////////////////////////
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using SD.Tools.Algorithmia.UtilityClasses;

namespace SD.Tools.Algorithmia.GeneralDataStructures
{
    /// <summary>
    /// Extension to the normal Dictionary. This class can store more than one value for every key. It keeps a HashSet for every Key value.
    /// Calling Add with the same Key and multiple values will store each value under the same Key in the Dictionary. Obtaining the values
    /// for a Key will return the HashSet with the Values of the Key. 
    /// </summary>
    /// <typeparam name="TKey">The type of the key.</typeparam>
    /// <typeparam name="TValue">The type of the value.</typeparam>
    public class MultiValueDictionary<TKey, TValue> : Dictionary<TKey, HashSet<TValue>>
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="MultiValueDictionary&lt;TKey, TValue&gt;"/> class.
        /// </summary>
        public MultiValueDictionary()
            : base()
        {
        }


        /// <summary>
        /// Adds the specified value under the specified key
        /// </summary>
        /// <param name="key">The key.</param>
        /// <param name="value">The value.</param>
        public void Add(TKey key, TValue value)
        {
            ArgumentVerifier.CantBeNull(key, "key");

            HashSet<TValue> container = null;
            if(!this.TryGetValue(key, out container))
            {
                container = new HashSet<TValue>();
                base.Add(key, container);
            }
            container.Add(value);
        }


        /// <summary>
        /// Determines whether this dictionary contains the specified value for the specified key 
        /// </summary>
        /// <param name="key">The key.</param>
        /// <param name="value">The value.</param>
        /// <returns>true if the value is stored for the specified key in this dictionary, false otherwise</returns>
        public bool ContainsValue(TKey key, TValue value)
        {
            ArgumentVerifier.CantBeNull(key, "key");
            bool toReturn = false;
            HashSet<TValue> values = null;
            if(this.TryGetValue(key, out values))
            {
                toReturn = values.Contains(value);
            }
            return toReturn;
        }


        /// <summary>
        /// Removes the specified value for the specified key. It will leave the key in the dictionary.
        /// </summary>
        /// <param name="key">The key.</param>
        /// <param name="value">The value.</param>
        public void Remove(TKey key, TValue value)
        {
            ArgumentVerifier.CantBeNull(key, "key");

            HashSet<TValue> container = null;
            if(this.TryGetValue(key, out container))
            {
                container.Remove(value);
                if(container.Count <= 0)
                {
                    this.Remove(key);
                }
            }
        }


        /// <summary>
        /// Merges the specified multivaluedictionary into this instance.
        /// </summary>
        /// <param name="toMergeWith">To merge with.</param>
        public void Merge(MultiValueDictionary<TKey, TValue> toMergeWith)
        { 
            if(toMergeWith==null)
            {
                return;
            }

            foreach(KeyValuePair<TKey, HashSet<TValue>> pair in toMergeWith)
            {
                foreach(TValue value in pair.Value)
                {
                    this.Add(pair.Key, value);
                }
            }
        }


        /// <summary>
        /// Gets the values for the key specified. This method is useful if you want to avoid an exception for key value retrieval and you can't use TryGetValue
        /// (e.g. in lambdas)
        /// </summary>
        /// <param name="key">The key.</param>
        /// <param name="returnEmptySet">if set to true and the key isn't found, an empty hashset is returned, otherwise, if the key isn't found, null is returned</param>
        /// <returns>
        /// This method will return null (or an empty set if returnEmptySet is true) if the key wasn't found, or
        /// the values if key was found.
        /// </returns>
        public HashSet<TValue> GetValues(TKey key, bool returnEmptySet)
        {
            HashSet<TValue> toReturn = null;
            if(!base.TryGetValue(key, out toReturn) && returnEmptySet)
            {
                toReturn = new HashSet<TValue>();
            }
            return toReturn;
        }
    }
}

Answer 2

ILookup may be good enough for you - but unfortunately there are no public implementations. You've basically got to go through Enumerable.ToLookup, as far as I'm aware. That means it's a "build once" kind of map - you can't add to it later. However, if that's all you need it's handy to be able to just use the built-in stuff.

Answer 3

In .NET 3.5, there is an ILookup<,> that represents this. The regular implementation (Lookup<,>) is immutable, but I have an EditableLookup<,> in MiscUtil that should do the job nicely.

Answer 4

you can find a c# multimap collection @ http://code.google.com/p/self-balancing-avl-tree/ . it's based on a self balancing avl tree which is also included with the code.

Answer 5

you can try this

C# MultiMap Generic Class

http://dotnetperls.com/multimap

Very simple multimap implementation.

Answer 6

Consider using BMultiMap<K,V> if your collection as a whole is large. It saves a lot of memory compared to Dictionary<K,List<V>>, especially if the number of values associated with each key is often small.