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

It doesn't seem to but there's an implementation here

Answer 2

Because it's almost christmas :)

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// http://www.sd.nl
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//////////////////////////////////////////////////////////////////////
// Contributers to the code:
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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 3

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 4

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 5

you can try this

C# MultiMap Generic Class

http://dotnetperls.com/multimap

Very simple multimap implementation.