public sealed class HashList<TKey, TValue> : VisitableHashtable<TKey, IList<TValue>>
{
    // Methods    
    public void Add(TKey key, ICollection<TValue> values);
    public void Add(TKey key, TValue value);
    public IEnumerator<TValue> GetValueEnumerator();
    public bool Remove(TValue item);
    public bool Remove(TKey key, TValue item);
    public void RemoveAll(TValue item);

    // Properties
    public int KeyCount { get; }
    public int ValueCount { get; }    
}

A HashList (or multi-dictionary) is a HashTable than can store multiple value for a specific key. It's built on the standard Dictionary class, and performs the same functions as the Dictionary<TKey, IList<TValue>> class, with prettier syntax.

More information on Hash Lists