Bidirectional 1 to 1 Dictionary in C#

I am looking for a generic, bidirectional 1 to 1 Dictionary class in C# (2), ie. a BiDictionaryOneToOne<T, S> which is guaranteed to only contain one of each value and key (up to RefEquals anyway), and which can be searched using either key or value. Anyone know of one, or should I just implement it myself? I can't believe that I'm the first person to need this...

There is a BiDictionary in the answers to this question, but it is not for unique elements (and also does not implement RemoveByFirst(T t) or RemoveBySecond(S s)).

Thanks!

OK, here is my attempt (building on Jon's - thanks), archived here and open for improvement :

/// <summary>
/// This is a dictionary guaranteed to have only one of each value and key. 
/// It may be searched either by TFirst or by TSecond, giving a unique answer because it is 1 to 1.
/// </summary>
/// <typeparam name="TFirst">The type of the "key"</typeparam>
/// <typeparam name="TSecond">The type of the "value"</typeparam>
public class BiDictionaryOneToOne<TFirst, TSecond>
{
    IDictionary<TFirst, TSecond> firstToSecond = new Dictionary<TFirst, TSecond>();
    IDictionary<TSecond, TFirst> secondToFirst = new Dictionary<TSecond, TFirst>();

    #region Exception throwing methods

    /// <summary>
    /// Tries to add the pair to the dictionary.
    /// Throws an exception if either element is already in the dictionary
    /// </summary>
    /// <param name="first"></param>
    /// <param name="second"></param>
    public void Add(TFirst first, TSecond second)
    {
        if (firstToSecond.ContainsKey(first) || secondToFirst.ContainsKey(second))
            throw new ArgumentException("Duplicate first or second");
        
        firstToSecond.Add(first, second);
        secondToFirst.Add(second, first);
    }

    /// <summary>
    /// Find the TSecond corresponding to the TFirst first
    /// Throws an exception if first is not in the dictionary.
    /// </summary>
    /// <param name="first">the key to search for</param>
    /// <returns>the value corresponding to first</returns>
    public TSecond GetByFirst(TFirst first)
    {
        TSecond second;
        if (!firstToSecond.TryGetValue(first, out second))
            throw new ArgumentException("first");
       
        return second; 
    }

    /// <summary>
    /// Find the TFirst corresponing to the Second second.
    /// Throws an exception if second is not in the dictionary.
    /// </summary>
    /// <param name="second">the key to search for</param>
    /// <returns>the value corresponding to second</returns>
    public TFirst GetBySecond(TSecond second)
    {
        TFirst first;
        if (!secondToFirst.TryGetValue(second, out first))
            throw new ArgumentException("second");
       
        return first; 
    }


    /// <summary>
    /// Remove the record containing first.
    /// If first is not in the dictionary, throws an Exception.
    /// </summary>
    /// <param name="first">the key of the record to delete</param>
    public void RemoveByFirst(TFirst first)
    {
        TSecond second;
        if (!firstToSecond.TryGetValue(first, out second))
            throw new ArgumentException("first");

        firstToSecond.Remove(first);
        secondToFirst.Remove(second);
    }

    /// <summary>
    /// Remove the record containing second.
    /// If second is not in the dictionary, throws an Exception.
    /// </summary>
    /// <param name="second">the key of the record to delete</param>
    public void RemoveBySecond(TSecond second)
    {
        TFirst first;
        if (!secondToFirst.TryGetValue(second, out first))
            throw new ArgumentException("second");

        secondToFirst.Remove(second);
        firstToSecond.Remove(first);
    }

    #endregion

    #region Try methods

    /// <summary>
    /// Tries to add the pair to the dictionary.
    /// Returns false if either element is already in the dictionary        
    /// </summary>
    /// <param name="first"></param>
    /// <param name="second"></param>
    /// <returns>true if successfully added, false if either element are already in the dictionary</returns>
    public Boolean TryAdd(TFirst first, TSecond second)
    {
        if (firstToSecond.ContainsKey(first) || secondToFirst.ContainsKey(second))
            return false;

        firstToSecond.Add(first, second);
        secondToFirst.Add(second, first);
        return true;
    }


    /// <summary>
    /// Find the TSecond corresponding to the TFirst first.
    /// Returns false if first is not in the dictionary.
    /// </summary>
    /// <param name="first">the key to search for</param>
    /// <param name="second">the corresponding value</param>
    /// <returns>true if first is in the dictionary, false otherwise</returns>
    public Boolean TryGetByFirst(TFirst first, out TSecond second)
    {
        return firstToSecond.TryGetValue(first, out second);
    }

    /// <summary>
    /// Find the TFirst corresponding to the TSecond second.
    /// Returns false if second is not in the dictionary.
    /// </summary>
    /// <param name="second">the key to search for</param>
    /// <param name="first">the corresponding value</param>
    /// <returns>true if second is in the dictionary, false otherwise</returns>
    public Boolean TryGetBySecond(TSecond second, out TFirst first)
    {
        return secondToFirst.TryGetValue(second, out first);
    }

    /// <summary>
    /// Remove the record containing first, if there is one.
    /// </summary>
    /// <param name="first"></param>
    /// <returns> If first is not in the dictionary, returns false, otherwise true</returns>
    public Boolean TryRemoveByFirst(TFirst first)
    {
        TSecond second;
        if (!firstToSecond.TryGetValue(first, out second))
            return false;

        firstToSecond.Remove(first);
        secondToFirst.Remove(second);
        return true;
    }

    /// <summary>
    /// Remove the record containing second, if there is one.
    /// </summary>
    /// <param name="second"></param>
    /// <returns> If second is not in the dictionary, returns false, otherwise true</returns>
    public Boolean TryRemoveBySecond(TSecond second)
    {
        TFirst first;
        if (!secondToFirst.TryGetValue(second, out first))
            return false;

        secondToFirst.Remove(second);
        firstToSecond.Remove(first);
        return true;
    }

    #endregion        
    
    /// <summary>
    /// The number of pairs stored in the dictionary
    /// </summary>
    public Int32 Count
    {
        get { return firstToSecond.Count; }
    }
    
    /// <summary>
    /// Removes all items from the dictionary.
    /// </summary>
    public void Clear()
    {
        firstToSecond.Clear();
        secondToFirst.Clear();
    }
}

A more complete implementation of bidirectional dictionary:

• Supports almost all interfaces of original Dictionary<TKey,TValue> (except infrastructure interfaces):
• IDictionary<TKey, TValue>
• IReadOnlyDictionary<TKey, TValue>
• IDictionary
• ICollection<KeyValuePair<TKey, TValue>> (this one and below are the base interfaces of the ones above)
• ICollection
• IReadOnlyCollection<KeyValuePair<TKey, TValue>>
• IEnumerable<KeyValuePair<TKey, TValue>>
• IEnumerable
• Serialization using SerializableAttribute.
• Debug view using DebuggerDisplayAttribute (with Count info) and DebuggerTypeProxyAttribute (for displaying key-value pairs in watches).
• Reverse dictionary is available as IDictionary<TValue, TKey> Reverse property and also implements all interfaces mentioned above. All operations on either dictionaries modify both.

Usage:

var dic = new BiDictionary<int, string>();
dic.Add(1, "1");
dic[2] = "2";
dic.Reverse.Add("3", 3);
dic.Reverse["4"] = 4;
dic.Clear();

Code is available in my private framework on GitHub: BiDictionary(TFirst,TSecond).cs (permalink, search).

Copy:

[Serializable]
[DebuggerDisplay ("Count = {Count}"), DebuggerTypeProxy (typeof(DictionaryDebugView<,>))]
public class BiDictionary<TFirst, TSecond> : IDictionary<TFirst, TSecond>, IReadOnlyDictionary<TFirst, TSecond>, IDictionary
{
    private readonly IDictionary<TFirst, TSecond> _firstToSecond = new Dictionary<TFirst, TSecond>();
    [NonSerialized]
    private readonly IDictionary<TSecond, TFirst> _secondToFirst = new Dictionary<TSecond, TFirst>();
    [NonSerialized]
    private readonly ReverseDictionary _reverseDictionary;

    public BiDictionary ()
    {
        _reverseDictionary = new ReverseDictionary(this);
    }

    public IDictionary<TSecond, TFirst> Reverse
    {
        get { return _reverseDictionary; }
    }

    public int Count
    {
        get { return _firstToSecond.Count; }
    }

    object ICollection.SyncRoot
    {
        get { return ((ICollection)_firstToSecond).SyncRoot; }
    }

    bool ICollection.IsSynchronized
    {
        get { return ((ICollection)_firstToSecond).IsSynchronized; }
    }

    bool IDictionary.IsFixedSize
    {
        get { return ((IDictionary)_firstToSecond).IsFixedSize; }
    }

    public bool IsReadOnly
    {
        get { return _firstToSecond.IsReadOnly || _secondToFirst.IsReadOnly; }
    }

    public TSecond this [TFirst key]
    {
        get { return _firstToSecond[key]; }
        set
        {
            _firstToSecond[key] = value;
            _secondToFirst[value] = key;
        }
    }

    object IDictionary.this [object key]
    {
        get { return ((IDictionary)_firstToSecond)[key]; }
        set
        {
            ((IDictionary)_firstToSecond)[key] = value;
            ((IDictionary)_secondToFirst)[value] = key;
        }
    }

    public ICollection<TFirst> Keys
    {
        get { return _firstToSecond.Keys; }
    }

    ICollection IDictionary.Keys
    {
        get { return ((IDictionary)_firstToSecond).Keys; }
    }

    IEnumerable<TFirst> IReadOnlyDictionary<TFirst, TSecond>.Keys
    {
        get { return ((IReadOnlyDictionary<TFirst, TSecond>)_firstToSecond).Keys; }
    }

    public ICollection<TSecond> Values
    {
        get { return _firstToSecond.Values; }
    }

    ICollection IDictionary.Values
    {
        get { return ((IDictionary)_firstToSecond).Values; }
    }

    IEnumerable<TSecond> IReadOnlyDictionary<TFirst, TSecond>.Values
    {
        get { return ((IReadOnlyDictionary<TFirst, TSecond>)_firstToSecond).Values; }
    }

    public IEnumerator<KeyValuePair<TFirst, TSecond>> GetEnumerator ()
    {
        return _firstToSecond.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator ()
    {
        return GetEnumerator();
    }

    IDictionaryEnumerator IDictionary.GetEnumerator ()
    {
        return ((IDictionary)_firstToSecond).GetEnumerator();
    }

    public void Add (TFirst key, TSecond value)
    {
        _firstToSecond.Add(key, value);
        _secondToFirst.Add(value, key);
    }

    void IDictionary.Add (object key, object value)
    {
        ((IDictionary)_firstToSecond).Add(key, value);
        ((IDictionary)_secondToFirst).Add(value, key);
    }

    public void Add (KeyValuePair<TFirst, TSecond> item)
    {
        _firstToSecond.Add(item);
        _secondToFirst.Add(item.Reverse());
    }

    public bool ContainsKey (TFirst key)
    {
        return _firstToSecond.ContainsKey(key);
    }

    public bool Contains (KeyValuePair<TFirst, TSecond> item)
    {
        return _firstToSecond.Contains(item);
    }

    public bool TryGetValue (TFirst key, out TSecond value)
    {
        return _firstToSecond.TryGetValue(key, out value);
    }

    public bool Remove (TFirst key)
    {
        TSecond value;
        if (_firstToSecond.TryGetValue(key, out value)) {
            _firstToSecond.Remove(key);
            _secondToFirst.Remove(value);
            return true;
        }
        else
            return false;
    }

    void IDictionary.Remove (object key)
    {
        var firstToSecond = (IDictionary)_firstToSecond;
        if (!firstToSecond.Contains(key))
            return;
        var value = firstToSecond[key];
        firstToSecond.Remove(key);
        ((IDictionary)_secondToFirst).Remove(value);
    }

    public bool Remove (KeyValuePair<TFirst, TSecond> item)
    {
        return _firstToSecond.Remove(item);
    }

    public bool Contains (object key)
    {
        return ((IDictionary)_firstToSecond).Contains(key);
    }

    public void Clear ()
    {
        _firstToSecond.Clear();
        _secondToFirst.Clear();
    }

    public void CopyTo (KeyValuePair<TFirst, TSecond>[] array, int arrayIndex)
    {
        _firstToSecond.CopyTo(array, arrayIndex);
    }

    void ICollection.CopyTo (Array array, int index)
    {
        ((IDictionary)_firstToSecond).CopyTo(array, index);
    }

    [OnDeserialized]
    internal void OnDeserialized (StreamingContext context)
    {
        _secondToFirst.Clear();
        foreach (var item in _firstToSecond)
            _secondToFirst.Add(item.Value, item.Key);
    }

    private class ReverseDictionary : IDictionary<TSecond, TFirst>, IReadOnlyDictionary<TSecond, TFirst>, IDictionary
    {
        private readonly BiDictionary<TFirst, TSecond> _owner;

        public ReverseDictionary (BiDictionary<TFirst, TSecond> owner)
        {
            _owner = owner;
        }

        public int Count
        {
            get { return _owner._secondToFirst.Count; }
        }

        object ICollection.SyncRoot
        {
            get { return ((ICollection)_owner._secondToFirst).SyncRoot; }
        }

        bool ICollection.IsSynchronized
        {
            get { return ((ICollection)_owner._secondToFirst).IsSynchronized; }
        }

        bool IDictionary.IsFixedSize
        {
            get { return ((IDictionary)_owner._secondToFirst).IsFixedSize; }
        }

        public bool IsReadOnly
        {
            get { return _owner._secondToFirst.IsReadOnly || _owner._firstToSecond.IsReadOnly; }
        }

        public TFirst this [TSecond key]
        {
            get { return _owner._secondToFirst[key]; }
            set
            {
                _owner._secondToFirst[key] = value;
                _owner._firstToSecond[value] = key;
            }
        }

        object IDictionary.this [object key]
        {
            get { return ((IDictionary)_owner._secondToFirst)[key]; }
            set
            {
                ((IDictionary)_owner._secondToFirst)[key] = value;
                ((IDictionary)_owner._firstToSecond)[value] = key;
            }
        }

        public ICollection<TSecond> Keys
        {
            get { return _owner._secondToFirst.Keys; }
        }

        ICollection IDictionary.Keys
        {
            get { return ((IDictionary)_owner._secondToFirst).Keys; }
        }

        IEnumerable<TSecond> IReadOnlyDictionary<TSecond, TFirst>.Keys
        {
            get { return ((IReadOnlyDictionary<TSecond, TFirst>)_owner._secondToFirst).Keys; }
        }

        public ICollection<TFirst> Values
        {
            get { return _owner._secondToFirst.Values; }
        }

        ICollection IDictionary.Values
        {
            get { return ((IDictionary)_owner._secondToFirst).Values; }
        }

        IEnumerable<TFirst> IReadOnlyDictionary<TSecond, TFirst>.Values
        {
            get { return ((IReadOnlyDictionary<TSecond, TFirst>)_owner._secondToFirst).Values; }
        }

        public IEnumerator<KeyValuePair<TSecond, TFirst>> GetEnumerator ()
        {
            return _owner._secondToFirst.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator ()
        {
            return GetEnumerator();
        }

        IDictionaryEnumerator IDictionary.GetEnumerator ()
        {
            return ((IDictionary)_owner._secondToFirst).GetEnumerator();
        }

        public void Add (TSecond key, TFirst value)
        {
            _owner._secondToFirst.Add(key, value);
            _owner._firstToSecond.Add(value, key);
        }

        void IDictionary.Add (object key, object value)
        {
            ((IDictionary)_owner._secondToFirst).Add(key, value);
            ((IDictionary)_owner._firstToSecond).Add(value, key);
        }

        public void Add (KeyValuePair<TSecond, TFirst> item)
        {
            _owner._secondToFirst.Add(item);
            _owner._firstToSecond.Add(item.Reverse());
        }

        public bool ContainsKey (TSecond key)
        {
            return _owner._secondToFirst.ContainsKey(key);
        }

        public bool Contains (KeyValuePair<TSecond, TFirst> item)
        {
            return _owner._secondToFirst.Contains(item);
        }

        public bool TryGetValue (TSecond key, out TFirst value)
        {
            return _owner._secondToFirst.TryGetValue(key, out value);
        }

        public bool Remove (TSecond key)
        {
            TFirst value;
            if (_owner._secondToFirst.TryGetValue(key, out value)) {
                _owner._secondToFirst.Remove(key);
                _owner._firstToSecond.Remove(value);
                return true;
            }
            else
                return false;
        }

        void IDictionary.Remove (object key)
        {
            var firstToSecond = (IDictionary)_owner._secondToFirst;
            if (!firstToSecond.Contains(key))
                return;
            var value = firstToSecond[key];
            firstToSecond.Remove(key);
            ((IDictionary)_owner._firstToSecond).Remove(value);
        }

        public bool Remove (KeyValuePair<TSecond, TFirst> item)
        {
            return _owner._secondToFirst.Remove(item);
        }

        public bool Contains (object key)
        {
            return ((IDictionary)_owner._secondToFirst).Contains(key);
        }

        public void Clear ()
        {
            _owner._secondToFirst.Clear();
            _owner._firstToSecond.Clear();
        }

        public void CopyTo (KeyValuePair<TSecond, TFirst>[] array, int arrayIndex)
        {
            _owner._secondToFirst.CopyTo(array, arrayIndex);
        }

        void ICollection.CopyTo (Array array, int index)
        {
            ((IDictionary)_owner._secondToFirst).CopyTo(array, index);
        }
    }
}

internal class DictionaryDebugView<TKey, TValue>
{
    private readonly IDictionary<TKey, TValue> _dictionary;

    [DebuggerBrowsable (DebuggerBrowsableState.RootHidden)]
    public KeyValuePair<TKey, TValue>[] Items
    {
        get
        {
            var array = new KeyValuePair<TKey, TValue>[_dictionary.Count];
            _dictionary.CopyTo(array, 0);
            return array;
        }
    }

    public DictionaryDebugView (IDictionary<TKey, TValue> dictionary)
    {
        if (dictionary == null)
            throw new ArgumentNullException("dictionary");
        _dictionary = dictionary;
    }
}

public static class KeyValuePairExts
{
    public static KeyValuePair<TValue, TKey> Reverse<TKey, TValue> (this KeyValuePair<TKey, TValue> @this)
    {
        return new KeyValuePair<TValue, TKey>(@this.Value, @this.Key);
    }
}

The question you refer to also shows a one-to-one implementation in this answer. Adding RemoveByFirst and RemoveBySecond would be trivial - as would implementing extra interfaces etc.

This is same as accepted answer, but I provided Update methods as well, and over all little more fleshed out:

public class BiDictionary<TKey1, TKey2> : IEnumerable<Tuple<TKey1, TKey2>>
{
    Dictionary<TKey1, TKey2> _forwards;
    Dictionary<TKey2, TKey1> _reverses;

    public int Count
    {
        get
        {
            if (_forwards.Count != _reverses.Count)
                throw new Exception("somewhere logic went wrong and your data got corrupt");

            return _forwards.Count;
        }
    }

    public ICollection<TKey1> Key1s
    {
        get { return _forwards.Keys; }
    }

    public ICollection<TKey2> Key2s
    {
        get { return _reverses.Keys; }
    }

    public BiDictionary(IEqualityComparer<TKey1> comparer1 = null, IEqualityComparer<TKey2> comparer2 = null)
    {
        _forwards = new Dictionary<TKey1, TKey2>(comparer1);
        _reverses = new Dictionary<TKey2, TKey1>(comparer2);
    }



    public bool ContainsKey1(TKey1 key)
    {
        return ContainsKey(key, _forwards);
    }

    private static bool ContainsKey<S, T>(S key, Dictionary<S, T> dict)
    {
        return dict.ContainsKey(key);
    }

    public bool ContainsKey2(TKey2 key)
    {
        return ContainsKey(key, _reverses);
    }

    public TKey2 GetValueByKey1(TKey1 key)
    {
        return GetValueByKey(key, _forwards);
    }

    private static T GetValueByKey<S, T>(S key, Dictionary<S, T> dict)
    {
        return dict[key];
    }

    public TKey1 GetValueByKey2(TKey2 key)
    {
        return GetValueByKey(key, _reverses);
    }

    public bool TryGetValueByKey1(TKey1 key, out TKey2 value)
    {
        return TryGetValue(key, _forwards, out value);
    }

    private static bool TryGetValue<S, T>(S key, Dictionary<S, T> dict, out T value)
    {
        return dict.TryGetValue(key, out value);
    }

    public bool TryGetValueByKey2(TKey2 key, out TKey1 value)
    {
        return TryGetValue(key, _reverses, out value);
    }

    public bool Add(TKey1 key1, TKey2 key2)
    {
        if (ContainsKey1(key1) || ContainsKey2(key2))   // very important
            return false;

        AddOrUpdate(key1, key2);
        return true;
    }

    public void AddOrUpdateByKey1(TKey1 key1, TKey2 key2)
    {
        if (!UpdateByKey1(key1, key2))
            AddOrUpdate(key1, key2);
    }

    // dont make this public; a dangerous method used cautiously in this class
    private void AddOrUpdate(TKey1 key1, TKey2 key2)
    {
        _forwards[key1] = key2;
        _reverses[key2] = key1;
    }

    public void AddOrUpdateKeyByKey2(TKey2 key2, TKey1 key1)
    {
        if (!UpdateByKey2(key2, key1))
            AddOrUpdate(key1, key2);
    }

    public bool UpdateKey1(TKey1 oldKey, TKey1 newKey)
    {
        return UpdateKey(oldKey, _forwards, newKey, (key1, key2) => AddOrUpdate(key1, key2));
    }

    private static bool UpdateKey<S, T>(S oldKey, Dictionary<S, T> dict, S newKey, Action<S, T> updater)
    {
        T otherKey;
        if (!TryGetValue(oldKey, dict, out otherKey) || ContainsKey(newKey, dict))
            return false;

        Remove(oldKey, dict);
        updater(newKey, otherKey);
        return true;
    }

    public bool UpdateKey2(TKey2 oldKey, TKey2 newKey)
    {
        return UpdateKey(oldKey, _reverses, newKey, (key1, key2) => AddOrUpdate(key2, key1));
    }

    public bool UpdateByKey1(TKey1 key1, TKey2 key2)
    {
        return UpdateByKey(key1, _forwards, _reverses, key2, (k1, k2) => AddOrUpdate(k1, k2));
    }

    private static bool UpdateByKey<S, T>(S key1, Dictionary<S, T> forwards, Dictionary<T, S> reverses, T key2,
                                          Action<S, T> updater)
    {
        T otherKey;
        if (!TryGetValue(key1, forwards, out otherKey) || ContainsKey(key2, reverses))
            return false;

        if (!Remove(otherKey, reverses))
            throw new Exception("somewhere logic went wrong and your data got corrupt");

        updater(key1, key2);
        return true;
    }

    public bool UpdateByKey2(TKey2 key2, TKey1 key1)
    {
        return UpdateByKey(key2, _reverses, _forwards, key1, (k1, k2) => AddOrUpdate(k2, k1));
    }

    public bool RemoveByKey1(TKey1 key)
    {
        return RemoveByKey(key, _forwards, _reverses);
    }

    private static bool RemoveByKey<S, T>(S key, Dictionary<S, T> keyDict, Dictionary<T, S> valueDict)
    {
        T otherKey;
        if (!TryGetValue(key, keyDict, out otherKey))
            return false;

        if (!Remove(key, keyDict) || !Remove(otherKey, valueDict))
            throw new Exception("somewhere logic went wrong and your data got corrupt");

        return true;
    }

    private static bool Remove<S, T>(S key, Dictionary<S, T> dict)
    {
        return dict.Remove(key);
    }

    public bool RemoveByKey2(TKey2 key)
    {
        return RemoveByKey(key, _reverses, _forwards);
    }

    public void Clear()
    {
        _forwards.Clear();
        _reverses.Clear();
    }

    public IEnumerator<Tuple<TKey1, TKey2>> GetEnumerator()
    {
        if (_forwards.Count != _reverses.Count)
            throw new Exception("somewhere logic went wrong and your data got corrupt");

        foreach (var item in _forwards)
            yield return Tuple.Create(item.Key, item.Value);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Similar to my answer here

Few things to note:

1. I have implemented only IEnumerable<>. I don't think ICollection<> makes sense here since the method names all could be way different for this special collection structure. Up to you to decide what should go inside IEnumerable<>. So now you have collection initializer syntax too, like

    var p = new BiDictionary<int, string> { 1, "a" }, { 2, "b" } };
   

2. I have attempted for some weird exceptions to be thrown here and there - just for data integrity. Just to be on the safer side so that you know if ever my code has bugs.

3. Performance: You can lookup for Value with either of the Keys, which means Get and Contains method require just 1 lookup (O(1)). Add requires 2 lookups and 2 adds. Update requires 1 lookup and 2 adds. Remove takes 3 lookups. All similar to accepted answer.

I have created such a class, using C5 collection classes.

public class Mapper<K,T> : IEnumerable<T>

{
    C5.TreeDictionary<K,T> KToTMap = new TreeDictionary<K,T>();
    C5.HashDictionary<T,K> TToKMap = new HashDictionary<T,K>();

    
    /// <summary>
    /// Initializes a new instance of the Mapper class.
    /// </summary>
    public Mapper()
    {
        KToTMap = new TreeDictionary<K,T>();
        TToKMap = new HashDictionary<T,K>();
    }


    public void Add(K key, T value)
    {
        KToTMap.Add(key, value);
        TToKMap.Add(value, key);
    }

    public bool ContainsKey(K key)
    {
        return KToTMap.Contains(key);
    }

    public int Count
    {
        get { return KToTMap.Count; }
    }
    

    public K this[T obj]
    {
        get
        {
            return TToKMap[obj];
        }
    }

    public T this[K obj]
    {
        get
        {
            return KToTMap[obj];
        }
    }

    public IEnumerator<T> GetEnumerator()
    {
        return KToTMap.Values.GetEnumerator();
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return KToTMap.Values.GetEnumerator();
    }
}

Another extension to the accepted answer. It implements IEnumerable so one can use foreach with that. I realize there are more answers with IEnumerable implementation but this one uses structs so it is garbage collector friendly. This is especially usefull in Unity engine (checked with the profiler).

/// <summary>
/// This is a dictionary guaranteed to have only one of each value and key. 
/// It may be searched either by TFirst or by TSecond, giving a unique answer because it is 1 to 1.
/// It implements garbage-collector-friendly IEnumerable.
/// </summary>
/// <typeparam name="TFirst">The type of the "key"</typeparam>
/// <typeparam name="TSecond">The type of the "value"</typeparam>
public class BiDictionary<TFirst, TSecond> : IEnumerable<BiDictionary<TFirst, TSecond>.Pair>
{


	public struct Pair
	{
		public TFirst  First;
		public TSecond Second;
	}


	public struct Enumerator : IEnumerator<Pair>, IEnumerator
	{

		public Enumerator(Dictionary<TFirst, TSecond>.Enumerator dictEnumerator)
		{
			_dictEnumerator = dictEnumerator;
		}

		public Pair Current
		{
			get
			{
				Pair pair;
				pair.First = _dictEnumerator.Current.Key;
				pair.Second = _dictEnumerator.Current.Value;
				return pair;
			}
		}

		object IEnumerator.Current
		{
			get
			{
				return Current;
			}
		}

		public void Dispose()
		{
			_dictEnumerator.Dispose();
		}

		public bool MoveNext()
		{
			return _dictEnumerator.MoveNext();
		}

		public void Reset()
		{
			throw new NotSupportedException();
		}

		private Dictionary<TFirst, TSecond>.Enumerator _dictEnumerator;
		
	}

	#region Exception throwing methods

	/// <summary>
	/// Tries to add the pair to the dictionary.
	/// Throws an exception if either element is already in the dictionary
	/// </summary>
	/// <param name="first"></param>
	/// <param name="second"></param>
	public void Add(TFirst first, TSecond second)
	{
		if (_firstToSecond.ContainsKey(first) || _secondToFirst.ContainsKey(second))
			throw new ArgumentException("Duplicate first or second");

		_firstToSecond.Add(first, second);
		_secondToFirst.Add(second, first);
	}

	/// <summary>
	/// Find the TSecond corresponding to the TFirst first
	/// Throws an exception if first is not in the dictionary.
	/// </summary>
	/// <param name="first">the key to search for</param>
	/// <returns>the value corresponding to first</returns>
	public TSecond GetByFirst(TFirst first)
	{
		TSecond second;
		if (!_firstToSecond.TryGetValue(first, out second))
			throw new ArgumentException("first");

		return second;
	}

	/// <summary>
	/// Find the TFirst corresponing to the Second second.
	/// Throws an exception if second is not in the dictionary.
	/// </summary>
	/// <param name="second">the key to search for</param>
	/// <returns>the value corresponding to second</returns>
	public TFirst GetBySecond(TSecond second)
	{
		TFirst first;
		if (!_secondToFirst.TryGetValue(second, out first))
			throw new ArgumentException("second");

		return first;
	}


	/// <summary>
	/// Remove the record containing first.
	/// If first is not in the dictionary, throws an Exception.
	/// </summary>
	/// <param name="first">the key of the record to delete</param>
	public void RemoveByFirst(TFirst first)
	{
		TSecond second;
		if (!_firstToSecond.TryGetValue(first, out second))
			throw new ArgumentException("first");

		_firstToSecond.Remove(first);
		_secondToFirst.Remove(second);
	}

	/// <summary>
	/// Remove the record containing second.
	/// If second is not in the dictionary, throws an Exception.
	/// </summary>
	/// <param name="second">the key of the record to delete</param>
	public void RemoveBySecond(TSecond second)
	{
		TFirst first;
		if (!_secondToFirst.TryGetValue(second, out first))
			throw new ArgumentException("second");

		_secondToFirst.Remove(second);
		_firstToSecond.Remove(first);
	}

	#endregion

	#region Try methods

	/// <summary>
	/// Tries to add the pair to the dictionary.
	/// Returns false if either element is already in the dictionary        
	/// </summary>
	/// <param name="first"></param>
	/// <param name="second"></param>
	/// <returns>true if successfully added, false if either element are already in the dictionary</returns>
	public bool TryAdd(TFirst first, TSecond second)
	{
		if (_firstToSecond.ContainsKey(first) || _secondToFirst.ContainsKey(second))
			return false;

		_firstToSecond.Add(first, second);
		_secondToFirst.Add(second, first);
		return true;
	}


	/// <summary>
	/// Find the TSecond corresponding to the TFirst first.
	/// Returns false if first is not in the dictionary.
	/// </summary>
	/// <param name="first">the key to search for</param>
	/// <param name="second">the corresponding value</param>
	/// <returns>true if first is in the dictionary, false otherwise</returns>
	public bool TryGetByFirst(TFirst first, out TSecond second)
	{
		return _firstToSecond.TryGetValue(first, out second);
	}

	/// <summary>
	/// Find the TFirst corresponding to the TSecond second.
	/// Returns false if second is not in the dictionary.
	/// </summary>
	/// <param name="second">the key to search for</param>
	/// <param name="first">the corresponding value</param>
	/// <returns>true if second is in the dictionary, false otherwise</returns>
	public bool TryGetBySecond(TSecond second, out TFirst first)
	{
		return _secondToFirst.TryGetValue(second, out first);
	}

	/// <summary>
	/// Remove the record containing first, if there is one.
	/// </summary>
	/// <param name="first"></param>
	/// <returns> If first is not in the dictionary, returns false, otherwise true</returns>
	public bool TryRemoveByFirst(TFirst first)
	{
		TSecond second;
		if (!_firstToSecond.TryGetValue(first, out second))
			return false;

		_firstToSecond.Remove(first);
		_secondToFirst.Remove(second);
		return true;
	}

	/// <summary>
	/// Remove the record containing second, if there is one.
	/// </summary>
	/// <param name="second"></param>
	/// <returns> If second is not in the dictionary, returns false, otherwise true</returns>
	public bool TryRemoveBySecond(TSecond second)
	{
		TFirst first;
		if (!_secondToFirst.TryGetValue(second, out first))
			return false;

		_secondToFirst.Remove(second);
		_firstToSecond.Remove(first);
		return true;
	}

	#endregion        

	/// <summary>
	/// The number of pairs stored in the dictionary
	/// </summary>
	public Int32 Count
	{
		get { return _firstToSecond.Count; }
	}

	/// <summary>
	/// Removes all items from the dictionary.
	/// </summary>
	public void Clear()
	{
		_firstToSecond.Clear();
		_secondToFirst.Clear();
	}


	public Enumerator GetEnumerator()
	{
		//enumerator.Reset(firstToSecond.GetEnumerator());
		return new Enumerator(_firstToSecond.GetEnumerator());
	}

	IEnumerator<Pair> IEnumerable<Pair>.GetEnumerator()
	{
		return GetEnumerator();
	}

	IEnumerator IEnumerable.GetEnumerator()
	{
		return GetEnumerator();
	}



	private Dictionary<TFirst, TSecond> _firstToSecond  = new Dictionary<TFirst, TSecond>();
	private Dictionary<TSecond, TFirst> _secondToFirst  = new Dictionary<TSecond, TFirst>();

}

A bit late, but here's an implementation I wrote a while back. It handles a few interesting edge cases, such as when the key overrides the equality check to perform partial equality. This results in the main dictionary storing A => 1 but the inverse storing 1 => A'.

You access the inverse dictionary via the Inverse property.

var map = new BidirectionalDictionary<int, int>();
map.Add(1, 2);
var result = map.Inverse[2]; // result is 1

---

//
// BidirectionalDictionary.cs
//
// Author:
//   Chris Chilvers <[email protected]>
//
// Copyright (c) 2009 Chris Chilvers
//
// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//

using System;
using System.Collections;
using System.Collections.Generic;

namespace Cadenza.Collections
{
    public class BidirectionalDictionary<TKey, TValue> : IDictionary<TKey, TValue>
    {
        private readonly IEqualityComparer<TKey> keyComparer;
        private readonly IEqualityComparer<TValue> valueComparer;
        private readonly Dictionary<TKey, TValue> keysToValues;
        private readonly Dictionary<TValue, TKey> valuesToKeys;
        private readonly BidirectionalDictionary<TValue, TKey> inverse;


        public BidirectionalDictionary () : this (10, null, null) {}

        public BidirectionalDictionary (int capacity) : this (capacity, null, null) {}

        public BidirectionalDictionary (IEqualityComparer<TKey> keyComparer, IEqualityComparer<TValue> valueComparer)
            : this (10, keyComparer, valueComparer)
        {
        }

        public BidirectionalDictionary (int capacity, IEqualityComparer<TKey> keyComparer, IEqualityComparer<TValue> valueComparer)
        {
            if (capacity < 0)
                throw new ArgumentOutOfRangeException ("capacity", capacity, "capacity cannot be less than 0");

            this.keyComparer = keyComparer ?? EqualityComparer<TKey>.Default;
            this.valueComparer = valueComparer ?? EqualityComparer<TValue>.Default;

            keysToValues = new Dictionary<TKey, TValue> (capacity, this.keyComparer);
            valuesToKeys = new Dictionary<TValue, TKey> (capacity, this.valueComparer);

            inverse = new BidirectionalDictionary<TValue, TKey> (this);
        }

        private BidirectionalDictionary (BidirectionalDictionary<TValue, TKey> inverse)
        {
            this.inverse = inverse;
            keyComparer = inverse.valueComparer;
            valueComparer = inverse.keyComparer;
            valuesToKeys = inverse.keysToValues;
            keysToValues = inverse.valuesToKeys;
        }


        public BidirectionalDictionary<TValue, TKey> Inverse {
            get { return inverse; }
        }


        public ICollection<TKey> Keys {
            get { return keysToValues.Keys; }
        }

        public ICollection<TValue> Values {
            get { return keysToValues.Values; }
        }

        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator ()
        {
            return keysToValues.GetEnumerator ();
        }

        IEnumerator IEnumerable.GetEnumerator ()
        {
            return GetEnumerator ();
        }

        void ICollection<KeyValuePair<TKey, TValue>>.CopyTo (KeyValuePair<TKey, TValue>[] array, int arrayIndex)
        {
            ((ICollection<KeyValuePair<TKey, TValue>>) keysToValues).CopyTo (array, arrayIndex);
        }


        public bool ContainsKey (TKey key)
        {
            if (key == null)
                throw new ArgumentNullException ("key");
            return keysToValues.ContainsKey (key);
        }

        public bool ContainsValue (TValue value)
        {
            if (value == null)
                throw new ArgumentNullException ("value");
            return valuesToKeys.ContainsKey (value);
        }

        bool ICollection<KeyValuePair<TKey, TValue>>.Contains (KeyValuePair<TKey, TValue> item)
        {
            return ((ICollection<KeyValuePair<TKey, TValue>>) keysToValues).Contains (item);
        }

        public bool TryGetKey (TValue value, out TKey key)
        {
            if (value == null)
                throw new ArgumentNullException ("value");
            return valuesToKeys.TryGetValue (value, out key);
        }

        public bool TryGetValue (TKey key, out TValue value)
        {
            if (key == null)
                throw new ArgumentNullException ("key");
            return keysToValues.TryGetValue (key, out value);
        }

        public TValue this[TKey key] {
            get { return keysToValues [key]; }
            set {
                if (key == null)
                    throw new ArgumentNullException ("key");
                if (value == null)
                    throw new ArgumentNullException ("value");

                //foo[5] = "bar"; foo[6] = "bar"; should not be valid
                //as it would have to remove foo[5], which is unexpected.
                if (ValueBelongsToOtherKey (key, value))
                    throw new ArgumentException ("Value already exists", "value");

                TValue oldValue;
                if (keysToValues.TryGetValue (key, out oldValue)) {
                    // Use the current key for this value to stay consistent
                    // with Dictionary<TKey, TValue> which does not alter
                    // the key if it exists.
                    TKey oldKey = valuesToKeys [oldValue];

                    keysToValues [oldKey] = value;
                    valuesToKeys.Remove (oldValue);
                    valuesToKeys [value] = oldKey;
                } else {
                    keysToValues [key] = value;
                    valuesToKeys [value] = key;
                }
            }
        }

        public int Count {
            get { return keysToValues.Count; }
        }

        bool ICollection<KeyValuePair<TKey, TValue>>.IsReadOnly {
            get { return false; }
        }


        public void Add (TKey key, TValue value)
        {
            if (key == null)
                throw new ArgumentNullException ("key");
            if (value == null)
                throw new ArgumentNullException ("value");

            if (keysToValues.ContainsKey (key))
                throw new ArgumentException ("Key already exists", "key");
            if (valuesToKeys.ContainsKey (value))
                throw new ArgumentException ("Value already exists", "value");

            keysToValues.Add (key, value);
            valuesToKeys.Add (value, key);
        }

        public void Replace (TKey key, TValue value)
        {
            if (key == null)
                throw new ArgumentNullException ("key");
            if (value == null)
                throw new ArgumentNullException ("value");

            // replaces a key value pair, if the key or value already exists those mappings will be replaced.
            // e.g. you have; a -> b, b -> a; c -> d, d -> c
            // you add the mapping; a -> d, d -> a
            // this will remove both of the original mappings
            Remove (key);
            inverse.Remove (value);
            Add (key, value);
        }

        void ICollection<KeyValuePair<TKey, TValue>>.Add (KeyValuePair<TKey, TValue> item)
        {
            Add (item.Key, item.Value);
        }

        public bool Remove (TKey key)
        {
            if (key == null)
                throw new ArgumentNullException ("key");

            TValue value;
            if (keysToValues.TryGetValue (key, out value)) {
                keysToValues.Remove (key);
                valuesToKeys.Remove (value);
                return true;
            }
            else {
                return false;
            }
        }

        bool ICollection<KeyValuePair<TKey, TValue>>.Remove (KeyValuePair<TKey, TValue> item)
        {
            bool removed = ((ICollection<KeyValuePair<TKey, TValue>>) keysToValues).Remove (item);
            if (removed)
                valuesToKeys.Remove (item.Value);
            return removed;
        }

        public void Clear ()
        {
            keysToValues.Clear ();
            valuesToKeys.Clear ();
        }


        private bool ValueBelongsToOtherKey (TKey key, TValue value)
        {
            TKey otherKey;
            if (valuesToKeys.TryGetValue (value, out otherKey))
                // if the keys are not equal the value belongs to another key
                return !keyComparer.Equals (key, otherKey);
            else
                // value doesn't exist in map, thus it cannot belong to another key
                return false;
        }
    }
}

Original source and tests on github.