builder.Append("}");
            return builder.ToString();
        }

        /// <summary>
        /// Display the contents of the dictionary in the debugger. This is intentionally private, it is called
        /// only from the debugger due to the presence of the DebuggerDisplay attribute. It is similar
        /// format to ToString(), but is limited to 250-300 characters or so, so as not to overload the debugger.
        /// </summary>
        /// <returns>The string representation of the items in the collection, similar in format to ToString().</returns>
        new internal string DebuggerDisplayString()
        {
            const int MAXLENGTH = 250;

            bool firstItem = true;

            System.Text.StringBuilder builder = new System.Text.StringBuilder();

            builder.Append("{");

            // Call ToString on each item and put it in.
            foreach (KeyValuePair<TKey, ICollection<TValue>> pair in this) {
                if (builder.Length >= MAXLENGTH) {
                    builder.Append(", ...");
                    break;
                }

                if (!firstItem)
                    builder.Append(", ");

                if (pair.Key == null)
                    builder.Append("null");
                else
                    builder.Append(pair.Key.ToString());

                builder.Append("->");

                // Put all values in a parenthesized list.
                builder.Append('(');

                bool firstValue = true;
                foreach (TValue val in pair.Value) {
                    if (!firstValue)
                        builder.Append(",");

                    if (val == null)
                        builder.Append("null");
                    else
                        builder.Append(val.ToString());

                    firstValue = false;
                }

                builder.Append(')');

                firstItem = false;
            }

            builder.Append("}");
            return builder.ToString();
        }


        /// <summary>
        /// Enumerate all the keys in the dictionary, and for each key, the collection of values for that key.
        /// </summary>
        /// <returns>An enumerator to enumerate all the key, ICollection&lt;value&gt; pairs in the dictionary.</returns>
        public override IEnumerator<KeyValuePair<TKey, ICollection<TValue>>> GetEnumerator()
        {
            using (IEnumerator<TKey> enumKeys = EnumerateKeys()) {
                while (enumKeys.MoveNext()) {
                    TKey key = enumKeys.Current;
                    yield return new KeyValuePair<TKey, ICollection<TValue>>(key, new ValuesForKeyCollection(this, key));
                }
            }
        }


        #region Keys and Values collections
        /// <summary>
        /// A private class that provides the ICollection&lt;TValue&gt; for a particular key. This is the collection
        /// that is returned from the indexer. The collections is read-write, live, and can be used to add, remove,
        /// etc. values from the multi-dictionary.
        /// </summary>
        [Serializable]
        private sealed class ValuesForKeyCollection : ReadOnlyCollectionBase<TValue>
        {
            private ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary;
            private TKey key;

            /// <summary>
            /// Constructor. Initializes this collection.
            /// </summary>
            /// <param name="myDictionary">Dictionary we're using.</param>
            /// <param name="key">The key we're looking at.</param>
            public ValuesForKeyCollection(ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary, TKey key)
            {
                this.myDictionary = myDictionary;
                this.key = key;
            }

            /// <summary>
            /// Get the number of values associated with the key.
            /// </summary>
            public override int Count
            {
                get
                {
                    return myDictionary.CountValues(key);
                }
            }

            /// <summary>
            /// A simple function that returns an IEnumerator&lt;TValue&gt; that
            /// doesn't yield any values. A helper.
            /// </summary>
            /// <returns>An IEnumerator&lt;TValue&gt; that yields no values.</returns>
            private IEnumerator<TValue> NoValues()
            {
                yield break;
            }

            /// <summary>
            /// Enumerate all the values associated with key.
            /// </summary>
            /// <returns>An IEnumerator&lt;TValue&gt; that enumerates all the values associated with key.</returns>
            public override IEnumerator<TValue> GetEnumerator()
            {
                IEnumerator<TValue> values;
                if (myDictionary.TryEnumerateValuesForKey(key, out values))
                    return values;
                else
                    return NoValues();
            }

            /// <summary>
            /// Determines if the given values is associated with key.
            /// </summary>
            /// <param name="item">Value to check for.</param>
            /// <returns>True if value is associated with key, false otherwise.</returns>
            public override bool Contains(TValue item)
            {
                return myDictionary.Contains(key, item);
            }
        }


        /// <summary>
        /// A private class that implements ICollection&lt;TKey&gt; and ICollection for the
        /// Keys collection. The collection is read-only.
        /// </summary>
        [Serializable]
        private sealed class KeysCollection : ReadOnlyCollectionBase<TKey>
        {
            private ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary;

            /// <summary>
            /// Constructor.
            /// </summary>
            /// <param name="myDictionary">The dictionary this is associated with.</param>
            public KeysCollection(ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary)
            {
                this.myDictionary = myDictionary;
            }

            public override int Count
            {
                get { return myDictionary.Count; }
            }

            public override IEnumerator<TKey> GetEnumerator()
            {
                return myDictionary.EnumerateKeys();
            }

            public override bool Contains(TKey key)
            {
                return myDictionary.ContainsKey(key);
            }
        }

        /// <summary>
        /// A private class that implements ICollection&lt;TValue&gt; and ICollection for the
        /// Values collection. The collection is read-only.
        /// </summary>
        [Serializable]
        private sealed class ValuesCollection : ReadOnlyCollectionBase<TValue>
        {
            private ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary;

            public ValuesCollection(ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary)
            {
                this.myDictionary = myDictionary;
            }

            public override int Count
            {
                get { return myDictionary.CountAllValues(); }
            }

            public override IEnumerator<TValue> GetEnumerator()
            {
                using (IEnumerator<TKey> enumKeys = myDictionary.EnumerateKeys()) {
                    while (enumKeys.MoveNext()) {
                        TKey key = enumKeys.Current;
                        IEnumerator<TValue> enumValues;
                        if (myDictionary.TryEnumerateValuesForKey(key, out enumValues)) {
                            using (enumValues) {
                                while (enumValues.MoveNext())
                                    yield return enumValues.Current;
                            }
                        }
                    }
                }
            }

            public override bool Contains(TValue value)
            {
                foreach (TValue v in this) {
                    if (myDictionary.EqualValues(v, value))
                        return true;
                }
                return false;
            }
        }

        /// <summary>
        /// A private class that implements ICollection&lt;IEnumerable&lt;TValue&gt;&gt; and ICollection for the
        /// Values collection on IDictionary. The collection is read-only.
        /// </summary>
        [Serializable]
        private sealed class EnumerableValuesCollection : ReadOnlyCollectionBase<ICollection<TValue>>
        {
            private ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary;

            public EnumerableValuesCollection(ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary)
            {
                this.myDictionary = myDictionary;
            }

            public override int Count
            {
                get { return myDictionary.Count; }
            }

            public override IEnumerator<ICollection<TValue>> GetEnumerator()
            {
                using (IEnumerator<TKey> enumKeys = myDictionary.EnumerateKeys()) {
                    while (enumKeys.MoveNext()) {
                        TKey key = enumKeys.Current;
                        yield return new ValuesForKeyCollection(myDictionary, key);
                    }
                }
            }

            public override bool Contains(ICollection<TValue> values)
            {
                if (values == null)
                    return false;
                TValue[] valueArray = Algorithms.ToArray(values);

                foreach (ICollection<TValue> v in this) {
                    if (v.Count != valueArray.Length)
                        continue;

                    // First check in order for efficiency.
                    if (Algorithms.EqualCollections(v, values, myDictionary.EqualValues))
                        return true;

                    // Now check not in order. We can't use Algorithms.EqualSets, because we don't 
                    // have an IEqualityComparer, just the ability to compare for equality. Unfortunately this is N squared,
                    // but there isn't a good choice here. We don't really expect this method to be used much.
                    bool[] found = new bool[valueArray.Length];
                    foreach (TValue x in v) {
                        for (int i = 0; i < valueArray.Length; ++i) {
                            if (!found[i] && myDictionary.EqualValues(x, valueArray[i])) 
                                found[i] = true;
                        }
                    }

                    if (Array.IndexOf(found, false) < 0)
                        return true;  // every item was found. The sets must be equal.
                }
                return false;
            }
        }

        /// <summary>
        /// A private class that implements ICollection&lt;KeyValuePair&lt;TKey,TValue&gt;&gt; and ICollection for the
        /// KeyValuePairs collection. The collection is read-only.
        /// </summary>
        [Serializable]
        private sealed class KeyValuePairsCollection : ReadOnlyCollectionBase<KeyValuePair<TKey, TValue>>
        {
            private ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary;

            public KeyValuePairsCollection(ReadOnlyMultiDictionaryBase<TKey, TValue> myDictionary)
            {
                this.myDictionary = myDictionary;
            }

            public override int Count
            {
                get { return myDictionary.CountAllValues(); }
            }

            public override IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
            {
                using (IEnumerator<TKey> enumKeys = myDictionary.EnumerateKeys()) {
                    while (enumKeys.MoveNext()) {
                        TKey key = enumKeys.Current;
                        IEnumerator<TValue> enumValues;
                        if (myDictionary.TryEnumerateValuesForKey(key, out enumValues)) {
                            using (enumValues) {
                                while (enumValues.MoveNext())
                                    yield return new KeyValuePair<TKey, TValue>(key, enumValues.Current);
                            }
                        }
                    }
                }
            }

            public override bool Contains(KeyValuePair<TKey, TValue> pair)
            {
                return myDictionary[pair.Key].Contains(pair.Value);
            }
        }

        #endregion



    }

}