int count = 0;
            IEnumerator<TValue> enumValues;

            if (TryEnumerateValuesForKey(key, out enumValues)) {
                using (enumValues) {
                    while (enumValues.MoveNext())
                        count += 1;
                }
            }

            return count;
        }

        /// <summary>
        /// Gets a total count of values in the collection. This default implementation
        /// is slow; it enumerates all of the keys in the dictionary and calls CountValues on each.
        /// A derived class may be able to supply a more efficient implementation.
        /// </summary>
        /// <returns>The total number of values associated with all keys in the dictionary.</returns>
        protected virtual int CountAllValues()
        {
            int count = 0;

            using (IEnumerator<TKey> enumKeys = EnumerateKeys()) {
                while (enumKeys.MoveNext()) {
                    TKey key = enumKeys.Current;
                    count += CountValues(key);
                }
            }

            return count;
        }

        /// <summary>
        /// Gets a read-only collection all the keys in this dictionary.
        /// </summary>
        /// <value>An readonly ICollection&lt;TKey&gt; of all the keys in this dictionary.</value>
        public virtual ICollection<TKey> Keys
        {
            get { return new KeysCollection(this); }
        }

        /// <summary>
        /// Gets a read-only collection of all the values in the dictionary. 
        /// </summary>
        /// <returns>A read-only ICollection&lt;TValue&gt; of all the values in the dictionary.</returns>
        public virtual ICollection<TValue> Values
        {
            get { return new ValuesCollection(this); }
        }

        /// <summary>
        /// Gets a read-only collection of all the value collections in the dictionary. 
        /// </summary>
        /// <returns>A read-only ICollection&lt;IEnumerable&lt;TValue&gt;&gt; of all the values in the dictionary.</returns>
        ICollection<ICollection<TValue>> IDictionary<TKey, ICollection<TValue>>.Values
        {
            get { return new EnumerableValuesCollection(this); }
        }

        /// <summary>
        /// Gets a read-only collection of all key-value pairs in the dictionary. If a key has multiple
        /// values associated with it, then a key-value pair is present for each value associated
        /// with the key.
        /// </summary>
        public virtual ICollection<KeyValuePair<TKey, TValue>> KeyValuePairs
        {
            get { return new KeyValuePairsCollection(this); }
        }

        /// <summary>
        /// Returns a collection of all of the values in the dictionary associated with <paramref name="key"/>,
        /// or changes the set of values associated with <paramref name="key"/>.
        /// If the key is not present in the dictionary, an ICollection enumerating no
        /// values is returned. The returned collection of values is read-write, and can be used to 
        /// modify the collection of values associated with the key.
        /// </summary>
        /// <param name="key">The key to get the values associated with.</param>
        /// <value>An ICollection&lt;TValue&gt; with all the values associated with <paramref name="key"/>.</value>
        public virtual ICollection<TValue> this[TKey key]
        {
            get
            {
                return new ValuesForKeyCollection(this, key);
            }
            set 
            {
                ReplaceMany(key, value);
            }
        }

        /// <summary>
        /// Gets a collection of all the values in the dictionary associated with <paramref name="key"/>,
        /// or changes the set of values associated with <paramref name="key"/>.
        /// If the key is not present in the dictionary, a KeyNotFound exception is thrown.
        /// </summary>
        /// <param name="key">The key to get the values associated with.</param>
        /// <value>An IEnumerable&lt;TValue&gt; that enumerates all the values associated with <paramref name="key"/>.</value>
        /// <exception cref="KeyNotFoundException">The given key is not present in the dictionary.</exception>
        ICollection<TValue> IDictionary<TKey, ICollection<TValue>>.this[TKey key] 
        {
            get
            {
                if (ContainsKey(key))
                    return new ValuesForKeyCollection(this, key);
                else
                    throw new KeyNotFoundException(Strings.KeyNotFound);
            }
            set
            {
                ReplaceMany(key, value);
            }
        }

        /// <summary>
        /// Replaces all values associated with <paramref name="key"/> with the single value <paramref name="value"/>.
        /// </summary>
        /// <remarks>This implementation simply calls Remove, followed by Add.</remarks>
        /// <param name="key">The key to associate with.</param>
        /// <param name="value">The new values to be associated with <paramref name="key"/>.</param>
        /// <returns>Returns true if some values were removed. Returns false if <paramref name="key"/> was not
        /// present in the dictionary before Replace was called.</returns>
        public virtual bool Replace(TKey key, TValue value)
        {
            bool removed = Remove(key);
            Add(key, value);
            return removed;
        }

        /// <summary>
        /// Replaces all values associated with <paramref name="key"/> with a new collection
        /// of values. If the collection does not permit duplicate values, and <paramref name="values"/> has duplicate
        /// items, then only the last of duplicates is added.
        /// </summary>
        /// <param name="key">The key to associate with.</param>
        /// <param name="values">The new values to be associated with <paramref name="key"/>.</param>
        /// <returns>Returns true if some values were removed. Returns false if <paramref name="key"/> was not
        /// present in the dictionary before Replace was called.</returns>
        public bool ReplaceMany(TKey key, IEnumerable<TValue> values)
        {
            bool removed = Remove(key);
            AddMany(key, values);
            return removed;
        }

        /// <summary>
        /// Shows the string representation of the dictionary. The string representation contains
        /// a list of the mappings in the dictionary.
        /// </summary>
        /// <returns>The string representation of the dictionary.</returns>
        public override string ToString()
        {
            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 (!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>
        /// 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 : CollectionBase<TValue>
        {