Как получить моментальный снимок ConcurrentDictionary в С#?

Просто вызовите метод ToArray().

Вот исходный код:

    /// <summary>
    /// Copies the key and value pairs stored in the <see cref="ConcurrentDictionary{TKey,TValue}"/> to a
    /// new array.
    /// </summary>
    /// <returns>A new array containing a snapshot of key and value pairs copied from the <see
    /// cref="ConcurrentDictionary{TKey,TValue}"/>.</returns>
    [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
    public KeyValuePair<TKey, TValue>[] ToArray()
    {
        int locksAcquired = 0;
        try
        {
            AcquireAllLocks(ref locksAcquired);
            int count = 0;
            checked
            {
                for (int i = 0; i < m_tables.m_locks.Length; i++)
                {
                    count += m_tables.m_countPerLock[i];
                }
            }

            KeyValuePair<TKey, TValue>[] array = new KeyValuePair<TKey, TValue>[count];

            CopyToPairs(array, 0);
            return array;
        }
        finally
        {
            ReleaseLocks(0, locksAcquired);
        }
    }