7

MSDN states that the enumerator returned from the ConcurrentDictionary does not represent a moment-in-time snapshot of ConcurrentDictionary.

Although it will be rarely needed in multi-threaded environment, but if one wants, what is the best way to get the moment-in-time snapshot of ConcurrentDictionary?

Alexander Abakumov
  • 13,617
  • 16
  • 88
  • 129
Ramy
  • 305
  • 1
  • 5
  • 10
  • 2
    How do you define a "moment-in-time" snapshot for a collection that can be changed on multiple threads in parallel? Iterating through the collection to gather the items that make it up takes time and during that time it may already be modified. It's not impossible to do this but it's quite complex and `ConcurrentDictionary` doesn't do it. – xxbbcc May 11 '17 at 20:29
  • 1
    @xxbbcc see the `GetEnumerator()` code for `ConcurrentBag`, `ConcurrentQueue` or `ConcurrentStack`, they all provide a moment in time snapshot of the enumerator. – Scott Chamberlain May 11 '17 at 20:31
  • [`ToArray`](http://referencesource.microsoft.com/#mscorlib/system/Collections/Concurrent/ConcurrentDictionary.cs,edad672303ee9ee3,references)? – Ivan Stoev May 11 '17 at 20:31
  • From [the documentation](https://msdn.microsoft.com/en-us/library/dd287144(v=vs.110).aspx) *"The enumeration represents a moment-in-time snapshot of the contents of the queue. It does not reflect any updates to the collection after GetEnumerator was called. The enumerator is safe to use concurrently with reads from and writes to the queue."* – Scott Chamberlain May 11 '17 at 20:32
  • @xxbbcc to get a non snapshot version you have to wrap the collection in a `BlockingCollection` then call `.GetConsumingEnumerable()` on it. It provides a enumerable with live data and removes items from the collection as items are returned to the enumerable. – Scott Chamberlain May 11 '17 at 20:35
  • @ScottChamberlain Those enumerators need to prevent mutations to the collection while they're being enumerated. Since there is no way for a consumer, trying to get a snapshot of the concurrent dictionary, to prevent anyone else from mutating it while we iterate it, we can't get such a snapshot. – Servy May 11 '17 at 20:35
  • 1
    @ScottChamberlain Your BlockignCollection proposal still wouldn't give you a snapshot in time. After pulling out one item some other user of the dictionary could remove an item (or add in a new one) messing up your iterator. – Servy May 11 '17 at 20:36
  • @Servy i don't follow your comment. the 3 classes I mention that perform a snapshot all allow adds and removes to the collection while you are enumerating the enumerable, you just don't see those adds or removes in the IEnumerable. also I never said BlockingCollection worked with ConcurrentDictionary or that it gave you a snapshot version – Scott Chamberlain May 11 '17 at 20:37
  • 1
    @IvanStoev For ConcurrentDictionary it doesn't, for other types it does. That was Scott's point. – Servy May 11 '17 at 20:37
  • @Servy why not for Concurrent Dictionary, it if does for other types? – Ramy May 11 '17 at 20:53
  • 1
    @Ramy Feel free to ask the developers of the class why the designed it the way they did. – Servy May 11 '17 at 20:54
  • `Although it will be rarely needed in multi-threaded environment`: this statement reads as completely opposite to requirements to 95% of use cases we might have. Why? Suppose we're iterating over `ConcurrentDictionary` and have passed an item with a key 3. Now, a concurrent thread replaces the item with the key 3. I've immediately got a Q: is it possible that we could iterate over the replaced item once again during the rest of our iteration? Or `ConcurrentDictionary`'s iterator guarantees a single iteration over the same key in case of the concurrent modifications? – Alexander Abakumov Aug 26 '23 at 19:49
  • I'd employ just the opposite rule in multi-threading environment: never iterate over concurrently modifiable collections unlike there's a good reason for doing that (for instance, some 'Producer-Consumer Queue' pattern). Otherwise, you could get an unpredictable behavior and hard-to-manage bugs. – Alexander Abakumov Aug 26 '23 at 20:02

1 Answers1

13

Just call ToArray() method.

Here is a source code:

    /// <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);
        }
    }
apocalypse
  • 5,764
  • 9
  • 47
  • 95
  • 3
    You could even chain it to a `foo.ToArray().ToDictionary(x=>x.Key, x=>x.Value);` to get it back in dictionary form. (Don't do just `foo.DoDictionary()` or it will use the `.GetEnumerator()` function instead of the `.ToArray()` function as the data source) – Scott Chamberlain May 11 '17 at 20:43
  • Depending on what you're after, `.Keys` and `.Values` will also give you a snapshot of their respective values. – xr280xr Mar 11 '22 at 17:29
  • There's an issue with this solution which is that ConcurrentDictionary will be blocked even for other READERS during this potentially relatively long loop (see, for instance, `ConcurrentDictionary.Count` property source code which waits for internal locks to become unlocked to proceed), though it shouldn't, since you're performing a NON-mutating operation on it. – Alexander Abakumov Aug 26 '23 at 20:38