Collection was modified; enumeration operation may not execute

2009-03-03 c# wcf concurrency dictionary thread-safety

I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur. Below is the code.

This is a WCF server in a Windows service. The method NotifySubscribers is called by the service whenever there is a data event (at random intervals, but not very often - about 800 times per day).

When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine.

Do you see a problem with this code? Do I need to make the dictionary thread-safe?

[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class SubscriptionServer : ISubscriptionServer
{
    private static IDictionary<Guid, Subscriber> subscribers;

    public SubscriptionServer()
    {            
        subscribers = new Dictionary<Guid, Subscriber>();
    }

    public void NotifySubscribers(DataRecord sr)
    {
        foreach(Subscriber s in subscribers.Values)
        {
            try
            {
                s.Callback.SignalData(sr);
            }
            catch (Exception e)
            {
                DCS.WriteToApplicationLog(e.Message, 
                  System.Diagnostics.EventLogEntryType.Error);

                UnsubscribeEvent(s.ClientId);
            }
        }
    }


    public Guid SubscribeEvent(string clientDescription)
    {
        Subscriber subscriber = new Subscriber();
        subscriber.Callback = OperationContext.Current.
                GetCallbackChannel<IDCSCallback>();

        subscribers.Add(subscriber.ClientId, subscriber);

        return subscriber.ClientId;
    }


    public void UnsubscribeEvent(Guid clientId)
    {
        try
        {
            subscribers.Remove(clientId);
        }
        catch(Exception e)
        {
            System.Diagnostics.Debug.WriteLine("Unsubscribe Error " + 
                    e.Message);
        }
    }
}

Answers

What's likely happening is that SignalData is indirectly changing the subscribers dictionary under the hood during the loop and leading to that message. You can verify this by changing

foreach(Subscriber s in subscribers.Values)

To

foreach(Subscriber s in subscribers.Values.ToList())

If I'm right, the problem will dissapear

Calling subscribers.Values.ToList() copies the values of subscribers.Values to a separate list at the start of the foreach. Nothing else has access to this list (it doesn't even have a variable name!), so nothing can modify it inside the loop.

When a subscriber unsubscribes you are changing contents of the collection of Subscribers during enumeration.

There are several ways to fix this, one being changing the for loop to use an explicit .ToList():

public void NotifySubscribers(DataRecord sr)  
{
    foreach(Subscriber s in subscribers.Values.ToList())
    {
                                              ^^^^^^^^^  
        ...

A more efficient way, in my opinion, is to have another list that you declare that you put anything that is "to be removed" into. Then after you finish your main loop (without the .ToList()), you do another loop over the "to be removed" list, removing each entry as it happens. So in your class you add:

private List<Guid> toBeRemoved = new List<Guid>();

Then you change it to:

public void NotifySubscribers(DataRecord sr)
{
    toBeRemoved.Clear();

    ...your unchanged code skipped...

   foreach ( Guid clientId in toBeRemoved )
   {
        try
        {
            subscribers.Remove(clientId);
        }
        catch(Exception e)
        {
            System.Diagnostics.Debug.WriteLine("Unsubscribe Error " + 
                e.Message);
        }
   }
}

...your unchanged code skipped...

public void UnsubscribeEvent(Guid clientId)
{
    toBeRemoved.Add( clientId );
}

This will not only solve your problem, it will prevent you from having to keep creating a list from your dictionary, which is expensive if there are a lot of subscribers in there. Assuming the list of subscribers to be removed on any given iteration is lower than the total number in the list, this should be faster. But of course feel free to profile it to be sure that's the case if there's any doubt in your specific usage situation.

Actually the problem seems to me that you are removing elements from the list and expecting to continue to read the list as if nothing had happened.

What you really need to do is to start from the end and back to the begining. Even if you remove elements from the list you will be able to continue reading it.

You can also lock your subscribers dictionary to prevent it from being modified whenever its being looped:

 lock (subscribers)
 {
         foreach (var subscriber in subscribers)
         {
               //do something
         }
 }

You can copy subscribers dictionary object to a same type of temporary dictionary object and then iterate the temporary dictionary object using foreach loop.

So a different way to solve this problem would be instead of removing the elements create a new dictionary and only add the elements you didnt want to remove then replace the original dictionary with the new one. I don't think this is too much of an efficiency problem because it does not increase the number of times you iterate over the structure.

I had the same issue, and it was solved when I used a for loop instead of foreach.

// foreach (var item in itemsToBeLast)
for (int i = 0; i < itemsToBeLast.Count; i++)
{
    var matchingItem = itemsToBeLast.FirstOrDefault(item => item.Detach);

   if (matchingItem != null)
   {
      itemsToBeLast.Remove(matchingItem);
      continue;
   }
   allItems.Add(itemsToBeLast[i]);// (attachDetachItem);
}

Why this error?

In general .Net collections do not support being enumerated and modified at the same time. If you try to modify the collection list during enumeration, it raises an exception. So the issue behind this error is, we can not modify the list/dictionary while we are looping through the same.

One of the solutions

If we iterate a dictionary using a list of its keys, in parallel we can modify the dictionary object, as we are iterating through the key-collection and not the dictionary(and iterating its key collection).

Example

//get key collection from dictionary into a list to loop through
List<int> keys = new List<int>(Dictionary.Keys);

// iterating key collection using a simple for-each loop
foreach (int key in keys)
{
  // Now we can perform any modification with values of the dictionary.
  Dictionary[key] = Dictionary[key] - 1;
}

Here is a blog post about this solution.

And for a deep dive in StackOverflow: Why this error occurs?

I've seen many options for this but to me this one was the best.

ListItemCollection collection = new ListItemCollection();
        foreach (ListItem item in ListBox1.Items)
        {
            if (item.Selected)
                collection.Add(item);
        }

Then simply loop through the collection.

Be aware that a ListItemCollection can contain duplicates. By default there is nothing preventing duplicates being added to the collection. To avoid duplicates you can do this:

ListItemCollection collection = new ListItemCollection();
            foreach (ListItem item in ListBox1.Items)
            {
                if (item.Selected && !collection.Contains(item))
                    collection.Add(item);
            }

InvalidOperationException- An InvalidOperationException has occurred. It reports a "collection was modified" in a foreach-loop

Use break statement, Once the object is removed.

ex:

ArrayList list = new ArrayList(); 

foreach (var item in list)
{
    if(condition)
    {
        list.remove(item);
        break;
    }
}

Okay so what helped me was iterating backwards. I was trying to remove an entry from a list but iterating upwards and it screwed up the loop because the entry didn't exist anymore:

for (int x = myList.Count - 1; x > -1; x--)
                        {

                            myList.RemoveAt(x);

                        }

There is one link where it elaborated very well & solution is also given. Try it if you got proper solution please post here so other can understand. Given solution is ok then like the post so other can try these solution.

for you reference original link :- https://bensonxion.wordpress.com/2012/05/07/serializing-an-ienumerable-produces-collection-was-modified-enumeration-operation-may-not-execute/

When we use .Net Serialization classes to serialize an object where its definition contains an Enumerable type, i.e. collection, you will be easily getting InvalidOperationException saying "Collection was modified; enumeration operation may not execute" where your coding is under multi-thread scenarios. The bottom cause is that serialization classes will iterate through collection via enumerator, as such, problem goes to trying to iterate through a collection while modifying it.

First solution, we can simply use lock as a synchronization solution to ensure that the operation to the List object can only be executed from one thread at a time. Obviously, you will get performance penalty that if you want to serialize a collection of that object, then for each of them, the lock will be applied.

Well, .Net 4.0 which makes dealing with multi-threading scenarios handy. for this serializing Collection field problem, I found we can just take benefit from ConcurrentQueue(Check MSDN)class, which is a thread-safe and FIFO collection and makes code lock-free.

Using this class, in its simplicity, the stuff you need to modify for your code are replacing Collection type with it, use Enqueue to add an element to the end of ConcurrentQueue, remove those lock code. Or, if the scenario you are working on do require collection stuff like List, you will need a few more code to adapt ConcurrentQueue into your fields.

BTW, ConcurrentQueue doesnât have a Clear method due to underlying algorithm which doesnât permit atomically clearing of the collection. so you have to do it yourself, the fastest way is to re-create a new empty ConcurrentQueue for a replacement.

The accepted answer is imprecise and incorrect in the worst case . If changes are made during ToList(), you can still end up with an error. Besides lock, which performance and thread-safety needs to be taken into consideration if you have a public member, a proper solution can be using immutable types.

In general, an immutable type means that you can't change the state of it once created. So your code should look like:

public class SubscriptionServer : ISubscriptionServer
{
    private static ImmutableDictionary<Guid, Subscriber> subscribers = ImmutableDictionary<Guid, Subscriber>.Empty;
    public void SubscribeEvent(string id)
    {
        subscribers = subscribers.Add(Guid.NewGuid(), new Subscriber());
    }
    public void NotifyEvent()
    {
        foreach(var sub in subscribers.Values)
        {
            //.....This is always safe
        }
    }
    //.........
}

This can be especially useful if you have a public member. Other classes can always foreach on the immutable types without worrying about the collection being modified.

I personally ran across this recently in unfamiliar code where it was in an open dbcontext with Linq's .Remove(item). I had a hell of a time locating the error debugging because item(s) got removed the first time iterating, and if I tried to step back and re-step over, the collection was empty, since i was in the same context!

Here is a specific scenario that warrants a specialized approach:

  1. The Dictionary is enumerated frequently.
  2. The Dictionary is modified infrequently.

In this scenario creating a copy of the Dictionary (or the Dictionary.Values) before every enumeration can be quite costly. My idea about solving this problem is to reuse the same cached copy in multiple enumerations, and watch an IEnumerator of the original Dictionary for exceptions. The enumerator will be cached along with the copied data, and interrogated before starting a new enumeration. In case of an exception the cached copy will be discarded, and a new one will be created. Here is my implementation of this idea:

public class SafeEnumerable<T> : IEnumerable<T>, IDisposable
{
    private IEnumerable<T> _source;
    private IEnumerator<T> _enumerator;
    private T[] _cached;

    public SafeEnumerable(IEnumerable<T> source)
    {
        _source = source ?? throw new ArgumentNullException(nameof(source));
    }

    public IEnumerator<T> GetEnumerator()
    {
        if (_source == null) throw new ObjectDisposedException(this.GetType().Name);
        if (_enumerator == null)
        {
            _enumerator = _source.GetEnumerator();
            _cached = _source.ToArray();
        }
        else
        {
            var modified = false;
            if (_source is ICollection collection) // C# 7 syntax
            {
                modified = _cached.Length != collection.Count;
            }
            if (!modified)
            {
                try
                {
                    _enumerator.MoveNext();
                }
                catch (InvalidOperationException)
                {
                    modified = true;
                }
            }
            if (modified)
            {
                _enumerator.Dispose();
                _enumerator = _source.GetEnumerator();
                _cached = _source.ToArray();
            }
        }
        foreach (var item in _cached)
        {
            yield return item;
        }
    }

    public void Dispose()
    {
        _enumerator?.Dispose();
        _enumerator = null;
        _cached = null;
        _source = null;
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

public static SafeEnumerable<T> AsSafeEnumerable<T>(this IEnumerable<T> source)
    => new SafeEnumerable<T>(source);

Usage example:

private static IDictionary<Guid, Subscriber> _subscribers;
private static SafeEnumerable<Subscriber> _safeSubscribers;

//...(in the constructor)
_subscribers = new Dictionary<Guid, Subscriber>();
_safeSubscribers = _subscribers.Values.AsSafeEnumerable();

// ...(elsewere)
foreach (var subscriber in _safeSubscribers)
{
    //...
}

Unfortunately this idea cannot be used currently with the class Dictionary in .NET Core 3.0, because this class does not throw a Collection was modified exception when enumerated and the methods Remove and Clear are invoked. All other containers I checked are behaving consistently. I checked systematically these classes: List<T>, Collection<T>, ObservableCollection<T>, HashSet<T>, SortedSet<T>, Dictionary<T,V> and SortedDictionary<T,V>. Only the two aforementioned methods of the Dictionary class in .NET Core are not invalidating the enumeration.


Update: I fixed the above problem by comparing also the lengths of the cached and the original collection. This fix assumes that the dictionary will be passed directly as an argument to the SafeEnumerable's constructor, and its identity will not be hidden by (for example) a projection like: dictionary.Select(e => e).AsSafeEnumerable().

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