Home/ Questions/The complete guide to the invalidoperationexception: collection was modified; enumeration operation may not execute issue
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Karina Lam
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Asked: 2022-05-18T07:00:28+00:00 In:

The complete guide to the invalidoperationexception: collection was modified; enumeration operation may not execute issue

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I get the error: invalidoperationexception: collection was modified; enumeration operation may not execute. when I try to run the program below:

[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);
  }
  }
 }

The error appears the system notifies as follows:

Collection was modified; enumeration operation may not execute

I tried to solve it with another sample. I got the reference in the community forum, but it still returned an invalid result. If someone knows the solution, please give me the support. Thanks!

enumeration operation may not execute
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2 Answers

  1. Best Answer
    Expert

    The cause: SignalData is most likely modifying the subscribers dictionary invisibly under the hood during the loop, resulting in that message.

    The solution: You can solve this problem by changing:

    foreach(Subscriber s in subscribers.Values)

    To

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

     

    • 24

  2. Unsubscribe a subscriber to change the contents of the Collection of Subscribers during Enumeration.

    This can be fixed in a number of ways, including changing the for loop to use an explicitly .ToList() 

    public void NotifySubscribers(DataRecord sr) 
{
 foreach(Subscriber s in subscribers.Values.ToList())
 {
 ^^^^^^^^^ 
 ...
    • 10

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