System.InvalidOperationExceptionCollection was modified after the enumerator was instantiated.

[KirillVolkov]

System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
System.Collections.Generic.Queue.Enumerator.MoveNext()
Quobject.SocketIoClientDotNet.Client.Manager.Cleanup()
Quobject.SocketIoClientDotNet.Client.Manager.<>c__DisplayClass3.b__1(object data)
Quobject.EngineIoClientDotNet.ComponentEmitter.ListenerImpl.Call(object[] args)
Quobject.EngineIoClientDotNet.ComponentEmitter.Emitter.Emit(string eventString, object[] args)
Quobject.SocketIoClientDotNet.Client.Manager.<>c__DisplayClass5.b__2()
Quobject.EngineIoClientDotNet.Thread.EasyTimer.<>c__DisplayClass1.b__0()
System.Threading.Tasks.AwaitTaskContinuation.InvokeAction(object state)
System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, object state, bool preserveSyncCtx)
System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, object state, bool preserveSyncCtx)
System.Threading.Tasks.AwaitTaskContinuation.RunCallback(ContextCallback callback, object state, ref Task currentTask)
System.Threading.Tasks.AwaitTaskContinuation.m__0(object s)
System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(object state)
System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, object state, bool preserveSyncCtx)
System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, object state, bool preserveSyncCtx)
System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
System.Threading.ThreadPoolWorkQueue.Dispatch()
System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

[jlewin]

A customer of ours reports the same exception on recent mono builds:

Unhandled Exception:
System.InvalidOperationException: Collection was modified after the enumerator was instantiated.
  at System.ThrowHelper.ThrowInvalidOperationException (ExceptionResource resource) <0x42160380 + 0x00027> in <filename unknown>:0 
  at System.Collections.Generic.Queue`1+Enumerator[T].MoveNext () <0x420909f0 + 0x0002b> in <filename unknown>:0 
  at Quobject.SocketIoClientDotNet.Client.Manager.Cleanup () <0x42090720 + 0x00077> in <filename unknown>:0 
  at Quobject.SocketIoClientDotNet.Client.Manager+<>c__DisplayClass50_0.<Open>b__1 (System.Object data) <0x4215fea0 + 0x0002b> in <filename unknown>:0 
  at Quobject.EngineIoClientDotNet.ComponentEmitter.ListenerImpl.Call (System.Object[] args) <0x420900b0 + 0x00052> in <filename unknown>:0 
  at Quobject.EngineIoClientDotNet.ComponentEmitter.Emitter.Emit (System.String eventString, System.Object[] args) <0x41fd52c0 + 0x00097> in <filename unknown>:0 
  at Quobject.SocketIoClientDotNet.Client.Manager+<>c__DisplayClass50_1.<Open>b__2 () <0x42160040 + 0x00153> in <filename unknown>:0 
  at Quobject.EngineIoClientDotNet.Thread.EasyTimer+<>c__DisplayClass2_0.<SetTimeout>b__0 () <0x42090f60 + 0x00040> in <filename unknown>:0 
  at System.Threading.Tasks.AwaitTaskContinuation.InvokeAction (System.Object state) <0x7fc2b664fd70 + 0x0003c> in <filename unknown>:0 
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, Boolean preserveSyncCtx) <0x7fc2b6658990 + 0x00178> in <filename unknown>:0 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, Boolean preserveSyncCtx) <0x7fc2b6658960 + 0x00020> in <filename unknown>:0 
  at System.Threading.Tasks.AwaitTaskContinuation.RunCallback (System.Threading.ContextCallback callback, System.Object state, System.Threading.Tasks.Task& currentTask) <0x7fc2b664fe80 + 0x0007f> in <filename unknown>:0 
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () <0x7fc2b65a42a0 + 0x00029> in <filename unknown>:0 
  at System.Threading.Tasks.AwaitTaskContinuation.<ThrowAsyncIfNecessary>m__0 (System.Object s) <0x7fc2b6650280 + 0x00041> in <filename unknown>:0 
  at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context (System.Object state) <0x7fc2b665ede0 + 0x00048> in <filename unknown>:0 
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, Boolean preserveSyncCtx) <0x7fc2b6658990 + 0x00178> in <filename unknown>:0 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, Boolean preserveSyncCtx) <0x7fc2b6658960 + 0x00020> in <filename unknown>:0 
  at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem () <0x7fc2b665ed70 + 0x00053> in <filename unknown>:0 
  at System.Threading.ThreadPoolWorkQueue.Dispatch () <0x7fc2b665d2e0 + 0x001d6> in <filename unknown>:0 
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback () <0x7fc2b665ebf0 + 0x00008> in <filename unknown>:0 

One interesting note from our logs is the following abnormal log messages:

Web exception occurred. Status code: SecureChannelFailure
Web exception occurred. Status code: SecureChannelFailure
Web exception occurred. Status code: SecureChannelFailure
Web exception occurred. Status code: Timeout
Web exception occurred. Status code: Timeout
Web exception occurred. Status code: Timeout

This error may occur when SSL is in an exceptional state and behaving abnormally