Bug 29344 - TableView Collection was modified enumeration operation may not execute
Summary: TableView Collection was modified enumeration operation may not execute
Alias: None
Product: Forms
Classification: Xamarin
Component: iOS ()
Version: 1.4.2
Hardware: PC Mac OS
: Normal critical
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2015-04-23 12:17 UTC by John Miller [MSFT]
Modified: 2015-06-17 13:37 UTC (History)
2 users (show)

Tags: u7k ac
Is this bug a regression?: ---
Last known good build:

Test Case (1.71 MB, application/zip)
2015-04-23 12:17 UTC, John Miller [MSFT]

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Description John Miller [MSFT] 2015-04-23 12:17:53 UTC
Created attachment 10880 [details]
Test Case


   Touching multiple EntryCells in a TableView while the Keyboard is open eventually causes a crash. 

**Steps to Reproduce:**

   1. Run the attached FormsGallery sample on an iOS Simulator
   2. Touch the "TableView for a form" row
   3. Touch an EntryCell to show the keyboard
   4. Scroll and touch other EntryCells
   5. Occasionally touch the Label of the EntryCell (caption text)
   6. Repeat step 4-5 until it crashes

   The above is sort of strange, so here is a video showing how I reproduced this: http://screencast.com/t/t1Pbln1KZ

**Actual Results:**

   2015-04-23 12:15:09.350 FormsTemplateiOS[22510:4058501] Unhandled managed exception: Collection was modified; enumeration operation may not execute. (System.InvalidOperationException)
  at System.Collections.Generic.List`1+Enumerator[System.Tuple`2[System.WeakReference,System.Action`2[System.Object,System.Object]]].MoveNext () [0x00063] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Collections.Generic/List.cs:815 
  at Xamarin.Forms.MessagingCenter.InnerSend (System.String message, System.Type senderType, System.Type argType, System.Object sender, System.Object args) [0x00000] in <filename unknown>:0 
  at Xamarin.Forms.MessagingCenter.Send[IVisualElementRenderer] (IVisualElementRenderer sender, System.String message) [0x00000] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.TableViewModelRenderer.RowSelected (UIKit.UITableView tableView, Foundation.NSIndexPath indexPath) [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, IntPtr principal, IntPtr delegate) [0x00005] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:62 
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00038] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:46 
  at FormsGallery.iOS.Application.Main (System.String[] args) [0x00008] in /Users/johnmiller/Downloads/xamarin-forms-samples-master/FormsGallery/FormsGallery/FormsGallery.iOS/Main.cs:17 
2015-04-23 12:15:09.351 FormsTemplateiOS[22510:4058501] critical: Stacktrace:

**Expected Results:**

   No crash.

**Build Date & Platform:**

   XF 1.4.2
   XI 8.9
Comment 1 Jason Smith [MSFT] 2015-05-20 21:07:20 UTC
Interesting catch:

For whoever ends up fixing this the offending line in the MessagingCenter is:

foreach (var action in actions) {

Should actually be:

foreach (var action in actions.ToList ()) {

or some cheaper equal.
Comment 2 Jason Smith [MSFT] 2015-05-20 21:09:03 UTC
For further reference, the issue is a callback from the MessagingCenter is causing either a sub or unsub to the offending message, thus invalidating the list. Proper behavior would need to ensure things that unsub mid processing loop dont end up getting the event.
Comment 3 Jason Smith [MSFT] 2015-06-01 12:17:21 UTC
This should be fixed in 1.4.3 final
Comment 4 Parmendra Kumar 2015-06-17 13:37:13 UTC
I have checked this issue with xamarin.forms. and its working fine at my end.

Hence closing this issue.

Environment info:

Xamarin Studio  5.9.3 (build 1)
Mono 4.0.1 ((detached/ed1d3ec)
GTK+ 2.24.23 (Raleigh theme)
Package version: 400010044
Xcode 6.2 (6776)
Xamarin.iOS (Enterprise Edition)
Xamarin.Android (Enterprise Edition)
Xamarin.Mac : (Enterprise Edition)
Mac OS X 10.9.4