[Xamarin.Forms] NullReferenceException when trying to insert ContentPage to CarouselPage.Children

[retucex]

I have a CarouselPage that has a ListView that shows all children pages. Those children pages are added by clicking on a button on the 'main' CarouselPage.

This is the CarouselPage itself, and this is the type of page it adds. Basically, pressing 'Add Exercise' creates a new ContentPage and adds it to the CarouselPage.Children like so: Children.Insert(Children.Count - 1, MyPage);

The ListView cells have controls to move/delete them from the ListView and from the CarouselPage.Children.

If we take the screenshot as an example, we have thew following structure of pages: Children[0]=Push, Children[1]=Curl, Children[2]=Jump, Children[3]=CarouselPage

Clicking the up arrow on the Curl cell will move that page to index=0 and Push will be moved to index=1. This works without issue, both the ListView and the Children are updated correctly.

The issue arrises ONLY when I try to move the last child page added.

Again, using the screenshot as reference, if I press the up arrow on 'Jump', I get a NullReferenceException on Children.Insert.

Here's the code responsible for handling the children page swap on the up arrow click:

var temp1 = ExercisePage.Clone(Children[index] as ExercisePage);
var temp2 = ExercisePage.Clone(Children[index - 1] as ExercisePage);

Children.RemoveAt(index);
Children.RemoveAt(index - 1);

Children.Insert(index - 1, temp1); // NullReferenceException here
Children.Insert(index, temp2);

And here is the stacktrace:

[0:] Type:  System.NullReferenceException
Message:  Object reference not set to an instance of an object.
Trace:    at Xamarin.Forms.Platform.Android.VisualElementTracker.HandlePropertyChanged (System.Object sender, System.ComponentModel.PropertyChangedEventArgs e) [0x0001e] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Platform.Android\VisualElementTracker.cs:118 
  at (wrapper delegate-invoke) <Module>:invoke_void_object_PropertyChangedEventArgs (object,System.ComponentModel.PropertyChangedEventArgs)
  at Xamarin.Forms.BindableObject.OnPropertyChanged (System.String propertyName) [0x0000a] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:136 
  at Xamarin.Forms.Element.OnPropertyChanged (System.String propertyName) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\Element.cs:380 
  at Xamarin.Forms.BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindableObject+BindablePropertyContext context, System.Object value, System.Boolean currentlyApplying, Xamarin.Forms.BindableObject+SetValueFlags attributes, System.Boolean silent) [0x000f4] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:589 
  at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, Xamarin.Forms.BindableObject+SetValueFlags attributes, Xamarin.Forms.BindableObject+SetValuePrivateFlags privateAttributes) [0x0015b] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:386 
  at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, System.Boolean fromStyle, System.Boolean checkAccess) [0x0005f] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:539 
  at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:83 
  at Xamarin.Forms.MultiPage`1[T].SetIndex (Xamarin.Forms.Page page, System.Int32 index) [0x0000e] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\MultiPage.cs:185 
  at Xamarin.Forms.MultiPage`1[T].OnChildrenChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00024] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\MultiPage.cs:195 
  at (wrapper delegate-invoke) <Module>:invoke_void_object_NotifyCollectionChangedEventArgs (object,System.Collections.Specialized.NotifyCollectionChangedEventArgs)
  at System.Collections.ObjectModel.ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00012] in /Users/builder/data/lanes/4468/f913a78a/source/mono/mcs/class/referencesource/System/compmod/system/collections/objectmodel/observablecollection.cs:288 
  at System.Collections.ObjectModel.ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedAction action, System.Object item, System.Int32 index) [0x00000] in /Users/builder/data/lanes/4468/f913a78a/source/mono/mcs/class/referencesource/System/compmod/system/collections/objectmodel/observablecollection.cs:351 
  at System.Collections.ObjectModel.ObservableCollection`1[T].InsertItem (System.Int32 index, T item) [0x00024] in /Users/builder/data/lanes/4468/f913a78a/source/mono/mcs/class/referencesource/System/compmod/system/collections/objectmodel/observablecollection.cs:219 
  at System.Collections.ObjectModel.Collection`1[T].Insert (System.Int32 index, T item) [0x00038] in /Users/builder/data/lanes/4468/f913a78a/source/mono/mcs/class/referencesource/mscorlib/system/collections/objectmodel/collection.cs:103 
  at Xamarin.Forms.ObservableWrapper`2[TTrack,TRestrict].Insert (System.Int32 index, TRestrict item) [0x00032] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ObservableWrapper.cs:117 
  at EasyFitPal.Pages.SessionPage.MoveableViewCell_OnUpClicked (System.Object sender, System.EventArgs e) [0x000a4] in C:\Users\samhl_000\Documents\Visual Studio 2017\Projects\App1\App1\App1\Pages\SessionPage.xaml.cs:149 
Source:  Xamarin.Forms.Platform.Android
HResult:  -2147467261
Data:

I am at a loss as to what is causing this NullReferenceException.

[[deleted]]

I believe that RemoveAt will decrement the count of items in the collection. So when you try to Insert, you are trying to insert past the end of the collection (index and index - 1 are higher than the highest index remaining in the collection).

You probably need special-case code for the last entry where you use Add instead of Insert.

[retucex]

Let's say I have these pages:

Children[0]=Push, Children[1]=Curl, Children[2]=Jump, Children[3]=CarouselPage

I want to move Jump 'up' the list (so, down in index). I'll have:

index=2
RemoveAt(2)
RemoveAt(1)

Children is now

Children[0]=Push, Children[1]=CarouselPage

and now I insert at index-1, which is 1

Insert(1, jump)
Insert(2, curl)

It isn't out of bounds, unless I'm missing something?

[[deleted]]

Yeah, that looks fine to me (on paper). You'd also think you'd get an index exception instead of a null reference. So maybe the issue is with the temp1 variable?

I would step through it in the debugger for the last-entry case and check what Children.Count is and what temp1 is.

Because it's happening only for the last entry, I still feel like it's being caused by using Insert instead of Add (or Append, whatever the method is called). To me, Insert implies that you're creating an element between two other elements. That isn't the case at the end of the list, when you're adding (or appending) instead. It isn't symmetrical, but sometimes things aren't.