CarouselView throws System.IndexOutOfRangeException when ObservableCollection is changed

Question

I have created a Xamarin Forms Shell App. The app has multiple tabs which route to different pages. On one of the pages I use a CarouselView.
The ItemsSource of the CarouselView is an ObservableCollection<ContentView> inside the ViewModel. The ObservableCollection gets updated via the OnAppearing()-method of the page by calling an update function in the ViewModel. The first time I navigate to the page the CarouselView works fine.
However, when I change the tab and then return to the tab with the CarouselView, the following exception is thrown:

System.IndexOutOfRangeException: 'Can't set CarouselView to position -1. ItemsSource has 3 items.'

This only happens on Android.

Steps to reproduce:

1. Create Xamarin Forms tabbed shell App
2. In the first tab add a CarouselView to the page
3. Update CarouselView inside OnAppearing()
4. Go to another tab
5. Return to tab with CarouselView in it

Any workaround would be highly appreciated. Thank you!

Answer 1

Hello,​

Welcome to our Microsoft Q&A platform!

I follow you steps, But I do not meet this issue.

I can run it normally like following screenshot.

I add my code to AboutPage.xaml

   <?xml version="1.0" encoding="utf-8" ?>  
   <ContentPage xmlns="http://xamarin.com/schemas/2014/forms"  
                xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"  
                x:Class="ShellCarouselView.Views.AboutPage"  
                xmlns:vm="clr-namespace:ShellCarouselView.ViewModels"  
                Title="{Binding Title}">  
         
       
       <ContentPage.Resources>  
           <ResourceDictionary>  
               <Color x:Key="Accent">#96d1ff</Color>  
           </ResourceDictionary>  
       </ContentPage.Resources>  
       <CarouselView ItemsSource="{Binding ItemsView}">  
           <CarouselView.ItemTemplate>  
               <DataTemplate>  
                   <ContentView Content="{Binding Content}" />  
               </DataTemplate>  
           </CarouselView.ItemTemplate>  
     
       </CarouselView>  
     
         
   </ContentPage>  

Here is background code of AboutPage.xaml

   public partial class AboutPage : ContentPage  
       {  
           AboutViewModel aboutViewModels;  
           public AboutPage()  
           {  
               InitializeComponent();  
     
                aboutViewModels = new AboutViewModel();  
     
               BindingContext = aboutViewModels;  
           }  
           public static int value = 1;  
           protected override void OnAppearing()  
           {  
               base.OnAppearing();  
     
             //  aboutViewModels.Items.Add(new Item() { Text = "tesx"+ (value++) });  
     
               aboutViewModels.ItemsView.Add(new View1());  
             
     
           }  
       }  

Here is my viewModel.

   public class AboutViewModel : BaseViewModel  
       {  
           public ObservableCollection<Item> Items { get; }  
     
           public ObservableCollection<ContentView> ItemsView { get; }  
           public AboutViewModel()  
           {  
               Items = new ObservableCollection<Item>();  
               ItemsView = new ObservableCollection<ContentView>();  
                 Title = "About";  
               OpenWebCommand = new Command(async () => await Browser.OpenAsync("https://aka.ms/xamarin-quickstart"));  
           }  
     
           public ICommand OpenWebCommand { get; }  
       }  

Best Regards,

Leon Lu

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Answer 2

Trapping the onpositionchanged did not trigger. or onpropertychanged actions showed the position=0 several times but the final time showed value =-1 . Editing the value was rejected by ide and kept showing -1. The binding I did was in the code not in XML page. Perhaps try the same steps but assign itemsource via code to replicate the error.