Xamarin XAML variable scope not working like expected - xamarin

Question

I have a really odd problem with variable scopes. A Listview named "TodoListView" is defined via xaml, and it's ItemSource populated from a SQListe database. Works. Inside the ListView I have a ViewCell to display the data row-wise.
<ContentPage ... x:Class="JanitorPro.MainPage" ...>
<StackLayout>
<ListView x:Name="TodoListView" Margin="20" ItemSelected="OnListItemSelected">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Horizontal" HorizontalOptions="FillAndExpand">
<Label Text="{Binding name}" VerticalTextAlignment="Center" HorizontalOptions="StartAndExpand" />
<Switch HorizontalOptions="End" IsToggled="{Binding done}" Toggled="DoneSwitchToggled"/>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
The codebehind looks like this (some irrelevant portions removed):
public partial class MainPage : ContentPage
{
public MainPage()
{
InitializeComponent();
}
protected override async void OnAppearing()
{
base.OnAppearing();
// load Database
TodoListView.ItemsSource = await App.TodoDatabase.GetItemsAsync("SELECT * FROM [TodoItem]");
}
async void OnReloadButtonClicked(object sender, EventArgs e)
{
Debug.WriteLine("Reload Button Click");
TodoListView.ItemsSource = await App.TodoDatabase.GetItemsAsync("SELECT * FROM [TodoItem]");
Debug.WriteLine("Reload done.");
}
async void OnListItemSelected(object sender, SelectedItemChangedEventArgs e)
{
if (e.SelectedItem != null)
{
await Navigation.PushAsync(new TodoItemPage
{
BindingContext = e.SelectedItem as TodoItem
});
}
}
private void DoneSwitchToggled(object sender, ToggledEventArgs e)
{
// TodoItem i = null;
TodoItem i = TodoListView.SelectedItem;
if (i != null)
{
Debug.WriteLine("Toggle: {0}", i.id);
}
}
}
}
The oddity has two stages. Before I inserted the DoneSwitchToggled event handler, every occurrance of TodoListView.ItemsSource got a red underline under TodoListView and a hint that "The name TodoListView does not exist in the current context". OK, I thought that VS was not smart enough to find a definition in the xaml file, because, despite of the warning, the program compiled and ran fine. TodoListView gets initialized and does correctly display the rows of the underlying database, so it does clearly exist at runtime.
Things went wild when I added the DoneSwitchToggled event handler to both XAML and the codebehind. All the sudden the program won't compile any longer but bail out with a CS0103 error "The name "TodoListView" does not exist in the current context". The error appears three times, with the line numbers pointing to the other occurrances of TodoListView in onAppearing() and OnReloadButtonClicked(). Huh? How the heck can the addition of a variable reference in an event handler render that variable invalid in completely different methods? OK, there was something fishy with the variable before (still don't know what ...), but it worked. Now it doesn't any more, whch doesn't make any sense for me. Furthermore, if I comment out the offending line in the DoneSwitchToggled event handler, and insert a dummy definition for i, like so:
TodoItem i = null;
// TodoItem i = TodoListView.SelectedItem;
everything is like before, VS still underlines the other appearances of TodoListView, but now the program builds and runs ok again.
Anyone who can explain this effect, and show me how correct my code? I think the objective is clear: DoneSwitchToggled is supposed to write back the switch value into the database (and do some other processing not shown in my stripped down sample), and though the "sender" object is correctly set to reference my button, I found no way to access the underlying data binding, since ToggledEventArgs unfortunately does seem to only pass the switch position "true" or "false", but - unlike the OnListItemSelected event handler - not pass any reference to the bound row through the second argument. So my idea was to use ListView.SelectedItem for this purpose.

Answer 1

Finally I figured it out myself. This seems to be a glitch in VS 2017. There is nothing wrong with TodoListView, so error CS0103 is misleading nonsense.
What VS really means is an error CS0266. TodoListView is defined by a generic list
List<TodoItem>
to access SelectedItem i need to typecast it:
TodoItem i = (TodoItem)TodoListView.SelectedItem;
This added, all errors are gone, app code builds OK.
Btw, unfortunately this approach to get at the item where the Switch has been flipped has proven not working, TodoListView does always return null for SelectedItem, seems that the ListView control doesn't see the button press. Need to find a different way to find the list item beneath the switch to get at the bound row id.