XAML bindings are flagging invalid binding failures in ListView - it is confused as to what the data context is...

Description

I have a list view which has its items source set to an observable collection in a top-level view model. The collection is a list of detail view models. I have explicitly set the data context in the item template to point to the detail view model, yet the error it flags indicates it is looking at the page's data context and not the context of a single element in the list. I hate to see any binding failures, but this one will not go away...

Here is the binding error listed (there are three, actually):

Data Context	EmailsViewModel
Binding Path	Email
Target		Label.Text
Target Type	String
Description	'Email' property not found on 'MobyClient.ViewModels.EmailsViewModel', target property: 'Microsoft.Maui.Controls.Label.Text'
File			\Pages\PageEmails.xaml
Line			268

Here is the EmailsViewModel class:

namespace MobyClient.ViewModels;

public partial class EmailsViewModel : ObservableObject {

	public EmailsViewModel (IMobyLogger logger, EmailService emailService) {

		Guard.IsNotNull (logger);
		Guard.IsNotNull (emailService);

		_logger = logger;
		_emailService = emailService;
	}

	public ObservableCollection<EmailViewModel> FilteredEmails { get; set; }
	public ObservableCollection<FolderViewModel> Folders { get; set; }

	private void InitializeViewModel () {
		if (_isInitialized) return;

		_filteredEmails = new List<Email> (_emailService.Emails);

		FilteredEmails = new ObservableCollection<EmailViewModel> ();
		Folders = new ObservableCollection<FolderViewModel> (_emailService.Folders);

		_logger.LogDebug ("Emails viewmodel is initialized.");

		_isInitialized = true;
	}

	private bool _isInitialized;
	private List<Email> _filteredEmails;
	private readonly IMobyLogger _logger;
	private readonly EmailService _emailService;
}

And here is the EmailViewModel:

namespace MobyClient.ViewModels;

public partial class EmailViewModel : ObservableObject {

	[ObservableProperty]
	private Email _email;
}

Looking at the xaml used to define the list view item template:

<ListView.ItemTemplate>
	<DataTemplate>
		<ViewCell>
			<Grid
				Margin="0,1,0,0"
				Padding="3,0">
				<Grid
					x:Name="FolderItemTemplateGrid"
					x:DataType="viewModels:EmailViewModel"
					ColumnDefinitions="Auto, *, Auto"
					ColumnSpacing="5"
					Margin="10,2"
					ZIndex="1000">
					<Label
						Grid.Column="0"
						FontFamily="SegoeUiRegular"
						FontSize="14"
						Text="{Binding Email.SenderEmail.ShortDisplayName}" />
					<Label
						Grid.Column="1"
						FontFamily="SegoeUiRegular"
						FontSize="14"
						Margin="10,0,0,0"
						MaximumWidthRequest="1000"
						Text="{Binding Email.Subject}" />
					<Label
						Grid.Column="2"
						FontFamily="SegoeUiRegular"
						FontSize="14"
						Margin="10,0,0,0"
						Text="{Binding Email.MessageDate}" />
				</Grid>
			</Grid>
		</ViewCell>
	</DataTemplate>
</ListView.ItemTemplate>

I am clearly setting the datatype for the item template to be of the EMAIL view model and NOT the EMAILS view model! Trust me when I say that the Email class DOES have the properties, SenderEmail.ShortName (from an EmailAddress class), Email.Subject, and Email.Message date.

So, why is the error pointing to the EmailsViewModel when I have set the datatype to be EmailViewModel? Intellisense works (which to my understanding is driven by the x:DataType property... I don't get it!

My code base has grown to a size where it is not feasible to give you a github repository, so the code snippets above will have to work.

Steps to Reproduce

I know you won't like this, but you will need to generate a simple app that has a listview with an itemtemplate defined as I listed above.

I can see no reason why these binding errors are being flagged....

Version with bug

6.0.400 (current)

Last version that worked well

Unknown/Other

Affected platforms

Windows, I was not able test on other platforms

Affected platform versions

net6.0-windows10.0.19041.0

Did you find any workaround?

No...

Relevant log output

No response

[rachelkang]

Hi, @mobycorp - Thank you for opening this issue. I understand that you may not be able to share your project, but would you be able to share a sample (in the form of a GitHub repository) that reproduces the issue you describe? That would help us tremendously in investigating this further and helping to debug this issue.

Hi @mobycorp. We have added the "s/needs-info" label to this issue, which indicates that we have an open question for you before we can take further action. This issue will be closed automatically in 7 days if we do not hear back from you by then - please feel free to re-open it if you come back to this issue after that time.

I am currently on a severe deadline and barely have time to eat dinner, so, no. I cannot supply you with example code. I indicated as much from above. You guys are smart guys and should have no problem reproducing what I have reported.

[StephaneDelcroix]

@mobycorp

• do you get that error at runtime or compile time
• running in DEBUG or RELEASE. does it happens on both ?
• how are you setting your list view item source
• are you sure EmailViewModel has a public Email property ?

We tried to come up with a repro, based on the scarce info you gave us, but that repro is working well. I'm sorry about your severe deadlines, and the impact on your work-life balance. We understand the sense of urgency, and this is why you got quick answers. If you really need this investigated, you definitely have to provide the smallest possible project that triggers this behaviour, and we will look at it. Thanks

Hi @mobycorp. We have added the "s/needs-repro" label to this issue, which indicates that we require steps and sample code to reproduce the issue before we can take further action. Please try to create a minimal sample project/solution or code samples which reproduce the issue, ideally as a GitHub repo that we can clone. See more details about creating repros here: https://github.com/dotnet/maui/blob/main/.github/repro.md

This issue will be closed automatically in 7 days if we do not hear back from you by then - please feel free to re-open it if you come back to this issue after that time.

I'm not trying to be difficult. I am on a tight deadline... And, Gerald, love your YouTube channel!! :-)

The errors are triggered by the display of content in the listview in the picture. One per row x3 because there are three properties in the view model. I tried dereferencing the three properties so that:

<Grid
	x:Name="FolderItemTemplateGrid"
	ColumnDefinitions="Auto, *, Auto"
	ColumnSpacing="5"
	Margin="10,2"
	ZIndex="1000">
	<Label
		Grid.Column="0"
		FontFamily="SegoeUiRegular"
		FontSize="14"
		Text="{Binding Email.SenderEmail.ShortDisplayName}" />
	<Label
		Grid.Column="1"
		FontFamily="SegoeUiRegular"
		FontSize="14"
		Margin="10,0,0,0"
		MaximumWidthRequest="1000"
		Text="{Binding Email.Subject}" />
	<Label
		Grid.Column="2"
		FontFamily="SegoeUiRegular"
		FontSize="14"
		Margin="10,0,0,0"
		Text="{Binding Email.MessageDate}" />
</Grid>

becomes:

<Grid
	x:Name="FolderItemTemplateGrid"
	ColumnDefinitions="Auto, *, Auto"
	ColumnSpacing="5"
	Margin="10,2"
	ZIndex="1000">
	<Label
		Grid.Column="0"
		FontFamily="SegoeUiRegular"
		FontSize="14"
		Text="{Binding ShortDisplayName}" />
	<Label
		Grid.Column="1"
		FontFamily="SegoeUiRegular"
		FontSize="14"
		Margin="10,0,0,0"
		MaximumWidthRequest="1000"
		Text="{Binding Subject}" />
	<Label
		Grid.Column="2"
		FontFamily="SegoeUiRegular"
		FontSize="14"
		Margin="10,0,0,0"
		Text="{Binding MessageDate}" />
</Grid>

but this did not change anything. The errors are still being generated...

@jfversluis, since I you didn't post the xaml you used to generate your example picture, I am not sure if you are doing things exactly like me. Did you create an item template that looks similar to the above. I know that HotReload NEVER works when I makes changes to code in a template and I am wondering if it is because of the item template... ???

This is my entire listview definition:

<ListView
	ItemsSource="{Binding FilteredEmails}"
	SelectionMode="Single">
	<ListView.ItemTemplate>
		<DataTemplate
			x:DataType="viewModels:EmailViewModel">
			<ViewCell>
				<Grid
					Margin="0,1,0,0"
					Padding="3,0">
					<Grid.GestureRecognizers>
						<TapGestureRecognizer
							Command="{Binding EmailSelectedCommand, Source={RelativeSource AncestorType={x:Type viewModels:EmailsViewModel}}}"
							CommandParameter="{Binding}" />
					</Grid.GestureRecognizers>
					<Rectangle
						BackgroundColor="{StaticResource Gray100}"
						Stroke="Gray"
						StrokeThickness="1"
						ZIndex="1" />
					<Grid
						x:Name="FolderItemTemplateGrid"
						ColumnDefinitions="Auto, *, Auto"
						ColumnSpacing="5"
						Margin="10,2"
						ZIndex="1000">
						<Label
							Grid.Column="0"
							FontFamily="SegoeUiRegular"
							FontSize="14"
							Text="{Binding Email.SenderEmail.ShortDisplayName}" />
						<Label
							Grid.Column="1"
							FontFamily="SegoeUiRegular"
							FontSize="14"
							Margin="10,0,0,0"
							MaximumWidthRequest="1000"
							Text="{Binding Email.Subject}" />
						<Label
							Grid.Column="2"
							FontFamily="SegoeUiRegular"
							FontSize="14"
							Margin="10,0,0,0"
							Text="{Binding Email.MessageDate}" />
					</Grid>
				</Grid>
			</ViewCell>
		</DataTemplate>
	</ListView.ItemTemplate>
</ListView>

There are two things to note here:

1. The tap gesture works so it knew enough to dereference the relative sourcing to the gesture handler
2. You can see that I declare the datatype at the DataTemplate level: x:DataType="viewModels:EmailViewModel" I know this works because intellisense was able to find all of the properties after I typed Email 'dot' field name (even the Email 'dot' SenderEmail 'dot'

Does any of that make sense?