XAML bindings are flagging invalid binding failures in ListView - it is confused as to what the data context is... #8433

mobycorp · 2022-06-29T20:58:45Z

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

The text was updated successfully, but these errors were encountered:

rachelkang · 2022-06-30T00:20:34Z

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.

ghost · 2022-06-30T00:20:50Z

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.

mobycorp · 2022-06-30T00:24:00Z

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 · 2022-06-30T08:20:59Z

@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

ghost · 2022-06-30T08:22:48Z

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.

mobycorp · 2022-06-30T13:16:39Z

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:

The tap gesture works so it knew enough to dereference the relative sourcing to the gesture handler
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?

jfversluis · 2022-06-30T13:39:26Z

Thanks for the kind words, appreciate it! 😄

The XAML I used is in that sample repo I posted. I didn't link it up the right way here, but you should be able to see my full reproduction here: https://github.com/jfversluis/Repro8433 for the bits and pieces you posted that is what it boiled down to according to me.

The thing that stands out in your opening post, is that the error states it's looking for the Email property on the EmailsViewModel notice the plural in this case. This is also reflected in your screenshot you posted in your last message. So it seems like something is mixed up in the scopes of things.

While reviewing your code I think I see what's up... Maybe. I see you set the x:DataType on the DataTemplate I vaguely recall doing that one time and it not working. Could you move that down to the ViewCell or the Grid maybe? Does that change it?

ghost · 2022-06-30T13:39:45Z

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.

mobycorp · 2022-06-30T13:52:02Z

@jfversluis You picked up on the exact issue that I am trying to describe.

The data context for the page is the EmailsViewModel (again, you noticed correctly it is the plural view model - the master if you will)
I explicitly set the data context for the item template by stating x:DataType="viewModels:EmailViewModel" (not plural - the detail view model)
I looked at your repo and the way you defined both view models looks correct.

Unless I don't understand fully what the x:DataType declaration does, I would think that would allow the parser to correctly map the properties in the detail view model (EmailViewModel)

mobycorp · 2022-06-30T13:55:00Z

@jfversluis Some additional detail... This is the code-behind of the content page:

namespace MobyClient.Pages;
public partial class PageEmails : ContentPage {
	public PageEmails (EmailsViewModel emailsViewModel) {
		Guard.IsNotNull (emailsViewModel);
		InitializeComponent ();
		BindingContext = emailsViewModel;
	}
}

mobycorp · 2022-06-30T13:59:40Z

@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

Here are the answers to your questions:

Runtime
Debug
I supplied code for the code-behind of the content page in a reply to Gerald below which is for EmailsViewModel (notice the plural of Emails - this is the master viewmodel) as the binding context
Yes, EmailViewModel (the detail viewmodel) has a public property Email

mobycorp · 2022-06-30T14:27:12Z

x:DataType is a compiled binding, it does not set the BindingContext, it should work even removing it.

What that does is to resolve the binding expressions at compile-time rather than runtime speeding up the binding process and also allows the intellisense in the xaml editor to suggest the properties for you reporting build errors when invalid properties are found.

Ok, that is exactly what I supposed x:DataType did. So, when in the xaml editor and I type Email.Subject and no squiggly lines are there AND it was able to find the properties and properties of properties, I would expect it to NOT report binding errors. That is the basic crux of what I am reporting.

PentaTech · 2022-06-30T14:34:36Z

If you remove the DataType in the ItemTemplate it will force the DataType you probably defined in your ContentPage expecting to find "Email" in EmailsViewModel, with "classic bindings" you should remove that too

Anyway the error shouldn't allow you to compile so that seems a bit different from what is happening to you...

As Gerald said, probably there's something about setting the DataType to the DataTemplate that gives you those errors at runtime

willshoemaker · 2022-07-06T20:58:24Z

I am experiencing the same issue and have created a repro here:
https://github.com/willshoemaker/MauiRepro8433

If the collection property on the view model is set prior to setting the view model as the binding context I do not experience a binding error, but if I set the value of the collection property later (in this case via a delayed Loaded event handler) there are binding errors for each property on each item in the collection. Note that despite the binding errors it appears to be displaying correctly, though it does not respond to XAML hot reload. I have only tested the issue on the Windows platform.

These errors occur both when the content of the ViewCell is in the initial page and also when it is in a separate ContentView

DOsterholz · 2023-09-14T07:16:51Z

I have the same problem here. My bindings work as expected, but the xaml compiler throws me error messages. However, I can launch and use the application as expected. The problem occurs in the ListView for me.

crazyfox55 · 2023-11-24T17:34:47Z

I have this issue after I upgraded from .Net 7 to .Net 8 as well as the responsiveness of opening my page is much slower.

DominikErnst · 2024-03-28T08:24:34Z

Any news about the Problem?

StephaneDelcroix · 2024-03-28T08:44:37Z

I am experiencing the same issue and have created a repro here: https://github.com/willshoemaker/MauiRepro8433

If the collection property on the view model is set prior to setting the view model as the binding context I do not experience a binding error, but if I set the value of the collection property later (in this case via a delayed Loaded event handler) there are binding errors for each property on each item in the collection. Note that despite the binding errors it appears to be displaying correctly, though it does not respond to XAML hot reload. I have only tested the issue on the Windows platform.

These errors occur both when the content of the ViewCell is in the initial page and also when it is in a separate ContentView

if you set the ItemsSource late, those Bindings Errors are normal, and expected, and should not prevent the proper display of the page.

I believe there's no problem here, the error are genuine at the time they're produced. I'm closing this old bug, if you have an issue that affects the rendering of the page, please open a new issue

ransagy · 2024-03-28T10:16:53Z

How exactly is it normal and expected to get errors and break hot reload everytime you want to refresh your binding? Is it not allowed to set the ItemSource late? is it documented as such?

I don't see how this issue is resolved in any way.

StephaneDelcroix · 2024-03-28T10:43:45Z

Bindings Errors aren't crashing the app, are they ? They only show up in the log window, as a diagnostic. It is normal to see them if you set your context late, cause at the time, there's indeed an error

But if you say HotReload breaks, there's indeed something to act on, and I'm reopenening this

/cc @chabiss

spadapet · 2024-03-28T16:37:03Z

@StephaneDelcroix the binding failures are not caused by the app setting the binding context late. The ListView is set up properly, but something internal to MAUI seems to be setting the binding context late for each item. It's out of the developer's control.

See my comment with a screenshot in this bug:

Databinding errors on ListView - What am I doing wrong? #19873 (comment)

This shouldn't affect hot reload though, so maybe something else is going on with hot reload.

jeroen-corteville · 2024-04-04T13:52:40Z

@StephaneDelcroix Hi, I think you can find more information here: #5511 (comment)

ajsuydam · 2024-06-20T19:00:43Z

@StephaneDelcroix @samhouts I'm going to add my voice to the others here - when I'm using list views, I constantly get binding errors in the log, even though it shows properly, and Hot Reload is touch and go under these circumstances. This is a serious issue for my project.

MarcelWolterbeek · 2024-08-15T08:59:07Z

Workaround to prevent the warnings (and InvalidCastExceptions):
In your ViewCell implementation add this code:

protected override void OnBindingContextChanged()
{
    if (BindingContext is TheViewModelForYourListItem)
        base.OnBindingContextChanged();
}

jo-ruch · 2024-11-05T14:47:10Z

Here is a generic solution to apply @MarcelWolterbeek 's solution

CustomViewCell.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ViewCell 
    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">
</ViewCell>

CustomViewCell.xaml.cs

namespace Your.Namespace.Here
{
    public partial class CustomViewCell<T> : ViewCell
    {
        protected override void OnBindingContextChanged()
        {
            if (BindingContext is T)
            {
                base.OnBindingContextChanged();
            }
        }
    }
}

Then this can be used as:

...

<ListView>
   <ListView.ItemTemplate>
        <DataTemplate>
            <ns:CustomViewCell x:TypeArguments="viewModels:ViewModelThatShouldBeUsed">
                 ....
             </ns:CustomViewCell>
        <DataTemplate>
   <ListView.ItemTemplate>
<ListView>

FireController1847 · 2024-12-10T11:13:31Z

Adding a +1 to this issue. The spam of warnings becomes quite unbearable when you have a list with a few hundred objects in it.

mobycorp added s/needs-verification Indicates that this issue needs initial verification before further triage will happen t/bug Something isn't working labels Jun 29, 2022

rachelkang added s/needs-info Issue needs more info from the author and removed s/needs-verification Indicates that this issue needs initial verification before further triage will happen labels Jun 30, 2022

ghost added s/needs-attention Issue has more information and needs another look and removed s/needs-info Issue needs more info from the author labels Jun 30, 2022

jfversluis added s/needs-repro Attach a solution or code which reproduces the issue and removed s/needs-attention Issue has more information and needs another look labels Jun 30, 2022

jfversluis added a commit to jfversluis/Repro8433 that referenced this issue Jun 30, 2022

https://github.com/dotnet/maui/issues/8433

bba1ab2

ghost added s/needs-attention Issue has more information and needs another look and removed s/needs-repro Attach a solution or code which reproduces the issue labels Jun 30, 2022

jfversluis added s/needs-info Issue needs more info from the author and removed s/needs-attention Issue has more information and needs another look labels Jun 30, 2022

ghost added s/needs-attention Issue has more information and needs another look and removed s/needs-info Issue needs more info from the author labels Jun 30, 2022

jsuarezruiz added the area-xaml XAML, CSS, Triggers, Behaviors label Jul 1, 2022

samhouts added this to the .NET 8 + Servicing milestone Sep 12, 2023

samhouts changed the title ~~XAML bindings are flagging invalid binding failures - it is confused as to what the data context is...~~ XAML bindings are flagging invalid binding failures in ListView - it is confused as to what the data context is... Sep 12, 2023

samhouts added the p/1 Work that is important, and has been scheduled for release in this or an upcoming sprint label Sep 18, 2023

samhouts modified the milestones: .NET 8 + Servicing, .NET 8 SR2 Oct 3, 2023

samhouts added this to MAUI SDK Ongoing Dec 21, 2023

github-project-automation bot moved this to Todo in MAUI SDK Ongoing Dec 21, 2023

Redth modified the milestones: .NET 8 SR2, .NET 8 + Servicing Jan 10, 2024

samhouts removed this from MAUI SDK Ongoing Jan 10, 2024

spadapet mentioned this issue Jan 16, 2024

Databinding errors on ListView - What am I doing wrong? #19873

Closed

samhouts added the platform/windows 🪟 label Feb 2, 2024

StephaneDelcroix self-assigned this Mar 28, 2024

StephaneDelcroix closed this as completed Mar 28, 2024

StephaneDelcroix reopened this Mar 28, 2024

samhouts removed the p/1 Work that is important, and has been scheduled for release in this or an upcoming sprint label Apr 23, 2024

PureWeen modified the milestones: .NET 8 + Servicing, Backlog Aug 2, 2024

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XAML bindings are flagging invalid binding failures in ListView - it is confused as to what the data context is... #8433

XAML bindings are flagging invalid binding failures in ListView - it is confused as to what the data context is... #8433

mobycorp commented Jun 29, 2022 •

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rachelkang commented Jun 30, 2022

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mobycorp commented Jun 30, 2022

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ghost commented Jun 30, 2022

mobycorp commented Jun 30, 2022 •

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jfversluis commented Jun 30, 2022

ghost commented Jun 30, 2022

mobycorp commented Jun 30, 2022

mobycorp commented Jun 30, 2022

mobycorp commented Jun 30, 2022

mobycorp commented Jun 30, 2022

PentaTech commented Jun 30, 2022 •

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willshoemaker commented Jul 6, 2022 •

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DOsterholz commented Sep 14, 2023

crazyfox55 commented Nov 24, 2023

DominikErnst commented Mar 28, 2024

StephaneDelcroix commented Mar 28, 2024

ransagy commented Mar 28, 2024

StephaneDelcroix commented Mar 28, 2024 •

edited

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spadapet commented Mar 28, 2024

jeroen-corteville commented Apr 4, 2024

ajsuydam commented Jun 20, 2024

MarcelWolterbeek commented Aug 15, 2024

jo-ruch commented Nov 5, 2024

FireController1847 commented Dec 10, 2024

XAML bindings are flagging invalid binding failures in ListView - it is confused as to what the data context is... #8433

XAML bindings are flagging invalid binding failures in ListView - it is confused as to what the data context is... #8433

Comments

mobycorp commented Jun 29, 2022 • edited Loading

Description

Steps to Reproduce

Version with bug

Last version that worked well

Affected platforms

Affected platform versions

Did you find any workaround?

Relevant log output

rachelkang commented Jun 30, 2022

ghost commented Jun 30, 2022

mobycorp commented Jun 30, 2022

StephaneDelcroix commented Jun 30, 2022

ghost commented Jun 30, 2022

mobycorp commented Jun 30, 2022 • edited Loading

jfversluis commented Jun 30, 2022

ghost commented Jun 30, 2022

mobycorp commented Jun 30, 2022

mobycorp commented Jun 30, 2022

mobycorp commented Jun 30, 2022

mobycorp commented Jun 30, 2022

PentaTech commented Jun 30, 2022 • edited Loading

willshoemaker commented Jul 6, 2022 • edited Loading

DOsterholz commented Sep 14, 2023

crazyfox55 commented Nov 24, 2023

DominikErnst commented Mar 28, 2024

StephaneDelcroix commented Mar 28, 2024

ransagy commented Mar 28, 2024

StephaneDelcroix commented Mar 28, 2024 • edited Loading

spadapet commented Mar 28, 2024

jeroen-corteville commented Apr 4, 2024

ajsuydam commented Jun 20, 2024

MarcelWolterbeek commented Aug 15, 2024

jo-ruch commented Nov 5, 2024

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