Picker ItemDisplayBinding doesn't work with compiled bindings

[davidortinau]

Description

When I use ItemDisplayBinding in 8.0.80 nightly or .NET 9 Preview 7, the picker doesn't populate as I'd expect.

I can get around it using a ToString override on the model, or by providing the x:DataType=models:VocabularyList.

The ContentPage does specify a x:DataType for the view model. This doesn't work:

<Picker x:Name="VocabPicker" 
    ItemsSource="{Binding VocabLists}"
    ItemDisplayBinding="{Binding Name}"                    
    SelectedItem="{Binding VocabList, Mode=TwoWay}" />

This does work:

<Picker x:Name="VocabPicker" 
    ItemsSource="{Binding VocabLists}"
    ItemDisplayBinding="{Binding Name, x:DataType=models:VocabularyList}"                    
    SelectedItem="{Binding VocabList, Mode=TwoWay}" />

Steps to Reproduce

https://github.com/davidortinau/SentenceStudio/blob/main/src/SentenceStudio/Pages/Lesson/LessonStartPage.xaml

My testing with the original code:

8.0.71 - Debug - Good
8.0.71 - Release - Good
8.0.80 - Debug - Sad
8.0.80 - Release - Good

Link to public reproduction project repository

No response

Version with bug

Nightly / CI build (Please specify exact version)

Is this a regression from previous behavior?

Yes, this used to work in .NET MAUI

Last version that worked well

8.0.61 SR6.1

Affected platforms

iOS, Android, Windows, macOS

Affected platform versions

No response

Did you find any workaround?

Yes, as described

Relevant log output

No response

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[simonrozsival]

This is likely caused by #22056 and it would also break with .NET 9 in Release mode where the binding would be compiled with the wrong source data type.

There is a mismatch between the declared x:DataType (<ContentPage ... x:DataType="lesson:LessonStartPageModel">) and the actual type of the binding context when the binding is applied (lesson:VocabularyList). Specifying the x:DataType for the binding is the right way to declare this binding going forward.

[StephaneDelcroix]

I did some testing, on the net9.0 branch (but should be similar on 'main'):

• ItemDisplayBinding is compiled if x:DataType is set on the {Binding}
• isn't compiled otherwise (without warning)

so,

• we shouldn't prevent the Binding to be applied if there's a DataType mismatch in this case...
• we should issue a compilation-time warning before falling back if DataType is missing

[hungchips]

This is still happening on MAUI 8.0.82.
And even I did it this way:
<Picker x:Name="VocabPicker" ItemsSource="{Binding VocabLists}" ItemDisplayBinding="{Binding Name, x:DataType=models:VocabularyList}" SelectedItem="{Binding VocabList, Mode=TwoWay}" />

But it still doesn't show up even though the Source has data.