XAML C# expression tries to assign a value to the read-only properties, but it should be based on the binding mode

[egvijayanand]

Description

XAML C# expression tries to assign a value to the read-only properties, but it should be based on the binding mode.

Try binding a ViewModel's Command property to the Button or a getter-only property to the Label.

Command:

private RelayCommand? incrementCommand;
public IRelayCommand IncrementCommand => incrementCommand ??= new RelayCommand(new Action(Increment));

[ObservableProperty]
public partial string CountText { get; private set; } = "Current count: 0";

XAML:

<Button Command="{IncrementCommand}" />
<Label Text="{CountText}" />

But ends up in a build error.

Error message: Property or indexer 'MainViewModel.IncrementCommand' cannot be assigned to -- it is read only.

It seems that it tries to assign a value to the read-only Command property. In general, command properties are intended to be read-only, featuring only a getter.

Steps to Reproduce

Try binding a getter-only, like Command, property in XAML using a C# expression.

Link to public reproduction project repository

Easy to reproduce.

Version with bug

10.0.40

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

Unknown/Other

Affected platforms

iOS, Android, Windows, macOS

Affected platform versions

All

Did you find any workaround?

Revert to the classic Binding extension.

Relevant log output

The Property or indexer 'MainViewModel.IncrementCommand' cannot be assigned to -- it is read only.
The property or indexer 'MainViewModel.CountText' cannot be used in this context because the set accessor is inaccessible.

[egvijayanand]

Only the TwoWay and OneWayToSource binding modes require the value to be assigned back to the source property using an accessible setter; for other binding types, it is sufficient to have a property with a publicly accessible getter.

[TamilarasanSF4853]

This issue has been validated in Visual Studio Code 1.109.3 and Visual Studio Insiders with MAUI versions 10.0.40. It is not reproduced on the Android, iOS, Mac, or Windows platforms from our end.

CommandIssue.zip

[egvijayanand]

This issue has been validated in Visual Studio Code 1.109.3 and Visual Studio Insiders with MAUI versions 10.0.40. It is not reproduced on the Android, iOS, Mac, or Windows platforms from our end.

Will provide a repro sample.

[egvijayanand]

Have uploaded a sample here to simulate the issue.

In the Views\MainPage.xaml, a Label and a Button use XAML C# expression. Here, the classic Binding operation works (commented code), whereas the XAML C# expression results in a build error.

It seems it tries to use the Setter method for all binding operations, but it should be based on the Binding mode.

[HarishKumarSF4517]

This issue has been validated using Visual Studio 18.3.0 and Visual Studio Code 1.109.4 with MAUI versions 10.0.0, 10.0.30, 10.0.31 and 10.0.40. It can be reproducible on 10.0.40 on Windows, Android, iOS and macOS platform.

10.0.40	Below 10.0.40