InvalidCastException when binding a child control's DataContext to a parent ViewModel property with Compiled Bindings enabled

[lindexi]

I have a MainWindow whose DataContext is set to a MainViewModel. The MainViewModel exposes a SubViewModel property. I want to pass this SubViewModel as the DataContext of a child FooUserControl, so that the control can bind to properties on SubViewModel using its own compiled x:DataType.

Here is the setup:

MainViewModel & SubViewModel:

public class MainViewModel
{
    public SubViewModel SubViewModel { get; } = new SubViewModel();
}

public class SubViewModel
{
    public string Name { get; set; } = "Foo";
}

MainWindow.axaml — the Window has x:DataType="MainViewModel" and passes SubViewModel to the child control via DataContext="{Binding Path=SubViewModel}":

<Window xmlns="https://github.com/avaloniaui"
        x:Class="LajaidemdawHalwelrearkea.MainWindow"
        x:DataType="viewModels:MainViewModel">
    <Window.DataContext>
        <viewModels:MainViewModel/>
    </Window.DataContext>
    <lajaidemdawHalwelrearkea:FooUserControl DataContext="{Binding Path=SubViewModel}"/>
</Window>

FooUserControl.axaml — the child control declares x:DataType="SubViewModel" and binds {Binding Name}:

<UserControl xmlns="https://github.com/avaloniaui"
             x:Class="LajaidemdawHalwelrearkea.FooUserControl"
             x:DataType="viewModels:SubViewModel">
    <TextBlock Text="{Binding Name}"/>
</UserControl>

When <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault> is set in the .csproj, the application throws the following exception once during startup:

System.InvalidCastException: Unable to cast object of type
'LajaidemdawHalwelrearkea.ViewModels.MainViewModel' to type
'LajaidemdawHalwelrearkea.ViewModels.SubViewModel'.

The exception occurs inside the compiled binding generated getter for the child control:

private static object LajaidemdawHalwelrearkea_002EViewModels_002ESubViewModel_002CLajaidemdawHalwelrearkea_002EName_0021Getter(object P_0)
{
    return ((SubViewModel)P_0).Name;
}

It appears that during initialization the compiled binding passes the parent's DataContext (MainViewModel) into the FooUserControl's getter before the child's own DataContext binding ({Binding Path=SubViewModel}) has taken effect, causing the cast to fail.

After this one-time exception, the application continues to run normally and the bindings work as expected.

My question: Is this a bug in the compiled bindings system, or am I using DataContext="{Binding Path=SubViewModel}" incorrectly with x:DataType on the child control? If there is a recommended pattern for passing a sub-viewmodel to a child control's DataContext when compiled bindings are enabled, I'd appreciate any guidance.

All my code is in github: https://gitlab.gz.cvte.cn/lindexi/InternalCode/tree/ff3f7aa8c453924511f49f07748a786ff47fc52e/AvaloniaIDemo/LajaidemdawHalwelrearkea

I can repro in Avalonia 12.0.3

[LaurentInSeattle]

I got a similar issue, it was resolved using ReflectionBinding instead of Binding.

HTH

[lindexi]

Thank you @LaurentInSeattle , but ReflectionBinding breaks the aot

[Necromanika]

I do not try it, but maybe the fact that MainWindow's DataContext is set via XAML with MainViewModel is the reason why your FooUserControl's DataContext is also initially set to this MainViewModel. And only after that FooUserControl's DataContext binding take effect, so you app continues to run normally

I haven't encountered such errors, even though I use a similar methods for passing ViewModels. However, I never expose ViewModels via XAML

Try to remove from MainWindow

    <Window.DataContext>
        <viewModels:MainViewModel/>
    </Window.DataContext>

And set Window DataContext from code (probably in OnFrameworkInitializationCompleted() mathod of App class)

[lindexi]

Thank you @Necromanika