Why validation does not working with indirect property of view model?

[LJN-hzleaper]

If a view model has a property A, which has a property B, ReactiveUI.Validation cannot validates property B. Following is my code. The first one of two textboxes display without any differences from plain textbox, the second one will display a tooltip when mouse hovered and have a red border. Can anyone explains?

public class MainViewModel : ReactiveValidationObject
{
    private readonly ObservableAsPropertyHelper<string> _helper;

    public string Msg
    {
        get => _helper.Value;
        set => Hello.Msg = value;
    }

    [Reactive]
    public Container Hello { get; set; } = new();

    public MainViewModel()
    {
        this.WhenAnyValue(x => x.Hello.Msg)
            .ToProperty(this, x => x.Msg, out _helper);
        this.ValidationRule(x => x.Hello.Msg,
            hi => false,
            "Hi, Hello.Msg.");
        this.ValidationRule(x => x.Msg,
            hi => false,
            "Hi, Msg.");
    }
}

public class Container 
{
    [Reactive]
    public string Msg { get; set; }
}

MainWindow.xaml :

<reactiveUi:ReactiveWindow
    x:TypeArguments="local:MainViewModel" x:Class="WpfPlayGround.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfPlayGround"
    xmlns:reactiveUi="http://reactiveui.net"
    xmlns:system="clr-namespace:System;assembly=System.Runtime"
    xmlns:views="clr-namespace:DoffReportSharp.Views"
    mc:Ignorable="d"
    Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:MainViewModel/>
    </Window.DataContext>
    <Window.Resources>
        <Style TargetType="TextBox" BasedOn="{StaticResource {x:Type TextBox}}">
            <Setter Property="Margin" Value="4"></Setter>
            <Setter Property="ToolTipService.InitialShowDelay" Value="0"/>
            <Setter Property="ToolTipService.IsEnabled" Value="False"></Setter>
            <Setter Property="ToolTip" Value=""/>
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="True">
                    <Setter
                        Property="ToolTip"
                        Value="{Binding 
                        RelativeSource={x:Static RelativeSource.Self},
                        Path=(Validation.Errors)/ErrorContent,
                        FallbackValue={x:Static system:String.Empty}}">
                    </Setter>
                    <Setter Property="ToolTipService.IsEnabled" Value="True"></Setter>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <StackPanel>
        <TextBox Text="{Binding Hello.Msg}"></TextBox>
        <TextBox Text="{Binding Msg}"></TextBox>
    </StackPanel>
</reactiveUi:ReactiveWindow>

[JakenVeina]

Is there supposed to be an image attached?

Can I assume Container actually subclasses ReactiveObject and you just omitted it?

The issue is likely that {Binding} is looking to extract validation data from Hello as the owner of the Msg property, and you didn't implement the validation there.