Why can't I bind command in DataContext?

[kyuranger]

There is a project in our company. We don’t know which version of Avalonia it was created from, but it has been continuously upgraded with the upgrade of Avalonia. Previously, the binding of Command in this project was similar to this:

<Window xmlns="https://github.com/avaloniaui"
        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"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AvaloniaApplication1.MainWindow"
		xmlns:VM="using:AvaloniaApplication1"
		x:DataType="VM:TestViewModel"
        Title="AvaloniaApplication1">
	<ItemsControl>
		<ItemsControl.ItemTemplate>
			<DataTemplate >
				<Button Command="{Binding $parent[Window].DataContext.TestCommand}">
					
				</Button>
			</DataTemplate>
		</ItemsControl.ItemTemplate>
		<ItemsControl.ItemsPanel>
			<ItemsPanelTemplate>
				<StackPanel></StackPanel>
			</ItemsPanelTemplate>
		</ItemsControl.ItemsPanel>
	</ItemsControl>
</Window>

ViewModel:

using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AvaloniaApplication1
{
    public partial class TestViewModel:ObservableObject
    {
        public RelayCommand TestCommand { get; }
    }
}

Recently, we upgraded this project to 11.2.5 and planned to add some new features (this project hasn't added new features for a long time). We added code similar to the above, but it prompted the following error:
Unable to resolve property or method of name 'TestCommand' on type 'System.Object'. Line 13, position 13.

But what confuses me is that the code that has been compiled successfully before never has such an error.

I thought there was something wrong with my code until I created a new Avalonia 11.2.5 project and added the above code to test it, and it prompted the same error.

I suspect that a certain version of Avalonia was changed, which caused this error to occur, but we don't know which version it was changed.

What I still don't understand is why the code that compiled successfully in the previous version of Avalonia did not prompt such an error.

How can I bind the Command of DataContext in the latest version of Avalonia?

[DoubleDBE]

I don't know the specifics when using $parent, but you can set x:Name="This" in the Window tag and bind the command like this;

Command="{Binding #This.((VM:TestViewModel)DataContext).TestCommand}"

If someone can tell me how to use $parent instead, that'd be great!

[tobyfirth]

I don't think a binding like that works if you are using compiled bindings which you might have enabled? There was also a refactor to the binding code in 11.1 which might have changed something.

Changing it to this should work though.

<Button Command="{Binding $parent[Window].((VM:TestViewModel)DataContext).TestCommand}">
					
</Button>

You could also do something like this as well.

<Button Command="{Binding $parent[ItemsControl].((VM:TestViewModel)DataContext).TestCommand}">
					
</Button>

The documentation here has some more information about how compiled bindings work.

[DoubleDBE]

Doesn't work as expected. also, what is t. ?

[tobyfirth]

An error I made when editing the code. The t should not be there.

[tobyfirth]

If you want to target a UserControl you also need to set the xmlns namespace for it xmlns:uc="using:MyApplication.MyControls" then use it in $parent.

<Button Command="{Binding $parent[uc:MyUserControl].((VM:TestViewModel)DataContext).TestCommand}">
					
</Button>

[kyuranger]

It works now according to your solution, but I still don't understand why the old code can still run without reporting any errors.