TemplateBinding to ICommand property not work

[Meloman19]

Describe the bug
TemplateBinding doesn't work for the ICommand property with Button inside template.

Steps to reproduce the bug

1. Create CustomControl:

class MyControl : Control
{
    public static readonly DependencyProperty MyCommandProperty =
        DependencyProperty.Register("MyCommand", typeof(System.Windows.Input.ICommand), typeof(MyControl), new PropertyMetadata(null));

    public System.Windows.Input.ICommand MyCommand
    {
        get { return (System.Windows.Input.ICommand)GetValue(MyCommandProperty); }
        set { SetValue(MyCommandProperty, value); }
    }
}

2. On root MainWindow:

<StackPanel>
    <StackPanel.Resources>
        <Style TargetType="local:MyControl">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="local:MyControl">
                        <Grid>
                            <Button Command="{TemplateBinding MyCommand}"
                                    Content="RUN"/>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </StackPanel.Resources>

    <local:MyControl x:Name="MyControl" />
    <TextBlock x:Name="TextBlock" />
</StackPanel>

3. On code behind MainWindow

public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        this.InitializeComponent();

        MyControl.MyCommand = new RelayCommand(DO); // Classic RelayCommand
    }

    private void DO(object obj)
    {
        TextBlock.Text = "PRESSED";
    }
}

Expected behavior
After clicking on the button, the text must become "PRESSED". But it's not.
Similar TemplateBindings to other properties work fine.

Workaround
Change
Command="{TemplateBinding MyCommand}"
To
Command="{Binding RelativeSource={RelativeSource Mode=TemplatedParent}, Path=MyCommand}"

Version Info

NuGet package version:
[Microsoft.ProjectReunion 0.5.0-prerelease]
[Microsoft.ProjectReunion.Foundation 0.5.0-prerelease]
[Microsoft.ProjectReunion.WinUI 0.5.0-prerelease]

Windows app type:

UWP	Win32
	Yes

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)	Yes
May 2020 Update (19041)
November 2019 Update (18363)	Yes
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

[huoyaoyuan]

Should be also #4414

[StephenLPeters]

@Meloman19 can you try the fix suggested in #4414 and confirm this is the same issue?

[Meloman19]

@StephenLPeters I tried and nothing changed.
I have no problem with style loading or template loading. Style loaded and Template is setted but TemplateBinding from MyControl.MyCommand to Button.CommandPropertynot worked. However TemplateBinding, for example, to Content is worked.

Example 2:

MyControl.cs

class MyControl : Control
{
   public MyControl()
   {
      DefaultStyleKey = typeof(MyControl);
   }

   public static readonly DependencyProperty MyCommandProperty =
         DependencyProperty.Register("MyCommand", typeof(System.Windows.Input.ICommand), typeof(MyControl), new PropertyMetadata(null));

   public System.Windows.Input.ICommand MyCommand
   {
      get { return (System.Windows.Input.ICommand)GetValue(MyCommandProperty); }
      set { SetValue(MyCommandProperty, value); }
   }

   public static readonly DependencyProperty MyContentProperty =
         DependencyProperty.Register("MyContent", typeof(object), typeof(MyControl), new PropertyMetadata(null));

   public object MyContent
   {
      get { return GetValue(MyContentProperty); }
      set { SetValue(MyContentProperty, value); }
   }
}

MainWindow.xaml

<StackPanel>
    <StackPanel.Resources>
        <Style x:Key="MyStyle"
               TargetType="local:MyControl">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="local:MyControl">
                        <Grid>
                            <Button Command="{TemplateBinding MyCommand}"
                                    Content="{TemplateBinding MyContent}"
                                    Click="Button_Click"/>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </StackPanel.Resources>

    <local:MyControl x:Name="MyControl"
                     Style="{StaticResource MyStyle}" />
</StackPanel>

MainWindow.cs

public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        this.InitializeComponent();

        MyControl.MyCommand = new RelayCommand(DO);
        MyControl.MyContent = "RUN";
    }

    private void DO(object obj)
    {
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        var button = sender as Button;

        bool hasCommand = button.Command != null;
        bool hasContent = button.Content != null;
    }
}

I see Button from Template and see "RUN" as content. If clicked on Button_Click I got hasCommand = false and hasContent = true.

[StephenLPeters]

Hmm, how about using {Binding RelativeParent={RelativeParent TemplatedParent} Path=MyCommand}

[Meloman19]

@StephenLPeters I already wrote about this :) RelativeSource works.

Workaround
Change
Command="{TemplateBinding MyCommand}"
To
Command="{Binding RelativeSource={RelativeSource Mode=TemplatedParent}, Path=MyCommand}"

I know this is a low priority bug (if at all it's bug). But this behavior differs from WPF and UWP.

[StephenLPeters]

Ah, sorry missed that... Hmm, this feels like it should work.... I know that there is a requirement for {TemplateBinding} that the DependencyObject you are using it on be in the visual tree. But that looks to be the case here (especially given that your template bindings to content work). @llongley suggests that a potential reason could be that the ICommand interface has a different namespace in native and managed dlls, maybe this is tripping up template binding. Would need some debugging.

[codendone]

Note: Still repros on the 0.5.5 release. (And, as noted above, this works properly on system UWP XAML.)

[Panda-Sharp]

Just started to play with WinUI, I started with 1.2, and I just hit this issue ... and I'm really surprised to see it here since March 2021!

[andreaslennartz]

Same issue as in #7003 - still present in 1.2, very annoying bug. Also very surprised to learn that this hasn't been fixed.

[Soap-141]

Any update on this?