Wpf: OnApplyTemplate not called on ContentControl

[mysteryx93]

Version: .NET Core 3
Windows 10 x64
It works in .NET Framework 4.8

I have a WPF user control deriving from ContentControl. It's working fine in .NET Framework 4.8. In .NET Core 3, however, OnApplyTemplate never gets called. (MediaPlayerWpf inherits from ContentControl)

public class MpvMediaPlayer : MediaPlayerWpf
{
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        // This never gets called.
    }
}

[weltkante]

Works for me in both 3.0 GA (3.0.0-rc1-19456-20) and an older preview 8 I had lying around (3.0.0-preview8-28376-27). I'm using this code and can put a breakpoint into OnApplyTemplate which gets hit correctly. Can you provide a reproduction example?

working code

```C#
namespace WpfApp1
{
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
}

public class MediaPlayerWpf : ContentControl
{
}

public class MpvMediaPlayer : MediaPlayerWpf
{
    public override void OnApplyTemplate()
    {
        // breakpoint gets hit (contrary to issue description)
        base.OnApplyTemplate();
    }
}

}

```XAML
<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApp1"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <local:MpvMediaPlayer />
    </Grid>
</Window>

[mysteryx93]

Reproduced the problem. The line causing OnApplyTemplate to never trigger is DefaultStyleKeyProperty. Comment that line and it gets triggered

namespace EmergenceGuardian.MpvPlayerUI
{
    public class MpvTest : Control
    {
        static MpvTest()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(MpvTest), new FrameworkPropertyMetadata(typeof(MpvTest)));
        }

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
        }
    }
}

It looks like it's somehow not finding or not binding the template. I put this in Themes\Generic.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                xmlns:local="clr-namespace:EmergenceGuardian.MpvPlayerUI">

    <Style TargetType="{x:Type local:MpvTest}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:MpvMediaPlayerHost}">
                    <Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
                        <Grid>
                        </Grid>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

[weltkante]

Thats probably a duplicate of #1699 - Desktop Framework had Properties\AssemblyInfo.cs in the project template which contained a [assembly: ThemeInfo(...)] attribute which is required for generic.xaml to work. The .NET Core template is missing this, you have to add that attribute manually for now.

[vatsan-madhavan]

The updated application templates in .NET Core 3.1 and .NET Core 5.0 should now address this.