WPF FindAncestor binding finds source in Release, fails in Debug - wpf

Question

I have a binding error that makes no sense. It always works in Release Mode. It sometimes -- only sometimes -- fails to find the source via FindAncestor in Debug Mode
Its for Style for a WPF Path that I use only when that Path is inside a specific custom control named LayerView. Below is the style. Note the 3 bindings that look for the parent source object LayerView are the ones that sometimes fail
<Style x:Key="LayerViewGuidePathStyle" TargetType="{x:Type Path}">
<Setter Property="Data">
<Setter.Value>
<MultiBinding Converter="{StaticResource CvtGuideOption}">
<Binding Source="{svc:ViewSettings}, Path=GuideOption}" />
<Binding RelativeSource="{RelativeSource AncestorType={x:Type ctrl:LayerView}}" Path="ScanWidth" Converter="{ctrl:LogValueConverter}"/>
<Binding RelativeSource="{RelativeSource AncestorType={x:Type ctrl:LayerView}}" Path="ScanHeight" Converter="{ctrl:LogValueConverter}"/>
<Binding RelativeSource="{RelativeSource AncestorType={x:Type ctrl:LayerView}}" Path="SceneTransform" Converter="{ctrl:LogValueConverter}"/>
</MultiBinding>
</Setter.Value>
</Setter>
</Style>
When they fail in Debug mode, this is the warning
System.Windows.Data Warning: 4 : Cannot find source for binding with
reference 'RelativeSource FindAncestor,
AncestorType='MyControls.LayerView', AncestorLevel='1''.
BindingExpression:Path=ScanHeight; DataItem=null; target element is
'Path' (Name='GuidePath'); target property is 'Data' (type 'Geometry')
Here is how I use this style inside my custom control LayerView
<ctrl:LayerView x:Name="MainLayerView" ItemsSource="{Binding Shapes}">
<ctrl:LayerView.Layers>
<Path x:Name="GuidePath" Style="{StaticResource LayerViewGuidePathStyle}" />
</ctrl:LayerView.Layers>
</ctrl:LayerView>
As you can probably tell, LayerView is an ItemsControl: Specifically a MultiSelector. My custom version merely adds non-dependency property named Layers which is a Collection<object>. The idea is that the user will set these statically in XAML, as I did above, and I will display them over the standard items of the control.
public class LayerView : MultiSelector
{
static LayerView()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(LayerView), new FrameworkPropertyMetadata(typeof(LayerView)));
}
private Collection<object>? _layers;
public Collection<object> Layers => _layers ??= new Collection<object>();
}
Finally, here is the ControlTemplate for LayerView. In addition to the required ItemsPresenter, I add an ItemsControl to show the contents of Layers as basically non-hit-detectable "overlays"
<ControlTemplate x:Key="LayerViewTemplate" TargetType="{x:Type gcl:LayerView}">
<Canvas x:Name="PART_MainCanvas" Background="Transparent">
<!-- We are an ItemsControl so present our items -->
<ItemsPresenter x:Name="PART_Items" />
<!-- Now present our custom layers. -->
<ItemsControl x:Name="PART_Layers"
ItemsSource="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Layers}"
IsHitTestVisible="False"/>
</Canvas>
</ControlTemplate
I have no moves here. I can usually fix bindings that fail, but I would expect consistent failure or success. Not a mix. And especially not intermittent failures It's almost as if it's a race condition.
Can anyone point me in the right direction as to why this would fail? Is my Layers property of the wrong type or is it missing some crucial attribute or something?

Answer 1

Without reproducing this error, of course, it is difficult to say exactly what the cause is. But I have some guesses.
The elements of the Layers collection are initialized BEFORE being added to the collection, but the binding is resolved after the element is loaded;
Layers is a regular CLR collection and information about loading ItemsControl (LayerView ) is not passed to it. Therefore, the elements of this collection, as I think, are initialized completely at once, including bindings. And this can happen before these items are included in the ItemsControl panel.
As I think, these errors also occur in Release, it's just that not all errors are caught in this mode. It is guaranteed that only errors that are critical for the operation of the application are caught. For example, exceptions.
At a minimum, you need to replace the collection with a FreezableCollection and implement a read-only DependecyProperty.
True, in this case, you can only include DependencyObject in it.
private static readonly DependencyPropertyKey LayersPropertyKey
= DependencyProperty.RegisterReadOnly(
nameof(Layers),
typeof(FreezableCollection<DependencyObject>),
typeof(LayerView),
new PropertyMetadata(null, (d, e) => ((LayerView)d).protectedLayers = (FreezableCollection<DependencyObject>)e.NewValue));
public static readonly DependencyProperty LayersProperty
= LayersPropertyKey.DependencyProperty;
public FreezableCollection<DependencyObject> Layers
{
get => (FreezableCollection<DependencyObject>)GetValue(LayersProperty);
}
// An optional auxiliary property for quickly obtaining a collection
// in the internal logic of the LayerView and its derivatives.
protected FreezableCollection<DependencyObject> protectedLayers {get; private set;}
// Initializing a Collection in a Constructor.
public LayerView() => SetValue(LayersPropertyKey, new FreezableCollection<DependencyObject>());
Supplement
Unfortunately, I could not immediately check your version - there was no "at hand" computer with the Studio.
Now I checked it.
I did not change the code of the LayerView element itself, I did not set the template in the style.
I got the Binding error only if the Element Template is not set.
<Window x:Class="Core2023.SO.Joe.BindingErrors.BindingErrorsWindow"
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:Core2023.SO.Joe.BindingErrors"
mc:Ignorable="d"
Title="BindingErrorsWindow" Height="150" Width="400">
<Window.Resources>
<ControlTemplate x:Key="LayerViewTemplate" TargetType="{x:Type local:LayerView}">
<ItemsControl x:Name="PART_Layers"
ItemsSource="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Layers}"/>
</ControlTemplate>
</Window.Resources>
<Grid>
<local:LayerView>
<local:LayerView.Layers>
<TextBlock Text="{Binding ActualHeight, RelativeSource={RelativeSource AncestorType={x:Type local:LayerView}}}"/>
</local:LayerView.Layers>
</local:LayerView>
</Grid>
</Window>
In order to accurately diagnose the cause of your problem, I need more complete code to reproduce the problem.