UserControl databinding inside ListView fails - wpf

Question

I have a custom control, here simplified:
<UserControl x:Class="WPF.TestControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WPF"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800">
<Grid>
<ContentPresenter Content="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Content}" />
</Grid>
</UserControl>
With usage:
<local:TestControl Grid.Row="0">
<Border BorderBrush="Red" BorderThickness="1">
<TextBlock Text="{Binding Path=Message, Mode=OneWay}"/>
</Border>
</local:TestControl>
So far so good, that works.
However, when used as part of a DataTemplate for a ListView, the binding no longer works:
<ListView ItemsSource="{Binding Path=Items}" Grid.Row="2">
<ListView.Resources>
<DataTemplate DataType="{x:Type local:TestViewModel}">
<local:TestControl>
<TextBlock Text="{Binding Path=Message, Mode=OneWay}"/>
</local:TestControl>
</DataTemplate>
</ListView.Resources>
</ListView>
What incantations are needed to make the databinding above work?

Answer 1

You are creating an infinite loop.
The UserControl already is a ContentControl as it derives from it. So as long as you don't override its ControlTemplate, you should not add a ContentPresenter to it, which binds to the TemplatedParent while being used inside a template.
The trigger of this misery is the TemplatedParent binding. The TestControl is part of a template. The template is applied to the ListViewItem, which is therefore the templated parent of TestControl.
Now the value of the binding source of the ContentPresenter inside the TestControl is the value of the ListViewItem.Content property, which holds the TestControl itself. This way you add the templated parent ListViewItem.Content, which is the TestControl, to the ContentPresenter of TestControl, which now contains the same TestControl (itself), which binds the templated parent ListViewItem.Content, which is the TestControl, to the ContentPresenter of TestControl, which now contains the same TestControl (itself), which binds the templated parent... StackOverflow.
If you want to use the UserControl like a ContentControl i.e. to display the visuals of other "external" controls, then use it like the ContentCobtrol it is:
TestControl
<UserControl x:Class="TestControl">
</UserControl>
DataTemplate
<DataTemplate DataType="{x:Type local:TestViewModel}">
<local:TestControl>
<TextBlock Text="{Binding Path=Message, Mode=OneWay}"/>
</local:TestControl>
</DataTemplate>
Or if the content is more complex and advanced than simple presentation, you should override the ControlTemplate:
TestControl
<UserControl x:Class="TestControl">
<UserControl.Template>
<ControlTemplate TargetType="local:TreeIndex">
<Border>
<Grid>
...
<ContentPresenter Content="{TemplateBinding Content}" />
</Grid>
</Border>
</ControlTemplate>
</UserControl.Template>
</UserControl>
DataTemplate
<DataTemplate DataType="{x:Type local:TestViewModel}">
<local:TestControl>
<TextBlock Text="{Binding Path=Message, Mode=OneWay}"/>
</local:TestControl>
</DataTemplate>