avalonia docs

Binding in a Control Template

When you're creating a control template and you want to bind to the templated parent you can use:

<TextBlock Name="tb" Text="{TemplateBinding Caption}"/>

<!-- Which is the same as -->
<TextBlock Name="tb" Text="{Binding Caption, RelativeSource={RelativeSource TemplatedParent}}"/>

Although the two syntaxes shown here are equivalent in this case, there is a difference. TemplateBinding accepts only a single property rather than a property path, so if you want to bind using a property path you must use the second syntax:

<!-- This WON'T work as TemplateBinding only accepts single properties -->
<TextBlock Name="tb" Text="{TemplateBinding Caption.Length}"/>

<!-- Instead this syntax must be used in this case -->
<TextBlock Name="tb" Text="{Binding Caption.Length, RelativeSource={RelativeSource TemplatedParent}}"/>