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 most cases, there are some differences:
  1. 1.
    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}}"/>
  2. 2.
    A TemplateBinding only supports OneWay mode for performance reasons (this is the same as WPF). This means a TemplateBinding is actually equivalent to {Binding RelativeSource={RelativeSource TemplatedParent}, Mode=OneWay}. If TwoWay binding is required in a control template, the full syntax is needed as shown below. Note that Binding will also use the default binding mode unlike TemplateBinding.
    {Binding RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}
  3. 3.
    TemplateBinding can only be used on IStyledElement.
<!-- This WON'T work as GeometryDrawing is not a IStyledElement. -->
<GeometryDrawing Brush="{TemplateBinding Foreground}"/>
<!-- Instead this syntax must be used in this case. -->
<GeometryDrawing Brush="{Binding Foreground, RelativeSource={RelativeSource TemplatedParent}}"/>