WPF old Control Template inside new Control Template

Question

I use a custom control from an external library, it has a property with ControlTemplate.

  <Style x:Key="{x:Type controls:HeaderDiscountButton}" TargetType="{x:Type controls:HeaderDiscountButton}" BasedOn="{StaticResource {x:Type core:ValueButton}}">
        <Setter Property="core:ButtonWithPopup.PopupTemplate">
            <Setter.Value>
                <ControlTemplate> 
                   some complicated xaml code
                </ControlTemplate>
          </Setter.Value>
        </Setter>
  </Style>

I would like to override it, but use old one in my new ControlTemplate, for example

  <Style x:Key="{x:Type controls:HeaderDiscountButton}" TargetType="{x:Type controls:HeaderDiscountButton}" BasedOn="{StaticResource {x:Type core:ValueButton}}">
        <Setter Property="core:ButtonWithPopup.PopupTemplate">
            <Setter.Value>
                <ControlTemplate> 
                   <StackPanel>
                      <Border>some complicated xaml code</Border>
                      <Border>some additional xaml code</Border>
                   </StackPanel>
                </ControlTemplate>
          </Setter.Value>
        </Setter>
  </Style>

I would like to avoid copying whole old code

Answer 1

This is not possible I am afraid. You can indeed base a Style on another Style and override specfic setters but you cannot "override" or inherit only a part of a ControlTemplate. Unfortunately you must (re)define the entire template as a whole.

So in this case you actually need to copy the (whole) original template and then edit it as per your requirements.