I have a DataTemplate that contains an Expander with a border in the header. I want the header border to have round corners when collapsed and straight bottom corners when expanded. What would best practice be for achieving this (bonus points for code samples as I am new to XAML)?

This is the template that holds the expander:

    <DataTemplate x:Key="A">
        <StackPanel>
            <Expander Name="ProjectExpander" Header="{Binding .}" HeaderTemplate="{StaticResource B}" >
                <StackPanel>
                    <Border CornerRadius="0,0,2,2">

This is the expander datatemplate:

    <DataTemplate x:Key="B">
        <Border x:Name="ProjectExpanderHeader"
                CornerRadius="{Binding local:ItemUserControl.ProjectHeaderBorderRadius, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContentPresenter}}}"
                Background="{StaticResource ItemGradient}"   
                HorizontalAlignment="{Binding HorizontalAlignment,
                                              RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContentPresenter}},
                                              Mode=OneWayToSource}">
            <local:ItemContentsUserControl Height="30"/>
        </Border>
    </DataTemplate>