HandyControl/src/Shared/HandyControl_Shared/Themes/Styles/Base/ExpanderBaseStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:hc="clr-namespace:HandyControl.Controls">

    <ControlTemplate x:Key="ExpanderLeftTemplate" TargetType="Expander">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>
            <Border x:Name="BorderHeader" Grid.Column="1" CornerRadius="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}">
                <ToggleButton VerticalAlignment="Stretch" VerticalContentAlignment="Stretch" Focusable="False" Padding="0,0,0,10" Foreground="{TemplateBinding Foreground}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" MinWidth="{TemplateBinding MinWidth}" Style="{StaticResource ToggleButtonCustom}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition />
                            <ColumnDefinition Width="32" />
                        </Grid.ColumnDefinitions>
                        <Grid.LayoutTransform>
                            <TransformGroup>
                                <RotateTransform Angle="-90"/>
                            </TransformGroup>
                        </Grid.LayoutTransform>
                        <ContentPresenter ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                        <Path Name="PathArrow" IsHitTestVisible="False" Grid.Column="1" Stretch="Uniform" Fill="{TemplateBinding Foreground}" Data="{StaticResource UpGeometry}" Margin="0,0,10,0" Width="12" Height="12" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" HorizontalAlignment="Right" />
                    </Grid>
                </ToggleButton>
            </Border>
            <ContentPresenter Name="ExpandSite" Visibility="Collapsed" Grid.Column="0" />
        </Grid>
        <ControlTemplate.Triggers>
            <Trigger Property="IsExpanded" Value="true">
                <Setter Property="CornerRadius" TargetName="BorderHeader" Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='0,1,1,0'}" />
                <Setter Property="Visibility" TargetName="ExpandSite" Value="Visible" />
                <Setter Property="Data" TargetName="PathArrow" Value="{StaticResource DownGeometry}" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="false">
                <Setter Property="Opacity" Value=".4" />
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <ControlTemplate x:Key="ExpanderUpTemplate" TargetType="Expander">
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition/>
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>
            <Border x:Name="BorderHeader" Grid.Row="1" CornerRadius="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}">
                <ToggleButton HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch" Focusable="False" Padding="10,0,0,0" Foreground="{TemplateBinding Foreground}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" MinHeight="{TemplateBinding MinHeight}" Style="{StaticResource ToggleButtonCustom}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition />
                            <ColumnDefinition Width="32" />
                        </Grid.ColumnDefinitions>
                        <ContentPresenter ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                        <Path Name="PathArrow" IsHitTestVisible="False" Grid.Column="1" Stretch="Uniform" Fill="{TemplateBinding Foreground}" Data="{StaticResource UpGeometry}" Margin="0,0,10,0" Width="12" Height="12" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" HorizontalAlignment="Right" />
                    </Grid>
                </ToggleButton>
            </Border>
            <ContentPresenter Name="ExpandSite" Visibility="Collapsed" Grid.Row="0" />
        </Grid>
        <ControlTemplate.Triggers>
            <Trigger Property="IsExpanded" Value="true">
                <Setter Property="CornerRadius" TargetName="BorderHeader" Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='0,0,1,1'}" />
                <Setter Property="Visibility" TargetName="ExpandSite" Value="Visible" />
                <Setter Property="Data" TargetName="PathArrow" Value="{StaticResource DownGeometry}" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="false">
                <Setter Property="Opacity" Value=".4" />
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <ControlTemplate x:Key="ExpanderRightTemplate" TargetType="Expander">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Border x:Name="BorderHeader" CornerRadius="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}">
                <ToggleButton VerticalAlignment="Stretch" VerticalContentAlignment="Stretch" Focusable="False" Padding="0,0,0,10" Foreground="{TemplateBinding Foreground}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" MinWidth="{TemplateBinding MinWidth}" Style="{StaticResource ToggleButtonCustom}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition />
                            <ColumnDefinition Width="32" />
                        </Grid.ColumnDefinitions>
                        <Grid.LayoutTransform>
                            <TransformGroup>
                                <RotateTransform Angle="-90"/>
                            </TransformGroup>
                        </Grid.LayoutTransform>
                        <ContentPresenter ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                        <Path Name="PathArrow" IsHitTestVisible="False" Grid.Column="1" Stretch="Uniform" Fill="{TemplateBinding Foreground}" Data="{StaticResource DownGeometry}" Margin="0,0,10,0" Width="12" Height="12" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" HorizontalAlignment="Right" />
                    </Grid>
                </ToggleButton>
            </Border>
            <ContentPresenter Name="ExpandSite" Visibility="Collapsed" Grid.Column="1" />
        </Grid>
        <ControlTemplate.Triggers>
            <Trigger Property="IsExpanded" Value="true">
                <Setter Property="CornerRadius" TargetName="BorderHeader" Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='1,0,0,1'}" />
                <Setter Property="Visibility" TargetName="ExpandSite" Value="Visible" />
                <Setter Property="Data" TargetName="PathArrow" Value="{StaticResource UpGeometry}" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="false">
                <Setter Property="Opacity" Value=".4" />
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <ControlTemplate x:Key="ExpanderDownTemplate" TargetType="Expander">
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
                <RowDefinition/>
            </Grid.RowDefinitions>
            <Border x:Name="BorderHeader" CornerRadius="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}">
                <ToggleButton HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch" Focusable="False" Padding="10,0,0,0" Foreground="{TemplateBinding Foreground}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Width="{TemplateBinding Width}" Height="{TemplateBinding MinHeight}" Style="{StaticResource ToggleButtonCustom}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition />
                            <ColumnDefinition Width="32" />
                        </Grid.ColumnDefinitions>
                        <ContentPresenter ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                        <Path Name="PathArrow" IsHitTestVisible="False" Grid.Column="1" Stretch="Uniform" Fill="{TemplateBinding Foreground}" Data="{StaticResource DownGeometry}" Margin="0,0,10,0" Width="12" Height="12" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" HorizontalAlignment="Right" />
                    </Grid>
                </ToggleButton>
            </Border>
            <ContentPresenter Name="ExpandSite" Visibility="Collapsed" Grid.Row="1" />
        </Grid>
        <ControlTemplate.Triggers>
            <Trigger Property="IsExpanded" Value="true">
                <Setter Property="CornerRadius" TargetName="BorderHeader" Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='1,1,0,0'}" />
                <Setter Property="Visibility" TargetName="ExpandSite" Value="Visible" />
                <Setter Property="Data" TargetName="PathArrow" Value="{StaticResource UpGeometry}" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="false">
                <Setter Property="Opacity" Value=".4" />
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <Style x:Key="ExpanderBaseStyle" TargetType="Expander">
        <Setter Property="Foreground" Value="{DynamicResource PrimaryTextBrush}" />
        <Setter Property="Background" Value="{DynamicResource SecondaryRegionBrush}" />
        <Setter Property="HorizontalContentAlignment" Value="Left" />
        <Setter Property="VerticalContentAlignment" Value="Center" />
        <Setter Property="BorderBrush" Value="{DynamicResource BorderBrush}" />
        <Setter Property="BorderThickness" Value="1" />
        <Setter Property="MinHeight" Value="{StaticResource DefaultControlHeight}" />
        <Setter Property="MinWidth" Value="{StaticResource DefaultControlHeight}" />
        <Setter Property="hc:BorderElement.CornerRadius" Value="{StaticResource DefaultCornerRadius}" />
        <Setter Property="Template" Value="{StaticResource ExpanderDownTemplate}"/>
        <Style.Triggers>
            <Trigger Property="ExpandDirection" Value="Left">
                <Setter Property="Template" Value="{StaticResource ExpanderLeftTemplate}"/>
            </Trigger>
            <Trigger Property="ExpandDirection" Value="Up">
                <Setter Property="Template" Value="{StaticResource ExpanderUpTemplate}"/>
            </Trigger>
            <Trigger Property="ExpandDirection" Value="Right">
                <Setter Property="Template" Value="{StaticResource ExpanderRightTemplate}"/>
            </Trigger>
        </Style.Triggers>
    </Style>

</ResourceDictionary>