<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="WpfApplication3.MainWindow"
    x:Name="Window"
    Title="MainWindow"
    Width="640" Height="480">

    <Window.Resources>
        <Style TargetType="{x:Type Expander}">
            <Style.Resources>
                <Style x:Key="ExpanderDownHeaderStyle" TargetType="{x:Type ToggleButton}">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type ToggleButton}">
                                <Border Padding="{TemplateBinding Padding}">
                                    <Grid Background="Transparent" SnapsToDevicePixels="False">
                                        <Grid.ColumnDefinitions>
                                            <ColumnDefinition Width="19"/>
                                            <ColumnDefinition Width="*"/>
                                        </Grid.ColumnDefinitions>
                                        <Ellipse x:Name="circle" HorizontalAlignment="Center" Height="19" Stroke="DarkGray" VerticalAlignment="Center" Width="19"/>
                                        <Path x:Name="arrow" Data="M 1,1.5 L 4.5,5 L 8,1.5" HorizontalAlignment="Center" SnapsToDevicePixels="false" Stroke="#666" StrokeThickness="2" VerticalAlignment="Center"/>
                                        <ContentPresenter Grid.Column="1" HorizontalAlignment="Left" Margin="4,0,0,0" RecognizesAccessKey="True" SnapsToDevicePixels="True" VerticalAlignment="Center"/>
                                    </Grid>
                                </Border>
                                <ControlTemplate.Triggers>
                                    <Trigger Property="IsChecked" Value="true">
                                        <Setter Property="Data" TargetName="arrow" Value="M 1,4.5  L 4.5,1  L 8,4.5"/>
                                    </Trigger>
                                    <Trigger Property="IsMouseOver" Value="true">
                                        <Setter Property="Stroke" TargetName="circle" Value="#FF3C7FB1"/>
                                        <Setter Property="Stroke" TargetName="arrow" Value="#222"/>
                                    </Trigger>
                                    <Trigger Property="IsPressed" Value="true">
                                        <Setter Property="Stroke" TargetName="circle" Value="#FF526C7B"/>
                                        <Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
                                        <Setter Property="Stroke" TargetName="arrow" Value="#FF003366"/>
                                    </Trigger>
                                </ControlTemplate.Triggers>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </Style.Resources>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Expander}">
                        <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="3" SnapsToDevicePixels="true">
                            <DockPanel>
                                <ToggleButton x:Name="HeaderSite" ContentTemplate="{TemplateBinding HeaderTemplate}" ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}" Content="{TemplateBinding Header}" DockPanel.Dock="Top" Foreground="{TemplateBinding Foreground}" FontWeight="{TemplateBinding FontWeight}" FontStyle="{TemplateBinding FontStyle}" FontStretch="{TemplateBinding FontStretch}" FontSize="{TemplateBinding FontSize}" FontFamily="{TemplateBinding FontFamily}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" MinWidth="0" MinHeight="0" Padding="{TemplateBinding Padding}" Style="{StaticResource ExpanderDownHeaderStyle}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
                                <ContentPresenter  x:Name="ExpandSite" DockPanel.Dock="Bottom" Focusable="false" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                                    <ContentPresenter.LayoutTransform>
                                        <ScaleTransform ScaleX="1" ScaleY="0"/>
                                    </ContentPresenter.LayoutTransform>
                                </ContentPresenter>
                            </DockPanel>
                        </Border>
                        <ControlTemplate.Triggers>
                            <EventTrigger RoutedEvent="Expander.Expanded">
                                <BeginStoryboard>
                                    <Storyboard Storyboard.TargetName="ExpandSite" Storyboard.TargetProperty="LayoutTransform.ScaleY">
                                        <DoubleAnimation To="1" Duration="0:0:0.2"/>
                                    </Storyboard>
                                </BeginStoryboard>
                            </EventTrigger>
                            <EventTrigger RoutedEvent="Expander.Collapsed">
                                <BeginStoryboard>
                                    <Storyboard Storyboard.TargetName="ExpandSite" Storyboard.TargetProperty="LayoutTransform.ScaleY">
                                        <DoubleAnimation To="0" Duration="0:0:0.2"/>
                                    </Storyboard>
                                </BeginStoryboard>
                            </EventTrigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>

    <Grid x:Name="LayoutRoot">
        <Expander>
            <Expander.Content>
                <StackPanel>
                    <TextBlock Text="Hello World !"/>
                    <TextBox/>
                </StackPanel>
            </Expander.Content>
        </Expander>
    </Grid>
</Window>