如何在WPF触发器中定位同级元素？

Question

我希望有一个按钮，单击该按钮时会显示一个下拉菜单。

到目前为止，我有这个：

    <dx:SimpleButton  Padding="1" Margin="1" HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch"
                      Command="{Binding CreateItem}" >
        <DockPanel HorizontalAlignment="Stretch" LastChildFill="True">
            <Image DockPanel.Dock="Left" Width="16" Height="16" Source="../Resources/Add.png" Margin="10" />
            <dx:SimpleButton Name="DropDownButton" DockPanel.Dock="Right" Padding="1" Margin="0,1,1,1" HorizontalContentAlignment="Center" VerticalContentAlignment="Center" BorderThickness="0" Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}">
                <dx:SimpleButton.Content>
                    <Image Width="16" Height="16" Source="../Resources/customDropDown.png"  />
                </dx:SimpleButton.Content>
                <dx:SimpleButton.ContextMenu>
                    <ContextMenu x:Name="DropDownMenu" PlacementRectangle="{Binding RelativeSource={RelativeSource Self}}">
                        <MenuItem Command="{Binding PasteItem}" Header="Paste" />
                    </ContextMenu>
                </dx:SimpleButton.ContextMenu>
                <dx:SimpleButton.Triggers>
                    <EventTrigger SourceName="DropDownButton" RoutedEvent="Button.Click">
                        <BeginStoryboard>
                            <Storyboard>
                                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DropDownMenu" Storyboard.TargetProperty="(ContextMenu.IsOpen)">
                                    <DiscreteObjectKeyFrame KeyTime="0:0:0">
                                        <DiscreteObjectKeyFrame.Value>
                                            <system:Boolean>True</system:Boolean>
                                        </DiscreteObjectKeyFrame.Value>
                                    </DiscreteObjectKeyFrame>
                                </ObjectAnimationUsingKeyFrames>
                            </Storyboard>
                        </BeginStoryboard>
                    </EventTrigger>
                </dx:SimpleButton.Triggers>
            </dx:SimpleButton>
            <TextBlock  VerticalAlignment="Center" Text="{Binding Path=Slot.Name}" /> 
        </DockPanel>
    </dx:SimpleButton>

但是当我单击按钮时，出现此错误：

'DropDownMenu' name cannot be found in the name scope of 'DevExpress.Xpf.Core.SimpleButton'.

我想念什么？