WPF ControlTemplate Expander issue

Question

The following control template code (from generic.xaml) gives me an error, 'The attached property ControlTemplate.Triggers' is not defined on 'Grid' or one of its base classes. Any help is appreciated.

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

    <Style TargetType="local:CustomExpander">
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
        <Setter Property="VerticalContentAlignment" Value="Stretch"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="local:CustomExpander">
                    <Grid Background="Transparent">
                        <Grid.Resources>
                            <LinearGradientBrush x:Key="ExpanderArrowFill" EndPoint="0,1" StartPoint="0,0">
                                <GradientStop Color="White" Offset="0"/>
                                <GradientStop Color="#FFBFBFBF" Offset="0.5"/>
                                <GradientStop Color="#FF878787" Offset="1"/>
                            </LinearGradientBrush>
                            <LinearGradientBrush x:Key="ExpanderArrowHoverFill" EndPoint="0,1" StartPoint="0,0">
                                <GradientStop Color="#FFF0F8FE" Offset="0"/>
                                <GradientStop Color="#FFE0F3FE" Offset="0.3"/>
                                <GradientStop Color="#FF6FA7C5" Offset="1"/>
                            </LinearGradientBrush>
                            <LinearGradientBrush x:Key="ExpanderArrowPressedFill" EndPoint="0,1" StartPoint="0,0">
                                <GradientStop Color="#FFDCF0FA" Offset="0"/>
                                <GradientStop Color="#FFC5E6F7" Offset="0.2"/>
                                <GradientStop Color="#FF5690D0" Offset="1"/>
                            </LinearGradientBrush>
                            <ControlTemplate x:Key="ExpanderDownHeaderTemplate" TargetType="ToggleButton">
                                <Grid Background="Transparent" Height="25">
                                    <VisualStateManager.VisualStateGroups>
                                        <VisualStateGroup x:Name="CheckStates">
                                            <VisualStateGroup.Transitions>
                                                <VisualTransition GeneratedDuration="00:00:00"/>
                                            </VisualStateGroup.Transitions>
                                            <VisualState x:Name="Checked">
                                                <Storyboard>
                                                    <DoubleAnimation BeginTime="0" Storyboard.TargetName="imgTransform" Storyboard.TargetProperty="Angle" From="90" To="180"  Duration="0:0:.2" />
                                                </Storyboard>
                                            </VisualState>
                                            <VisualState x:Name="Unchecked">
                                                <Storyboard>
                                                    <DoubleAnimation BeginTime="0" Storyboard.TargetName="imgTransform" Storyboard.TargetProperty="Angle" From="180" To="90"  Duration="0:0:.2" />
                                                </Storyboard>
                                            </VisualState>
                                        </VisualStateGroup>
                                    </VisualStateManager.VisualStateGroups>
                                    <Border Padding="{TemplateBinding Padding}">
                                        <Grid Background="Transparent">
                                            <Grid.ColumnDefinitions>
                                                <ColumnDefinition Width="10"/>
                                                <ColumnDefinition Width="*"/>
                                            </Grid.ColumnDefinitions>
                                            <Grid HorizontalAlignment="Left" VerticalAlignment="Center">
                                                <Grid x:Name="grdHeaderGrid"  Height="19" Width="Auto" HorizontalAlignment="Left" VerticalAlignment="Center">
                                                    <Path Data="M 0,2.5 3,2.5 1.5,0 Z" Stroke="Black" StrokeThickness="5" Fill="Black"  HorizontalAlignment="Left" VerticalAlignment="Center">
                                                        <Path.RenderTransform>
                                                            <RotateTransform x:Name="imgTransform" Angle="90" CenterX="0" CenterY="0" />
                                                        </Path.RenderTransform>
                                                    </Path>
                                                </Grid>
                                            </Grid>
                                            <ContentPresenter  x:Name="header" HorizontalAlignment="Stretch" Margin="4,0,0,0"  VerticalAlignment="Center" Grid.Column="1" Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}"/>
                                        </Grid>
                                    </Border>
                                    <Rectangle x:Name="FocusVisualElement" Stroke="#FF448DCA" StrokeThickness="1" IsHitTestVisible="false" Visibility="Collapsed"/>
                                </Grid>
                            </ControlTemplate>
                        </Grid.Resources>

                            <Border x:Name="Background" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="0">
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition x:Name="cd0" Width="Auto"/>
                                        <ColumnDefinition x:Name="cd1" Width="Auto"/>
                                    </Grid.ColumnDefinitions>
                                    <Grid.RowDefinitions>
                                        <RowDefinition x:Name="rd0" Height="Auto"/>
                                        <RowDefinition x:Name="rd1" Height="Auto"/>
                                    </Grid.RowDefinitions>

                                    <Image x:Name="NormalImage" Source="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=NormalImageSource}"
                                           HorizontalAlignment="Center" VerticalAlignment="Center" Stretch="UniformToFill"/>

                                    <Image x:Name="PressedImage" Source="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=PressedImageSource}"
                                           HorizontalAlignment="Center" VerticalAlignment="Center" Stretch="UniformToFill" Visibility="Collapsed"/>

                                    <Image x:Name="DisabledImage" Source="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DisabledImageSource}"
                                           HorizontalAlignment="Center" VerticalAlignment="Center" Stretch="UniformToFill" Visibility="Collapsed"/>

                                    <!--<Image x:Name="MouseOverImage" Source="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=MouseOverImageSource}"
                                           HorizontalAlignment="Center" VerticalAlignment="Center" Stretch="UniformToFill" Visibility="Collapsed" />-->

                                    <ToggleButton x:Name="ExpanderButton" FontFamily="{TemplateBinding FontFamily}" FontSize="{TemplateBinding FontSize}" FontStretch="{TemplateBinding FontStretch}" FontStyle="{TemplateBinding FontStyle}" FontWeight="{TemplateBinding FontWeight}" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Padding="{TemplateBinding Padding}" Template="{StaticResource ExpanderDownHeaderTemplate}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" MinHeight="0" MinWidth="0" Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" Grid.Column="0" Grid.Row="0" IsChecked="{TemplateBinding IsExpanded}" Margin="{TemplateBinding HeaderMargin}"/>
                                    <ContentControl x:Name="ExpandSite" FontFamily="{TemplateBinding FontFamily}" FontSize="{TemplateBinding FontSize}" FontStretch="{TemplateBinding FontStretch}" FontStyle="{TemplateBinding FontStyle}" FontWeight="{TemplateBinding FontWeight}" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Visibility="Collapsed"  Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}" Grid.Column="0" Grid.Row="1"/>
                                </Grid>
                            </Border>
                            <ControlTemplate.Triggers>
                                <Trigger Property="IsExpanded" Value="True">
                                    <Setter TargetName="ExpandSite" 
                                                  Property="Visibility" 
                                                  Value="Visible"/>
                                </Trigger>
                            </ControlTemplate.Triggers>


                        <Border x:Name="DisabledVisualElement" IsHitTestVisible="false" Opacity="0" Background="#A5FFFFFF" CornerRadius="0"/>
                        <Border x:Name="FocusVisualElement" IsHitTestVisible="false" Visibility="Collapsed" BorderThickness="1" CornerRadius="0">
                            <Border.BorderBrush>
                                <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                                    <GradientStop Color="#FFA3AEB9"/>
                                    <GradientStop Color="#FF8399A9" Offset="0.375"/>
                                    <GradientStop Color="#FF718597" Offset="0.375"/>
                                    <GradientStop Color="#FF617584" Offset="1"/>
                                </LinearGradientBrush>
                            </Border.BorderBrush>
                        </Border>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

The following code snippetcode has some issue, I don't know where to place it.

                    <ControlTemplate.Triggers>
                        <Trigger Property="IsExpanded" Value="True">
                            <Setter TargetName="ExpandSite" 
                                          Property="Visibility" 
                                          Value="Visible"/>
                        </Trigger>
                    </ControlTemplate.Triggers>

Answer 1

You have the <ControlTemplate.Triggers> element inside the <Grid> instead of directly inside the <ControlTemplate>. Move it down after the line that has </Grid> and before the line that has </ControlTemplate> so that it is a direct child of the <ControlTemplate> element.