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Connecting The ComboBox Popup Border to Main ComboBox Border

I'm trying to match the Style of a ComboBox I'm creating to a design I have.
I'm unable to connect the Popup border to the Main Border that shows the selected ComboBox item.
I took this ComboBox Template from a site, changed some properties to match the design's style, things were coming along good until I realized I can't connect these 2 aforementioned borders. I'm not exactly sure how we could go about doing it.
I also thought about adding 2 Rows and making the ComboBox Popup Rowspawn but that didn't work either.
Creating a StackOverflow post as I ran out of ideas, any ideas are welcome.
Thank you in advance.
Edit: I was able to use VerticalOffset to move the Popup up, where I want it to be but I can't tuck it underneath the Border I create within the ToggleButton template. Panel.ZIndex usage didn't help either. Still working on a way to accomplish this task.
Design:
My ComboBox:
Here's the part I have an issue with (Along with other parts that I'll address myself later):
Here's my code:
<ControlTemplate x:Key="ComboBoxToggleButton" TargetType="{x:Type ToggleButton}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="30"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Border x:Name="Border"
Grid.ColumnSpan="2"
CornerRadius="4"
Background="#FFFFFF">
<Border.Effect>
<DropShadowEffect ShadowDepth="0"
Color="Black"
Opacity="0.29"
BlurRadius="5"/>
</Border.Effect>
</Border>
<Border Grid.Column="0"
CornerRadius="4,0,0,4"
Margin="0,0,1,0"
Background="#FFFFFF"
BorderBrush="LightGray"
BorderThickness="0,0,1.7,0">
</Border>
<Canvas Width="30"
Height="30"
Grid.Column="1"
Margin="9,-18,0,0">
<Canvas.LayoutTransform>
<MatrixTransform Matrix="1,0,0,-1,0,0" />
</Canvas.LayoutTransform>
<Path Data="M131.6625 270.6375A21.1875 21.1875 0 0 0 168.4125 270.6375L296.98125 51.88125C305.55 37.29375 295.25625 18.75 278.60625 18.75H21.45C4.78125 18.75 -5.49375 37.3125 3.075 51.88125L131.6625 270.6375z"
Stroke="{x:Null}"
Fill="#707070"
Stretch="Uniform"
Width="10"
Height="10"
SnapsToDevicePixels="False"
UseLayoutRounding="False"
StrokeThickness="0"
StrokeStartLineCap="flat"
StrokeEndLineCap="flat"
StrokeLineJoin="miter"/>
</Canvas>
</Grid>
</ControlTemplate>
<ControlTemplate x:Key="ComboBoxTextBox" TargetType="{x:Type TextBox}">
<Border x:Name="PART_ContentHost" Focusable="False" Background="{TemplateBinding Background}" />
</ControlTemplate>
<Style x:Key="{x:Type ComboBox}" TargetType="{x:Type ComboBox}">
<Setter Property="SnapsToDevicePixels" Value="true"/>
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Visible"/>
<Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
<Setter Property="MinWidth" Value="120"/>
<Setter Property="MinHeight" Value="20"/>
<Setter Property="Foreground" Value="#707070"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBox}">
<Grid>
<ToggleButton
Name="ToggleButton"
Template="{StaticResource ComboBoxToggleButton}"
Grid.Column="2"
Focusable="false"
IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
ClickMode="Press">
</ToggleButton>
<ContentPresenter Name="ContentSite" IsHitTestVisible="False" Content="{TemplateBinding SelectionBoxItem}"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
Margin="15,3,23,3"
VerticalAlignment="Center"
HorizontalAlignment="Left" />
<TextBox x:Name="PART_EditableTextBox"
Style="{x:Null}"
Template="{StaticResource ComboBoxTextBox}"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Margin="15,3,23,3"
Focusable="True"
Background="#FF3F3F3F"
Foreground="Green"
Visibility="Hidden"
IsReadOnly="{TemplateBinding IsReadOnly}"/>
<Popup
Name="Popup"
Placement="Bottom"
IsOpen="{TemplateBinding IsDropDownOpen}"
AllowsTransparency="True"
Focusable="False"
PopupAnimation="Slide">
<Grid Name="DropDown"
SnapsToDevicePixels="True"
MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
Margin="15,15,15,15">
<Border
x:Name="DropDownBorder"
Background="White"
BorderThickness="0"
BorderBrush="#888888">
<Border.Effect>
<DropShadowEffect ShadowDepth="0"
Color="Black"
Opacity="0.29"
BlurRadius="5"/>
</Border.Effect>
</Border>
<ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasItems" Value="false">
<Setter TargetName="DropDownBorder" Property="MinHeight" Value="95"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="#888888"/>
</Trigger>
<Trigger Property="IsGrouping" Value="true">
<Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
</Trigger>
<Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="true">
<Setter TargetName="DropDownBorder" Property="CornerRadius" Value="0"/>
<Setter TargetName="DropDownBorder" Property="Margin" Value="0,2,0,0"/>
</Trigger>
<Trigger Property="IsEditable" Value="true">
<Setter Property="IsTabStop" Value="false"/>
<Setter TargetName="PART_EditableTextBox" Property="Visibility" Value="Visible"/>
<Setter TargetName="ContentSite" Property="Visibility" Value="Hidden"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
</Style.Triggers>
</Style>
<!-- SimpleStyles: ComboBoxItem -->
<Style x:Key="{x:Type ComboBoxItem}" TargetType="{x:Type ComboBoxItem}">
<Setter Property="SnapsToDevicePixels" Value="true"/>
<Setter Property="Foreground" Value="#707070"/>
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBoxItem}">
<Border Name="Border"
Padding="2"
SnapsToDevicePixels="true">
<ContentPresenter Margin="8,3,23,3"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

WPF TabItem header as TextBlock

I would put a TextBlock inside the header of my TabItem as I have the problem concerning the missing underscore. I changed my style as follows:
<Style x:Key="TabItemDistintaStyle" TargetType="{x:Type TabItem}">
<Setter Property="FocusVisualStyle" Value="{StaticResource TabItemFocusVisual}"/>
<Setter Property="Foreground" Value="White"/>
<Setter Property="Padding" Value="6,1,6,1"/>
<Setter Property="BorderBrush" Value="{StaticResource TabControlNormalBorderBrush}"/>
<Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
<Setter Property="VerticalContentAlignment" Value="Stretch"/>
<Setter Property="HeaderTemplate">
<Setter.Value>
<DataTemplate>
<TextBlock />
</DataTemplate>
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<Grid SnapsToDevicePixels="true">
<Border x:Name="Bd" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1,1,1,0" Padding="{TemplateBinding Padding}" CornerRadius="6,210,0,0">
<DockPanel>
<ContentPresenter x:Name="Content" DockPanel.Dock="Left" HorizontalAlignment="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" VerticalAlignment="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" ContentSource="Header" RecognizesAccessKey="True"/>
<Button x:Name="cmdClose" DockPanel.Dock="Right"
Command="ApplicationCommands.Close"
Margin="7,0,3,0" Style="{DynamicResource ButtonDistintaCloseStyle}" RenderTransformOrigin="0.5,0.5">
<Button.RenderTransform>
<TransformGroup>
<ScaleTransform/>
<SkewTransform/>
<RotateTransform/>
<TranslateTransform/>
</TransformGroup>
</Button.RenderTransform>
</Button>
</DockPanel>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
But it does not work, as the first underscore is still missing. What's wrong?

You need bind (set) your TextBlock's Text with TabItem's "Header" property,
i think this sample helps you:
<Window x:Class="MyWpfAppSample1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:MyWpfAppSample1"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800" WindowStartupLocation="CenterOwner">
<Window.Resources>
<Style x:Key="MyTabItemStyle"
TargetType="{x:Type TabItem}">
<Setter Property="FocusVisualStyle" Value="{x:Null}" />
<Setter Property="HorizontalContentAlignment" Value="Stretch" />
<Setter Property="VerticalContentAlignment" Value="Stretch" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<Grid x:Name="Root"
Width="180"
Height="45"
Margin="0,0,0,0"
SnapsToDevicePixels="true">
<TextBlock x:Name="contentPresenter"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Focusable="False"
FontSize="14"
Foreground="LightCoral"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
Text="{TemplateBinding Header}" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid>
<TabControl Margin="0,5,0,0"
FocusVisualStyle="{x:Null}">
<TabItem Header="My First Tab"
IsSelected="{Binding FirstTabItemSelected}"
Style="{DynamicResource MyTabItemStyle}">
Tab Item1
</TabItem>
<TabItem Header="My Second Tab"
IsSelected="{Binding SecondTabItemSelected}"
Style="{DynamicResource MyTabItemStyle}">
TabItem2
</TabItem>
</TabControl>
</Grid>
</Window>
Result:

How to give Expander Full Width of Column in a Grid

I have an Expander and it should receive the full width of the column but I cant get it to work.
I tried to add the Horizontal(Content)Alignment = stretch on the TextBlock and on the Expander itself even on the Grid, but it is not working.
What I need is that the Expander takes about 90% of the width and the rest are assigned to the buttons as in the following example:
I want to display e.g. a name and when you press on it, expands down and shows additional information
and if the buttons are pressed, then the commands behind the buttons will be executed (no commands are binded in the example).
<ListView
ItemsSource="{Binding log}"
SelectionMode="Multiple"
Style="{StaticResource ListViewStyle}">
<ListView.ItemTemplate>
<DataTemplate>
<Border BorderThickness="0,0,0,1" BorderBrush="Gray">
<Grid HorizontalAlignment="Stretch">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100*" />
<ColumnDefinition Width="5*" />
<ColumnDefinition Width="5*" />
</Grid.ColumnDefinitions>
<Expander Grid.Column="0"
HorizontalAlignment="Stretch"
IsExpanded="{Binding Mode=TwoWay, Path=IsSelected, RelativeSource={RelativeSource AncestorType=ListBoxItem, Mode=FindAncestor}}"
Style="{StaticResource ExpanderStyle}">
<Expander.Header>
<TextBlock Text="{Binding Name}" Foreground="White"
HorizontalAlignment="Stretch" />
</Expander.Header>
<TextBlock
VerticalAlignment="Center"
FontSize="16"
Foreground="White"
Text="{Binding Description}" />
</Expander>
<Button Grid.Column="1"
Style="{StaticResource IconButton}">
<Button.Background>
<ImageBrush ImageSource="../icons/edit.png"
Stretch="None" />
</Button.Background>
</Button>
<dialogButton:ConfirmButton Grid.Column="2"
Question="{x:Static language:Strings.confirm}"
Style="{DynamicResource IconButton}"
Margin="0,0,10,0">
<dialogButton:ConfirmButton.Background>
<ImageBrush ImageSource="../icons/delete.png"
Stretch="None" />
</dialogButton:ConfirmButton.Background>
</dialogButton:ConfirmButton>
</Grid>
</Border>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
Styles
<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">
<ContentPresenter HorizontalAlignment="Stretch" Margin="0" RecognizesAccessKey="True" SnapsToDevicePixels="True" VerticalAlignment="Center"/>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ExpanderStyle" TargetType="{x:Type Expander}">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
<Setter Property="VerticalContentAlignment" Value="Stretch"/>
<Setter Property="BorderBrush" Value="Transparent"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<Border Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="3" SnapsToDevicePixels="true">
<DockPanel>
<ToggleButton x:Name="HeaderSite" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}" DockPanel.Dock="Top" FontStyle="{TemplateBinding FontStyle}" FontStretch="{TemplateBinding FontStretch}" FontWeight="{TemplateBinding FontWeight}" FocusVisualStyle="{StaticResource ExpanderHeaderFocusVisual}" FontFamily="{TemplateBinding FontFamily}" Foreground="{TemplateBinding Foreground}" FontSize="{TemplateBinding FontSize}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" MinHeight="0" MinWidth="0" Margin="1" 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}" Visibility="Collapsed"/>
</DockPanel>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="true">
<Setter Property="Visibility" TargetName="ExpandSite" Value="Visible"/>
</Trigger>
<Trigger Property="ExpandDirection" Value="Down">
<Setter Property="DockPanel.Dock" TargetName="ExpandSite" Value="Bottom"/>
<Setter Property="DockPanel.Dock" TargetName="HeaderSite" Value="Top"/>
<Setter Property="Background" Value="Transparent" />
<Setter Property="Style" TargetName="HeaderSite" Value="{StaticResource ExpanderDownHeaderStyle}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
I also tried things from the following Threads
wpf - Header of Expander fit contents width?
Problem here is that the Buttons are pushed out of the view
Best Solution would be if the Expander just takes the width of the column so that I can adjust the widths of the element by the Grid.ColumnDefinition.

The issue is that the item container, a ListViewItem does not stretch its content by default. You have to create an item container style in order to set the HorizontalContentAlignment to Stretch.
<ListView.ItemContainerStyle>
<Style TargetType="{x:Type ListViewItem}">
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
</Style>
</ListView.ItemContainerStyle>

Wpf textbox does not change borderbrush when trigger mouse over?

In my WPF application I want to to change borderbrush color for a TextBox when mouse enter, but it does not change color to the one I want, but it change to sky blue (I guess it is standard color).
Here is my XAML code:
<Window x:Class="OnePlayApp.Views.LoginDialog"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:OnePlayApp.Views"
WindowStyle="None"
ResizeMode="NoResize"
Title="LoginDialog" Height="350" Width="550" Foreground="Black">
<Grid Background="#282828">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="24*"/>
<ColumnDefinition Width="43*"/>
<ColumnDefinition Width="43*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="59*"/>
<RowDefinition Height="50*"/>
<RowDefinition Height="50*"/>
<RowDefinition Height="191*"/>
</Grid.RowDefinitions>
<Image Margin="9,10,173,6" Source="/Resources/logo.png" Grid.ColumnSpan="2"/>
<Label x:Name="AccountName" Content="Account name" Margin="6,10,1,13" Foreground="#FFAFADAD" HorizontalContentAlignment="Right" Grid.Row="1"/>
<Label x:Name="Password" Content="Password
" Margin="6,10,1,13" Foreground="#FFAFADAD" HorizontalContentAlignment="Right" Grid.Row="2"/>
<TextBox x:Name="username" Margin="6,11,15,8" Background="#FF282828" Foreground="White" FontSize="15" Text="Ahsep12015#one.com" FontFamily="Yu Gothic UI Semibold" Grid.Column="1" Grid.Row="1" Grid.ColumnSpan="2" VerticalContentAlignment="Center" BorderThickness="10">
<TextBox.Style>
<Style TargetType="{x:Type TextBox}">
<Setter Property="BorderBrush" Value="Green" />
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="BorderBrush" Value="Red" />
</Trigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>
<PasswordBox x:Name="password" Margin="6,8,15,13" Background="#FF282828" Password="123456789" FontSize="18" Grid.Column="1" Grid.Row="2" Grid.ColumnSpan="2"/>
<Button x:Name="LoginBtn" Content="Login" Margin="6,15,15,146" Click="LoginBtn_Click" FontSize="16" FontFamily="Yu Gothic UI Semibold" Foreground="White" Grid.Column="1" Grid.Row="3">
<Button.Style>
<Style TargetType="{x:Type Button}">
<Setter Property="Background" Value="#FFAFADAD"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border Background="{TemplateBinding Background}">
<ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="#79B539"/>
</Trigger>
</Style.Triggers>
</Style>
</Button.Style>
</Button>
<Button x:Name="CancelBtn" Content="Cancel" Margin="6,15,15,146" Click="CancelBtn_Click" FontSize="16" FontFamily="Yu Gothic UI Semibold" Foreground="White" Grid.Column="2" Grid.Row="3">
<Button.Style>
<Style TargetType="{x:Type Button}">
<Setter Property="Background" Value="#FFAFADAD"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border Background="{TemplateBinding Background}">
<ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="#79B539"/>
</Trigger>
</Style.Triggers>
</Style>
</Button.Style>
</Button>
</Grid>
</Window>

I had the same issue on Windows 8, and the only way I was able to fix this issue (which appears to be a bug in WPF) was with a ControlTemplate. This is the standard control template for a TextBox as coughed up by Blend. I changed the BorderBrush colors, added it to my global TextBox style, and it works great. There may be a better solution out there, but this is what works for me.
<Style TargetType="{x:Type TextBox}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBoxBase}">
<Border x:Name="border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
<ScrollViewer x:Name="PART_ContentHost" Focusable="False" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Opacity" TargetName="border" Value="0.56"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="BorderBrush" TargetName="border" Value="Red"/>
</Trigger>
<Trigger Property="IsKeyboardFocused" Value="True">
<Setter Property="BorderBrush" TargetName="border" Value="Orange"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

wpf - how to use dataTemplate for treeViewItems

The following code throws an exception:
<TreeView
ItemsSource="{Binding TreeRootInstance}"
x:Name="Htree"
ItemTemplate="CellTemplate"
SelectedItemChanged="HTree_OnSelectedItemChanged"
KeyDown="HTree_KeyDown">
<TreeView.Resources>
<DataTemplate x:Key="CellTemplate">
<Border>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="auto"/>
<ColumnDefinition Width="auto"/>
</Grid.ColumnDefinitions>
<ToggleButton x:Name="Expander"
HorizontalAlignment="Right"
Focusable="False"
Grid.Column="1"
IsChecked="{Binding Path=IsExpanded, RelativeSource={RelativeSource AncestorType={x:Type vw:TreeViewItem}}}"
ClickMode="Press"/>
<TextBlock
Text="{Binding Name}"
Grid.Column="0" />
</Grid>
</Border>
<DataTemplate.Triggers>
<DataTrigger Binding="{Binding Path=HasItems, RelativeSource={RelativeSource AncestorType={x:Type TreeViewItem}}}" Value="False">
<Setter TargetName="Expander" Property="Visibility" Value="Hidden"/>
</DataTrigger>
</DataTemplate.Triggers>
</DataTemplate>
<Style TargetType="{x:Type TreeViewItem}" x:Key="aa">
<Setter Property="IsTabStop" Value="True"/>
<Setter Property="TabIndex" Value="0"/>
<Setter Property="IsExpanded" Value="{Binding Path=IsExpanded,Mode=TwoWay}"/>
<Setter Property="ItemsSource" Value="{Binding Children}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TreeViewItem}">
<StackPanel>
<Border Name="Bd">
</Border>
<ItemsPresenter x:Name="ItemsHost" />
</StackPanel>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="false">
<Setter TargetName="ItemsHost"
Property="Visibility"
Value="Collapsed"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style TargetType="{x:Type TreeView}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TreeView}">
<Border BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="ItemContainerStyle" Value="{StaticResource aa}" />
</Style>
</TreeView.Resources>
</TreeView>
(I've omitted some of the code but the main hirerchy is still there...)
The exception is a xaml parse exception, on ItemsControl.ItemTemplate.
Could you please tell me how to make this code work?
Thanks,
Li

You can't use a plain string as a resource locator, you need to use either a StaticResource markup extension, i.e., ItemTempate="{StaticResource CellTemplate}"

The ItemTemplate line should read:
ItemTemplate="{StaticResource CellTemplate}"