How can i make TreeViewItem expand horizontally to the right?

Question

I'm trying to create a treeview that expands horizontally to the right so that it expands like this:

  Parent1 -> Child1 

            Child2 

            Child3 

            Child4 

            Child5 

  Parent2 -> Child1 

            Child2 

            Child3 

            Child4 

            Child5 

Please help!! Thank you!!

Answer 1

You have to change the style of TreeViewItem.

This is the default style: (I copy pasted this from microsoft homepage)

<!--=================================================================
      TreeViewItem
  ==================================================================-->
<Style x:Key="ExpandCollapseToggleStyle" TargetType="ToggleButton">
  <Setter Property="Focusable" Value="False"/>
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="ToggleButton">
        <Grid
          Width="15"
          Height="13"
          Background="Transparent">
          <Path x:Name="ExpandPath"
            HorizontalAlignment="Left" 
            VerticalAlignment="Center" 
            Margin="1,1,1,1"
            Fill="{StaticResource GlyphBrush}"
            Data="M 4 0 L 8 4 L 4 8 Z"/>
        </Grid>
        <ControlTemplate.Triggers>
          <Trigger Property="IsChecked"
               Value="True">
            <Setter Property="Data"
                TargetName="ExpandPath"
                Value="M 0 4 L 8 4 L 4 8 Z"/>
          </Trigger>
        </ControlTemplate.Triggers>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>
<Style x:Key="TreeViewItemFocusVisual">
  <Setter Property="Control.Template">
    <Setter.Value>
      <ControlTemplate>
        <Border>
          <Rectangle Margin="0,0,0,0"
                 StrokeThickness="5"
                 Stroke="Black"
                 StrokeDashArray="1 2"
                 Opacity="0"/>
        </Border>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>
<Style x:Key="{x:Type TreeViewItem}"
     TargetType="{x:Type TreeViewItem}">
  <Setter Property="Background"
      Value="Transparent"/>
  <Setter Property="HorizontalContentAlignment"
      Value="{Binding Path=HorizontalContentAlignment,
              RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
  <Setter Property="VerticalContentAlignment"
      Value="{Binding Path=VerticalContentAlignment,
              RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
  <Setter Property="Padding"
      Value="1,0,0,0"/>
  <Setter Property="Foreground"
      Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
  <Setter Property="FocusVisualStyle"
      Value="{StaticResource TreeViewItemFocusVisual}"/>
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="{x:Type TreeViewItem}">
        <Grid>
          <Grid.ColumnDefinitions>
            <ColumnDefinition MinWidth="19"
                      Width="Auto"/>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
          </Grid.ColumnDefinitions>
          <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition/>
          </Grid.RowDefinitions>
          <ToggleButton x:Name="Expander"
                  Style="{StaticResource ExpandCollapseToggleStyle}"
                  IsChecked="{Binding Path=IsExpanded,
                              RelativeSource={RelativeSource TemplatedParent}}"
                  ClickMode="Press"/>
          <Border Name="Bd"
              Grid.Column="1"
              Background="{TemplateBinding Background}"
              BorderBrush="{TemplateBinding BorderBrush}"
              BorderThickness="{TemplateBinding BorderThickness}"
              Padding="{TemplateBinding Padding}">
            <ContentPresenter x:Name="PART_Header"
                      ContentSource="Header"
                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"/>
          </Border>
          <ItemsPresenter x:Name="ItemsHost"
                  Grid.Row="1"
                  Grid.Column="1"
                  Grid.ColumnSpan="2"/>
        </Grid>
        <ControlTemplate.Triggers>
          <Trigger Property="IsExpanded"
               Value="false">
            <Setter TargetName="ItemsHost"
                Property="Visibility"
                Value="Collapsed"/>
          </Trigger>
          <Trigger Property="HasItems"
               Value="false">
            <Setter TargetName="Expander"
                Property="Visibility"
                Value="Hidden"/>
          </Trigger>
          <MultiTrigger>
            <MultiTrigger.Conditions>
              <Condition Property="HasHeader"
                     Value="false"/>
              <Condition Property="Width"
                     Value="Auto"/>
            </MultiTrigger.Conditions>
            <Setter TargetName="PART_Header"
                Property="MinWidth"
                Value="75"/>
          </MultiTrigger>
          <MultiTrigger>
            <MultiTrigger.Conditions>
              <Condition Property="HasHeader"
                     Value="false"/>
              <Condition Property="Height"
                     Value="Auto"/>
            </MultiTrigger.Conditions>
            <Setter TargetName="PART_Header"
                Property="MinHeight"
                Value="19"/>
          </MultiTrigger>
          <Trigger Property="IsSelected"
               Value="true">
            <Setter TargetName="Bd"
                Property="Background"
                Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
            <Setter Property="Foreground"
                Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
          </Trigger>
          <MultiTrigger>
            <MultiTrigger.Conditions>
              <Condition Property="IsSelected"
                     Value="true"/>
              <Condition Property="IsSelectionActive"
                     Value="false"/>
            </MultiTrigger.Conditions>
            <Setter TargetName="Bd"
                Property="Background"
                Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
            <Setter Property="Foreground"
                Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
          </MultiTrigger>
          <Trigger Property="IsEnabled"
               Value="false">
            <Setter Property="Foreground"
                Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
          </Trigger>
        </ControlTemplate.Triggers>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

Take a look at this part:

      <Grid.ColumnDefinitions>
        <ColumnDefinition MinWidth="19"
                  Width="Auto"/>
        <ColumnDefinition Width="Auto"/>
        <ColumnDefinition Width="*"/>
      </Grid.ColumnDefinitions>
      <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition/>
      </Grid.RowDefinitions>

The ItemsPresenter is placed by default in Grid.Colum = 1 and Grid.Row = 1. That is how you become the tree alike look.

If you want the ItemsPreseter to be layouted next to Header, just set the ItemsPresenter in Grid.Column = 2 and Grid.Row = 0.

You will not need the second row anywys.

Try it out and let us know if you got it right.

Feel free to mark this as answer of vote it up if you find this post helpful.