#111 – The Visual Tree

A visual tree in WPF breaks down the logical tree into lower-level visual elements.  Where elements in a logical tree are typically controls, the visual tree contains all of the underlying visual elements that make up the control.  All elements in a visual tree derive from Visual or Visual3D.

As an example, the visual tree for the following XAML:

 <Window x:Class="WpfApplication4.MainWindow"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml>
     <StackPanel>
         <Button Content="Click Me" />
         <TextBox />
         <ListBox>
             <ListBoxItem Content="Barley"/>
             <ListBoxItem Content="Oats"/>
         </ListBox>
     </StackPanel>
 </Window>

looks like this:

Window
    Border
        AdornerDecorator
            ContentPresenter
                StackPanel
                    Button
                        ButtonChrome
                            ContentPresenter
                                TextBlock
                    TextBox
                        ListBoxChrome
                            ScrollViewer
                                Grid
                                    Rectangle
                                    ScrollContentPresenter
                                        TextBoxView
                                            TextBoxLineDrawingVisual
                                        AdornerLayer
                                    Scrollbar
                                    Scrollbar
                    ListBox
                        Border
                            ScrollViewer
                                Grid
                                    Rectangle
                                    ScrollContentPresenter
                                        ItemsPresenter
                                            VirtualizingStackPanel
                                                ListBoxItem
                                                    Border
                                                        ContentPresenter
                                                            TextBlock
                                                ListBoxItem
                                                    Border
                                                        ContentPresenter
                                                            TextBlock
                                        AdornerLayer
                                    ScrollBar
                                    Scrollbar
            AdornerLayer
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#110 – An Application for Viewing a WPF Logical Tree

Here’s a little WPF application that can load .xaml files and then display the underlying logical tree.  It uses the LogicalTreeHelper.GetChildren method to recursively descend through the logical tree and display it in a TreeView control.

You can download an executable version of the application from: DisplayWpfTrees.zip

You can find a more detailed explanation of the application at: An Application to Let You View WPF Logical Trees

You can find full source code at the WPFLogicalTree project on Codeplex.

#109 – Navigating the Logical Tree in Code

You can use the LogicalTreeHelper.GetChildren method to traverse a logical tree and enumerate all of the objects in the tree.

Every element in a logical tree is a DependencyObject, so you pass a top-level object that derives from DependencyObject into the GetChildren method.  The method returns a collection of children of that object.

 // Enumerate each immediate child of main window.  (Does NOT descend down tree)
 foreach (Object obj in LogicalTreeHelper.GetChildren(mainWindow as DependencyObject))
     Debug.WriteLine(obj.ToString());

You could extend this example and descend down the logical tree by calling GetChildren on the children returned from the first call.

#108 – The Logical Tree

In WPF, the logical tree is the hierarchy of elements that make up your user interface.  If your user interface is defined in XAML, the logical tree is the set of elements from the XAML, organized into a tree based on their parent/child relationships.

The logical tree can also be thought of as a model that describes the relationships between objects at runtime.  Knowing the logical tree can be helpful in understanding:

  • Resource lookup
  • Property inheritance
  • Event routing

As an example, the logical tree for the following XAML:

 <Window x:Class="WpfApplication4.MainWindow"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
     Title="A window.." Height="350" Width="525">
     <StackPanel>
         <Button Content="Click Me" Height="23" HorizontalAlignment="Left" Width="75" Click="button1_Click" />
         <TextBox />
         <ListBox>
             <ListBoxItem Content="Barley"/>
             <ListBoxItem Content="Oats"/>
         </ListBox>
     </StackPanel>
 </Window>

looks like this:

#107 – Markup Extensions in the XAML Namespace

Some markup extensions are part of the extensions to XAML added for WPF (e.g. StaticResource).  But some markup extensions are part of the XAML vocabulary itself, typically prefixed with x:.  They are listed below:

  • x:Array – Allows including an array of objects in XAML
  • x:Null – A null value
  • x:Reference – A reference to another element defined in XAML  (XAML 2009)
  • x:Static – References a static element in code, e.g. value of a static property
  • x:Type – Specifies a .NET type

#106 – Set Property Value to Point to Another Object

There are times when you’d like to set a property value on one element to point to an instance of another element and do this in XAML, rather than in code.

An example of this is the CommandTarget property, which is used to indicate the control that should be the target of the command being initiated from a control.

For example, if you have a Button that executes a Paste command and you want the contents pasted in a TextBox, you’d set the button’s CommandTarget property to point to the TextBox.

This is done in XAML using the Binding markup extension, setting its ElementName property to point to the desired control.

 <Button Content="Paste" Command="ApplicationCommands.Paste"
     CommandTarget="{Binding ElementName=myTextBox}" />
 <TextBox Name="myTextBox"/>

XAML 2009 allows using the x:Reference markup extension, but this is not supported in compiled XAML in WPF.

 <Button Content="Paste" Command="ApplicationCommands.Paste"
     CommandTarget="{x:Reference myTextBox}" />
 <TextBox Name="myTextBox"/>

#105 – Viewing BAML as XAML

BAML is simply a compiled binary version of a XAML fragment.  The XAML elements are converted into equivalent binary objects.  This means that translating back from BAML to XAML is straightforward.

The simplest way to view a particular .baml file as XAML is to use the .NET Reflector tool.  After you download the tool, download the BamlViewer add-in for Reflector.  You’ll have to install the add-in (View | Add-Ins | Add).

Once installed, you just open the executable that contains the BAML, stored as a resource.  Then open the BAML Viewer from the Tools menu.

The BAML Viewer window will open and you can then navigate to the .baml that you want to examine (found embedded as a resource).  When you select the .baml file in the upper pane, the equivalent XAML will be displayed in the lower pane.