Weird behavior of DataContext Inheritance

Actually there are several questions in this post, though all of them are about DataContext inheritance. I think you will have fun with these questions, if anyone can explain what's going on here, it will be greatly appreciated, however it's really not that easy to answer.

I created a CustomControl which derives from ContentControl by adding a Gutter property, so that the user of this control can specify two part of this control Content and Gutter. Here is my codes:

public class BizField : ContentControl   {       static BizField()       {           DefaultStyleKeyProperty.OverrideMetadata(typeof(BizField), new FrameworkPropertyMetadata(typeof(BizField)));       }       public object Gutter       {           get { return (object)GetValue(GutterProperty); }           set { SetValue(GutterProperty, value); }       }       // Using a DependencyProperty as the backing store for Gutter.  This enables animation, styling, binding, etc...       public static readonly DependencyProperty GutterProperty =           DependencyProperty.Register("Gutter",                           typeof(object),                            typeof(BizField),                            new UIPropertyMetadata(null,                               new PropertyChangedCallback(OnGutterChanged)));       private static void OnGutterChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)       {           var me = sender as BizField;           me.OnGutterChanged(e);       }       private void OnGutterChanged(DependencyPropertyChangedEventArgs e)       {                }   }

The template is quite simple:

 

    <Style TargetType="{x:Type local:BizField}">        <Setter Property="Template">            <Setter.Value>                <ControlTemplate TargetType="{x:Type local:BizField}">                    <Border Background="{TemplateBinding Background}"                            BorderBrush="{TemplateBinding BorderBrush}"                            BorderThickness="{TemplateBinding BorderThickness}">                        <StackPanel>                            <ContentPresenter/>                            <ContentPresenter ContentSource="Gutter"/>                        </StackPanel>                    </Border>                </ControlTemplate>            </Setter.Value>        </Setter>    </Style>

Now if I use it in the main window, it works as expected:

 

<Window x:Class="DataContexPropagate.MainWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        xmlns:local="clr-namespace:DataContexPropagate"        Title="MainWindow" Height="350" Width="525">    <Grid>        <local:BizField>            <Button Content="Click one"/>            <local:BizField.Gutter>                <Button Content="{Binding}"/>            </local:BizField.Gutter>        </local:BizField>    </Grid></Window>

public partial class MainWindow : Window  {      public MainWindow()      {          //Notice DataContext is set before parsing the baml          this.DataContext = "Hello";                 InitializeComponent();               }  }

 

The button defined in the Gutter shows "Hello". Now I try to add the gutter as BizField's logical child.

    private void OnGutterChanged(DependencyPropertyChangedEventArgs e)   {      this.AddLogicalChild(Gutter);   }

It still works fine, but if I set the DataContext after loading xaml, it will broke.

public partial class MainWindow : Window{    public MainWindow()    {                     InitializeComponent();        //Set datacontext after parsing baml        this.DataContext = "Hello";           }}

If you run the application, the button show nothing, apparently DataContext Inheritance doesn' t work properly now. but if I change the button to a TextBlock, it will work. what's the magic with TextBlock?

 

<Window x:Class="DataContexPropagate.MainWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        xmlns:local="clr-namespace:DataContexPropagate"        Title="MainWindow" Height="350" Width="525">    <Grid>        <local:BizField>            <Button Content="Click one"/>            <local:BizField.Gutter>                <TextBlock Text="{Binding}"/>            </local:BizField.Gutter>        </local:BizField>    </Grid></Window>

To make the databinding works for the button, I have to add following codes to BizControl.

 

protected override System.Collections.IEnumerator LogicalChildren     {         get         {             yield return Content;             yield return Gutter;         }     }

According to this example, you will find that "setting datacontext before or after parsing Baml does matter". But I don't why and more intersting what happened to the TextBlock?

 

 

Full source codes for BizField:

 

public class BizField : ContentControl   {       static BizField()       {           DefaultStyleKeyProperty.OverrideMetadata(typeof(BizField), new FrameworkPropertyMetadata(typeof(BizField)));       }       public object Gutter       {           get { return (object)GetValue(GutterProperty); }           set { SetValue(GutterProperty, value); }       }       // Using a DependencyProperty as the backing store for Gutter.  This enables animation, styling, binding, etc...       public static readonly DependencyProperty GutterProperty =           DependencyProperty.Register("Gutter",                           typeof(object),                           typeof(BizField),                           new UIPropertyMetadata(null,                               new PropertyChangedCallback(OnGutterChanged)));       private static void OnGutterChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)       {           var me = sender as BizField;           me.OnGutterChanged(e);       }       private void OnGutterChanged(DependencyPropertyChangedEventArgs e)       {           //Comment this out, it will search for the visual parent.           this.AddLogicalChild(Gutter);       }       //Only by adding this method, it can work properly.       protected override System.Collections.IEnumerator LogicalChildren       {           get           {               yield return Content;               yield return Gutter;           }       }   }

<Window x:Class="DataContexPropagate.MainWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        xmlns:local="clr-namespace:DataContexPropagate"        Title="MainWindow" Height="350" Width="525">    <Grid>        <local:BizField>            <Button Content="Click one"/>            <local:BizField.Gutter>                <Button Content="{Binding}"/>                <!--<TextBlock Text="{Binding}"/>-->            </local:BizField.Gutter>        </local:BizField>    </Grid></Window>

 

public partial class MainWindow : Window{    public MainWindow()    {        //Notice DataContext is set before parsing the baml        //this.DataContext = "Hello";                InitializeComponent();        //Set datacontext after parsing baml        this.DataContext = "Hello";           }}