Question

I’m trying to make a user control that has a few DependencyProperties which are forward to child controls in the user control. After a few trys I got this to work. For testing a made a small example.

In the example I have a user control named Ctrl that just contains a TextBox and exposes the Text property of the TextBox. This control is used in a window which contains a TextBox and my custom Ctrl which are bound to the same property.

user control

XAML

<UserControl x:Class="trying.Ctrl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    DataContext="{Binding RelativeSource={RelativeSource self}}"
    Height="Auto" Width="Auto">
    <TextBox Text="{Binding MyText}" />
</UserControl>

Code behind

using System.Windows;
using System.Windows.Controls;

namespace trying
{
    public partial class Ctrl : UserControl
    {
        public static readonly DependencyProperty MyTextProperty = DependencyProperty.Register(
            "MyText",
            typeof( string ),
            typeof( UserControl ),
            new FrameworkPropertyMetadata( default(string), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
        public string MyText
        {
            get { return (string)GetValue( MyTextProperty ); }
            set { SetValue( MyTextProperty, value ); }
        }

        public Ctrl()
        {
            InitializeComponent();
        }
    }
}

window

XAML

<Window x:Class="trying.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:cc="clr-namespace:trying"
    DataContext="{Binding RelativeSource={RelativeSource self}}"
    Title="Window1" Height="300" Width="300">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <TextBox Grid.Row="0" VerticalAlignment="Center" HorizontalAlignment="Center" Text="{Binding DisplayText}" />
        <cc:Ctrl Grid.Row="1" VerticalAlignment="Center" HorizontalAlignment="Center" MyText="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DisplayText}" />
    </Grid>
</Window>

code behind

using System.Windows;
using System.ComponentModel;

namespace trying
{
    public partial class Window1 : Window, INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        protected void NotifyPropertyChanged( string propertyName )
        {
            if( PropertyChanged != null )
                PropertyChanged( this, new PropertyChangedEventArgs( propertyName ) );
        }

        private string m_displayText = "asdf";
        public string DisplayText
        {
            get { return m_displayText; }
            set
            {
                m_displayText = value;
                NotifyPropertyChanged( "DisplayText" );
            }
        }

        public Window1()
        {
            InitializeComponent();
        }
    }
}

problem

How I posted the code it works. My question is now: What have I done wrong that I have to use the binding

 MyText="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}

When binding the MyText property of Ctrl and can not just the simple binding like I use when binding the original TextBox?

If I don’t bind this way it won’t work, and I get the warning

  System.Windows.Data Error: 39 : BindingExpression path error: 'DisplayText' property
  not found on 'object' ''Ctrl' (Name='')'. BindingExpression:Path=DisplayText; 
  DataItem='Ctrl' (Name=''); target element is 'Ctrl' (Name=''); target property
  is 'MyText' (type 'String')

What do I have to change that binding is possible like with the original TextBox?

during execution.

Answer 1

The UserControl‘s DataContext is pointing to itself, so any binding on an instance of the control will be looking at the Ctrl instance rather than the inherited DataContext.

Try setting the DataContext further down the tree:

<UserControl 
   x:Class="trying.Ctrl" x:Name="root"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
   Height="Auto" Width="Auto"
>
   <TextBox 
      DataContext="{Binding ElementName=root}"
      Text="{Binding MyText}" 
   />
</UserControl>