Binding to UserControl DependencyProperty

Question

I have created a UserControl with some DependencyProperties (in the example here only one string property). When I instantiate the Usercontrol, I can set the property of the UserControl and it is shown as expected. When I am trying to replace the static text by Binding, nothing is displayed.

My UserControl looks as follows:

<User Control x:Class="TestUserControBinding.MyUserControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="30" d:DesignWidth="100">
    <Grid>
    <Label Content="{Binding MyText}"/>
  </Grid>
</UserControl>

The Code Behind is:

namespace TestUserControBinding {

  public partial class MyUserControl : UserControl {
    public MyUserControl() {
      InitializeComponent();
      this.DataContext = this;
    }

    public static readonly DependencyProperty MyTextProperty = 
                   DependencyProperty.Register(
                         "MyText", 
                          typeof(string), 
                          typeof(MyUserControl));

    public string MyText {
      get {
        return (string)GetValue(MyTextProperty);
      }
      set {
        SetValue(MyTextProperty, value);
      }
    }// MyText

  }
}

When I try this in my MainWindow, everything is as expected:

<Window x:Class="TestUserControBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:TestUserControBinding"
        Title="MainWindow" Height="350" Width="525">
  <StackPanel>
    <local:MyUserControl MyText="Hello World!"/>
  </StackPanel>
</Window>

But this doesn't work:

<Window x:Class="TestUserControBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:TestUserControBinding"
        Title="MainWindow" Height="350" Width="525">
  <StackPanel>
    <local:MyUserControl MyText="{Binding Path=Text}"/>
    <Label Content="{Binding Path=Text}"/>
  </StackPanel>
</Window>

The behaviour of the label is correct, so there is no Problem with the Property "Text"

What is my mistake? I Ponder for hours, but can't find anything I have forgotten.

Answer 1

With the following binding in your UserControl:

<Label Content="{Binding MyText}"/>

I'm not sure how setting the text directly to the MyText property works. You must be setting the DataContext on the UserControl somewhere for this to work.

Regardless, this binding is the issue - as I understand your scenario, you don't want to bind to the DataContext of the UserControl because that will not necessarily have a MyText property. You want to bind to the UserControl itself, and specifically the DependencyProperty you created. To do that, you need to use a RelativeSource binding, like the following:

<Label Content="{Binding RelativeSource={RelativeSource AncestorType={x:Type local:MyUserControl}}, Path=MyText}"/>

This will navigate up the visual tree to MyUserControl and then find the MyText property there. It will not be dependent on the DataContext, which will change based on where you place the UserControl.

In this case, local refers to a namespace you'll need to define in the UserControl:

<UserControl x:Class="TestUserControBinding.MyUserControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:local="clr-namespace:TestUserControBinding"
         ...>

And your second example should work at that point.