Question

The task: implement the simplest Dependency Property ever, which can be used in xaml like that:

<uc:MyUserControl1 MyTextProperty="{Binding Text}"/>

I think that this answer is quite close. For better readability i copy all my code here (mostly from that answer above).

<UserControl x:Class="Test.UserControls.MyUserControl1"
             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" 
             DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <Grid>
        <!-- Text is being bound to outward representative property;
             Note the DataContext of the UserControl -->
        <TextBox Text="{Binding MyTextProperty}"/>
    </Grid>
</UserControl>

and

public partial class MyUserControl1 : UserControl
{
    // The dependency property which will be accessible on the UserControl
    public static readonly DependencyProperty MyTextPropertyProperty =
        DependencyProperty.Register("MyTextProperty", typeof(string), typeof(MyUserControl1), new UIPropertyMetadata(String.Empty));
    public string MyTextProperty
    {
        get { return (string)GetValue(MyTextPropertyProperty); }
        set { SetValue(MyTextPropertyProperty, value); }
    }

    public MyUserControl1()
    {
        InitializeComponent();
    }
}

And this is my MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:uc="clr-namespace:WpfApplication1"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel Orientation="Vertical">
        <uc:MyUserControl1 MyTextProperty="my text goes here"/>
        <Button Click="ButtonBase_OnClick" Content="click"/>
    </StackPanel>
</Window>

So far, everything works. However, i find this quite not usefull. What i’d need is

<uc:MyUserControl1 MyTextProperty="{Binding Text}"/>

and being able to change this by setting a DataContext (as you usually do in MVVM)

So i replace the line as above and add my code behind as follows:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
        Text = "Initial Text";
        DataContext = this;
    }
    private string _Text;
    public string Text
    {
        get { return _Text; }
        set
        {
            if (value != _Text)
            {
                _Text = value;
                NotifyPropertyChanged("Text");
            }
        }
    }

    private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
    {
        Text = "clicked";
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(String info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

Neither the "initial Text" nor the "clicked" is displayed… ever. So my question is how to implement a dept. property correctly to be used with

<uc:MyUserControl1 MyTextProperty="{Binding Text}"/>

Answer 1

The Text property is located on the DataContext of the MainWindow not of the UserControl.

So change this line <uc:MyUserControl1 MyTextProperty="{Binding Text}"/> into this:

<uc:MyUserControl1 MyTextProperty="{Binding Text, ElementName=MyMainWindow}"/>

Which will tell the Binding that you’re talking about the Text element located in you MainWindow. Of course, since in this example I used ElementName, you’re going to want to name your window MyMainWindow…

So add this to your MainWindow:

<Window  Name="MyMainWindow" ..... />

If you rather not name your window, you can use the RelativeSource FindAncestor binding like this:

<wpfApplication6:MyUserControl1 MyTextProperty="{Binding Text, RelativeSource={RelativeSource FindAncestor, AncestorType=Window}}"/>

In both ways, you are asking to find the property named ‘Text’ in the DataContext of the window.