c# - Issue with DependencyProperty binding

Question

I created a small File Browser Control:

<UserControl x:Class="Test.UserControls.FileBrowserControl"
             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="44" d:DesignWidth="461" Name="Control">
    <Grid Margin="3">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>
        <TextBox  Margin="3" Text="{Binding SelectedFile}" IsReadOnly="True" TextWrapping="Wrap" />
        <Button HorizontalAlignment="Right" Margin="3" Width="100" Content="Browse" Grid.Column="1" Command="{Binding BrowseCommand}" />
    </Grid>
</UserControl>

With the following code behind:

public partial class FileBrowserControl : UserControl
{
    public ICommand BrowseCommand { get; set; }
    //The dependency property
    public static DependencyProperty SelectedFileProperty = DependencyProperty.Register("SelectedFile",
        typeof(string),typeof(FileBrowserControl), new PropertyMetadata(String.Empty));
    public string SelectedFile { get{ return (string)GetValue(SelectedFileProperty);}  set{ SetValue(SelectedFileProperty, value);}}
    //For my first test, this is a static string
    public string Filter { get; set; }

    public FileBrowserControl()
    {
        InitializeComponent();
        BrowseCommand = new RelayCommand(Browse);
        Control.DataContext = this;
    }
    private void Browse()
    {
        SaveFileDialog dialog = new SaveFileDialog();
        if (Filter != null)
        {
            dialog.Filter = Filter;
        }
        if (dialog.ShowDialog() == true)
        {
            SelectedFile = dialog.FileName;
        }
    }
}

And I use it like this:

<userControls:FileBrowserControl SelectedFile="{Binding SelectedFile}" Filter="XSLT File (*.xsl)|*.xsl|All Files (*.*)|*.*"/>

(SelectedFile is Property of the ViewModel of the usercontrol using this control)

Currently the issue is that when I click on Browse, the textbox in the usercontrol is correctly updating, but the SelectedFile property of the viewmodel parent control is not set(no call to the set property).

If I set the Mode of the binding to TwoWay, I got this exception:

An unhandled exception of type 'System.StackOverflowException' occurred in Unknown Module.

So what did I do wrong?

Answer 1

The primary problem is that you set your UserControl's DataContext to itself in its constructor:

DataContext = this;

You should not do that, because it breaks any DataContext based Bindings, i.e. to a view model instance that is provided by property value inheritance of the DataContext property

Instead you would change the binding in the UserControl's XAML like this:

<TextBox Text="{Binding SelectedFile,
                RelativeSource={RelativeSource AncestorType=UserControl}}" />
    

Now, when you use your UserControl and write a binding like

<userControls:FileBrowserControl SelectedFile="{Binding SelectedFile}" />

the SelectedFile property gets bound to a SelectedFile property in your view model, which should be in the DataContext inherited from a parent control.