Why Doesn't My Parent ViewModel's `SelectedFile` Property Update When Using a DependencyProperty in a UserControl?

DependencyProperty binding problem and solution

This example demonstrates a file browser control that displays the path of the selected file in a text box and allows browsing files.

<code class="language-xml"><UserControl ... x:Class="Test.UserControls.FileBrowserControl">
    <Grid ...>
        <TextBox Text="{Binding SelectedFile}" />
    </Grid>
</UserControl></code>

The use of controls is as follows:

<code class="language-csharp">public class FileBrowserControl : UserControl
{
    public ICommand BrowseCommand { get; set; }
    public static DependencyProperty SelectedFileProperty = DependencyProperty.Register("SelectedFile", ...);
    public string SelectedFile { get => (string)GetValue(SelectedFileProperty); set => SetValue(SelectedFileProperty, value); }
    public string Filter { get; set; }

    public FileBrowserControl()
    {
        ...
        this.DataContext = this; // 将 DataContext 设置为 FileBrowserControl 实例。
    }

    private void Browse()
    {
        ...
        SelectedFile = dialog.FileName; // 更新 FileBrowserControl 实例的 SelectedFile 属性。
    }
}</code>

Question:

<code class="language-xml"><FileBrowserControl Filter="XSLT File (*.xsl)|*.xsl|All Files (*.*)|*.*" SelectedFile="{Binding SelectedFile}" /></code>

When clicking the "Browse" button, the text box in the file browser control will be updated correctly, but the SELECTEDFILE property of the parent control ViewModel will not be set.

The root cause:

The problem is that the DataContext is set to itself in the constructor of FileBrowserControl to itself:

This conflict with the inheritance bound to the ViewModel, because it destroys any data binding attributes binding binding using RelativeSource.

Solution:

<code class="language-csharp">this.DataContext = this;</code>

In order to solve this problem, the binding in the XAML of UserControl should be modified:

This can ensure that the SelectedFile property is bound to the SELECTEDFILE attribute in the parent control part, which inherits the DataContext of its father's control. By specifying

, the binding will find the parent level of UserControl up, so as to find the correct

attribute.

<code class="language-xml"><TextBox Text="{Binding SelectedFile, RelativeSource={RelativeSource AncestorType=UserControl}}" /></code>

Through the above modification, the

attributes of the parent ViewModel will be updated correctly. RelativeSource AncestorType=UserControl

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