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.

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

The use of controls is as follows:

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。
    }
}

<：> Question:

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

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.

<本> 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.

<决> Solution:

this.DataContext = this;

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.

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

Through the above modification, the

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

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