WPF - ValidationRule is not being called

Question

I got this Xaml of a TextBlock:

<TextBlock VerticalAlignment="Center">
       <TextBlock.Text>
             <Binding Path="FilesPath" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">
                  <Binding.ValidationRules>
                        <viewModel:ExtensionRule></viewModel:ExtensionRule>
                   </Binding.ValidationRules>
              </Binding>
         </TextBlock.Text>
 </TextBlock>

In the ViewModel:

    private string _filesPath;
    public string FilesPath
    {
        set 
        { 
            _filesPath = value;
            OnPropertyChange("FilesPath");
        }
        get { return _filesPath; }
    }

    private void OnPropertyChange(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

and the validation rule is this:

public class ExtensionRule : ValidationRule
{
    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        string filePath = String.Empty;
        filePath = (string)value;
        if (String.IsNullOrEmpty(filePath))
        {
            return new ValidationResult(false, "Must give a path");
        }

        if (!File.Exists(filePath))
        {
            return new ValidationResult(false, "File not found");
        }
        string ext = Path.GetExtension(filePath);
        if (!ext.ToLower().Contains("txt"))
        {
            return new ValidationResult(false, "given file does not end with the \".txt\" file extenstion");
        }
        return new ValidationResult(true, null);
    }
}

and the FilesPath property is being updated by another event: (vm is the viewModel var)

private void BrowseButton_Click(object sender, RoutedEventArgs e)
    {
        // Create OpenFileDialog 
        OpenFileDialog dlg = new OpenFileDialog();

        // Set filter for file extension and default file extension 
        dlg.DefaultExt = ".txt";
        dlg.Filter = "txt Files (*.txt)|*.txt";

        // Display OpenFileDialog by calling ShowDialog method 
        bool? result = dlg.ShowDialog();

        // Get the selected file name and display in a TextBox 
        if (result == true)
        {
            // Open document 
            string filename = dlg.FileName;
            vm.FilesPath = filename;
        }
    }

Why doesn't the ValidationRule being called when i choose a file by the file dialog?

Answer 1

According to this MSDN Library article validation rules are only checked when transferring data from binding's target property (TextBlock.Text in your case) to source property (your vm.FilesPath property) - the aim here is to validate user input from, for example, a TextBox. In order to give validation feedback from source property to the control owning the target property (the TextBlock control) your view model should implement either IDataErrorInfo or INotifyDataErrorInfo.