WPF Validation not fired

I have a simple textbox and a button on a WPF form. When I click the button an OpenFolderDialog opens and I choose a folder. That SelectedPath is then shown in the textbox. This all works fine.

Then I 开发者_如何学编程decided I wanted validation on the textbox to check if the directory exists, because you can just paste some path in the textbox too. When my program starts the textbox shows a red border around it because the validation sees an empty textbox. For now I can live with that.

There are two problems:

1. When I select a valid folder through the dialog, the PropertyChnged is fired, but is null and thus the validation never runs and the red border is still shown.
2. When I just paste a valid directory in it, nothing is fired at all and the red border is still shown.

What am I doing wrong?

Below my code. I'm new to WPF so I appreciate every help I can get.

<TextBox Grid.ColumnSpan="2" Grid.Row="1" x:Name="textBoxFolder" Margin="2,4">
  <TextBox.Text>
    <Binding Path="this.MovieFolder" UpdateSourceTrigger="PropertyChanged">
      <Binding.ValidationRules>
        <!--  Validation rule set to run when binding target is updated. -->
        <Rules:MandatoryInputRule ValidatesOnTargetUpdated="True" />
      </Binding.ValidationRules>
    </Binding>
  </TextBox.Text>
</TextBox>

And here is my c# code:

public partial class MainWindow : Window, INotifyPropertyChanged
{
  private string _movieFolder;
  public string MovieFolder
  {
    get { return _movieFolder; }
    set
    {
      _movieFolder = value;
      OnNotifyPropertyChanged("MovieFolder");
    }
  }

  public MainWindow()
  {
    InitializeComponent();
    //textBoxFolder.DataContext = MovieFolder;

  }


  public event PropertyChangedEventHandler PropertyChanged;
  private void OnNotifyPropertyChanged(string propertyName)
  {
    if (PropertyChanged != null)
    {
      PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
  }

  private void buttonSearchFolder_Click(object sender, RoutedEventArgs e)
  {
    FolderBrowserDialog folderBrowserDialog = new FolderBrowserDialog();
    folderBrowserDialog.ShowDialog();

    MovieFolder = folderBrowserDialog.SelectedPath;
    textBoxFolder.Text = MovieFolder;
  }

  private void MenuItemClose_Click(object sender, RoutedEventArgs e)
  {
    this.Close();
  }

  private void Window_Loaded(object sender, RoutedEventArgs e)
  {

  }
}

public class MandatoryInputRule : ValidationRule
{
  public override ValidationResult Validate(object value, CultureInfo cultureInfo)
  {
    if (value != null)
    {
      string input = value as string;

      if (Directory.Exists(input))
        return new ValidationResult(true, null);
    }

    return new ValidationResult(false, "Not a valid folder.");
  }
}

Your binding path is just wrong, you can not bind via this (it will look for a property called this). It works as expected if the binding is correct.