MultiTrigger doesn't seem to follow conditions

I have a multitrigger, one of whose conditions is

<Condition Property="Validation.HasError"
    Value="True"/>

Outside the trigger, I set the error template to null, and only in one of the trigger's setters do I have

<Setter Property="ToolTip" 
Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>

But for some reason sometimes I still get an exception that the index is out of range, i.e. that I'm trying to access element 0 of an empty error collection.

This seems to happen with controls whose backing data I try to do something with, but I'm not sure of what's causing it to try to create the tooltip even without any errors.

EDIT: I've removed as much stuff as possible and the following is what is left. The exception arises when typing something into the data-bound textbox and then switching focus.

In App.xaml, under application resources

        <Style TargetType="{x:Type TextBox}">
        <Style.Triggers>
            <Trigger Property="Validation.HasError"
                       Value="True">
                <Setter Property="ToolTip" 
                        Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
            </Trigger>
        </Style.Triggers>
    </Style>

In MainWindow.xaml.cs (this is the startup window)

    public MainWindow()
{
    InitializeComponent();
    this.DataContext = new Bar();
}

In MainWindow.xaml, the relevant binding

        <TextBox Text="{Binding Foo, ValidatesOnDataErrors=True}"/>

In Bar.cs

public sealed class Bar : IDataErrorInfo, INotifyPr开发者_JAVA百科opertyChanged
{
// Standard PropertyChanged, OnPropertyChanged removed

// Foo does have a backing field and notifies in the
// setter but those have been removed for brevity
public string Foo {get;set;}

public string Error
{
    get { return ""; }
}

public string this[string columnName]
{
    get
    {
        switch (columnName)
        {
            case "Foo":
                if (string.IsNullOrEmpty(Foo))
                    return "The foo must not be empty.";

                return "";

            default:
                throw new ArgumentException("columnName");
        }
    }
}
}

---

Instead of "Path=(Validation.Errors)[0].ErrorContent", use "Path=(Validation.Errors).CurrentItem.ErrorContent". No idea why this prevents it, considering that if there is an error there's no reason for the error collection to be null or empty or whatever, but it works.

---

Sometimes the errors collection will be null.

Add a converter to the Xaml

<Setter Property="ToolTip" 
        Value="{Binding RelativeSource={RelativeSource Self}, 
        Path=(Validation.Errors),
        Converter={StaticResource ValidationErrorConverter}}"/>

Your converter should look like this:

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
  ReadOnlyObservableCollection<ValidationError> errors = value as ReadOnlyObservableCollection<ValidationError>;

  if (errors == null)
  {
      return String.Empty;
  }

  return errors.Count > 0 ? errors[0].ErrorContent : String.Empty;
}