WPF Binding - Default value for empty string

Question

Is there a standard way to set a default or fallback value for a WPF binding if the bound string is empty?

Answer 1

I was under the impression that FallbackValue provides a value when the binding fails and TargetNullValue provides a value when the bound value is null.

To do what you want you will either need a converter (possibly with a parameter) to convert an empty string to a target value, or put the logic in your view model.

I would probably go with a converter something like this (not tested).

public class EmptyStringConverter : MarkupExtension, IValueConverter
{  
    public object Convert(object value, Type targetType, 
                          object parameter, CultureInfo culture)
    {
        return string.IsNullOrEmpty(value as string) ? parameter : value;
    }

    public object ConvertBack(object value, Type targetType, 
                              object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }
}

Answer 2

DataTrigger is the way i do it like this:

<TextBox>
  <TextBox.Style>
        <Style TargetType="{x:Type TextBox}"  BasedOn="{StaticResource ReadOnlyTextBox}">
            <Setter Property="Text" Value="{Binding Name}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Path=Name.Length, FallbackValue=0, TargetNullValue=0}" Value="0">
                    <Setter Property="Text" Value="{x:Static local:ApplicationLabels.NoValueMessage}"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>

Answer 3

You should create a converter for this, which implements IValueConverter

public class StringEmptyConverter : IValueConverter {

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
      return string.IsNullOrEmpty((string)value) ? parameter : value;
    }

public object ConvertBack(
      object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
      throw new NotSupportedException();
    }

}

Then in xaml you'd just provide the converter to the binding, (xxx just represents your Window / UserControl / Style ... where the binding is)

<xxx.Resources>
<local:StringEmptyConverter x:Key="StringEmptyConverter" />
</xxx.Resources>
<TextBlock Text="{Binding Name, Converter={StaticResource StringEmptyConverter}, ConverterParameter='Placeholder Text'}" />

Answer 4

You could use a converter and do the respective validation on it.

Binding="{Binding Path=Name, Converter={StaticResource nameToOtherNameConverter}}"

and in your converter

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (!string.IsNullOrEmpty(value.ToString()))
        { /*do something and return your new value*/ }