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?

<TextBlock Text="{Binding Name, FallbackValue='Unnamed'" />

The FallbackValue only seems to kick in when Name is null, but not when it is set to String.Empty.

there is a much simple answer at [enter link description here][1] [1]: http://stackoverflow.com/questions/16612622/give-textblock-default-value-if-result-returns-null — FLICKER, Feb 05 '15 at 19:07
there is a much simple solution at [TextBox Default Value if Binding returns null][1] [1]: http://stackoverflow.com/questions/16612622/give-textblock-default-value-if-result-returns-null — FLICKER, Feb 05 '15 at 19:09

score 82 · Answer 1 · answered Mar 22 '13 at 10:21

82

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>

answered Mar 22 '13 at 10:21

iltzortz

2,342
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I like this approach because it let's me bind the default value to another property in my ViewModel. Thanks :) – Daniel Gruszczyk Oct 06 '17 at 16:51
1

you can probably also do `Name` and empty string instead of `Name.length`, totally love this approach :) – EricG Oct 20 '17 at 15:41
Seeing as `Name` is generally a `string`, then `Name.Length` would be of type `Int32`. For this reason, there is no need specify `TargetNullValue` because the the expression will never evaluate to `null`, granted it evaluates at all. `FallbackValue` is the property pulling the weight here. – Nicholas Miller Sep 11 '18 at 21:13
1

ugly way to do this... should be 1 liner for such a simple thing. – Konrad Jun 27 '19 at 09:09

Phil · Accepted Answer · 2013-03-22T10:18:29.973

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;
    }
}

Viv · Answer 3 · 2013-03-22T11:32:24.247

9

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'}" />

edited Mar 22 '13 at 11:32

answered Mar 22 '13 at 10:15

Viv

17,170
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I need to specify a different default string for every binding, but I could pass in the string as the ConverterParameter. – kroimon Mar 22 '13 at 11:14
@Viv Why binding to Name property, not to Text? – Andrzej Dec 30 '14 at 13:47
@Andrzej huh Name is the property the OP had mentioned in his question. – Viv Dec 30 '14 at 13:49

score 0 · Answer 4 · answered Mar 22 '13 at 10:15

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*/ }

WPF Binding - Default value for empty string

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