How do I conditionally format a WPF TextBlock?

Question

I have a WPF TextBlock bound to a string. If that string is empty, I want the TextBlock to display a warning message in another colour.

This is easy to do in code, I was wondering if there was a elegant WPF pure XAML solution for it? I have investigated Style Triggers, but the syntax doesn't come naturally to me.

Thanks!

Answer 1

Adding some details to Daniel's (slightly short) answer as some of the needed DataTrigger stuff is not really trivial (like {x:Null} ):

<TextBlock Text="{Binding MyTextProperty}">
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding MyTextProperty}" Value="{x:Null}">
                    <Setter Property="Text" Value="Hey, the text should not be empty!"/>
                    <Setter Property="Foreground" Value="Red"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

BTW: Did this completely from memory, did not check it in VS or Blend , so please excuse if there are errors in there. However, you should be able to sort them out yourself. What counts is the idea. Good luck!

Answer 2

You can use Converter for this. Simply Create class with IValueConverter. After in dataBinding use this converter

For example your XAML

<Window x:Class="WpfApplication4.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:lib="clr-namespace:WpfApplication4"
    Title="Window1" Height="300" Width="300">

<Window.Resources>
    <lib:TextBlockDataConveter x:Key="DataConverter"/>
    <lib:TextBlockForegroundConverter x:Key="ColorConverter"/>
</Window.Resources>

<Grid>
    <TextBlock Text="{Binding Path=message, Converter ={StaticResource DataConverter}}" Foreground="{Binding message, Converter={StaticResource ColorConverter}}"/>
</Grid>

and your converters:

public class TextBlockDataConveter:IValueConverter
{
    #region IValueConverter Members

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value == null)
        {
            return "Error Message";
        }
        else
        {
            return value;
        }
    }

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

    #endregion
}

class TextBlockForegroundConverter:IValueConverter
{

    #region IValueConverter Members

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value == null)
        {

            SolidColorBrush brush = new SolidColorBrush();
            brush.Color = Colors.Red;
            return brush;
        }
        else
        {
            SolidColorBrush brush = new SolidColorBrush();
            brush.Color = Colors.Black;
            return brush;

        }
    }

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

    #endregion
}

it works . Check it.