WPF Datatrigger not firing when expected

Question

I have the following XAML:

<TextBlock Text="{Binding ElementName=EditListBox, Path=SelectedItems.Count}" Margin="0,0,5,0"/> <TextBlock Text="items selected">     <TextBlock.Style>         <Style TargetType="{x:Type TextBlock}">             <Style.Triggers>                 <DataTrigger Binding="{Binding ElementName=EditListBox, Path=SelectedItems.Count}" Value="1">                     <Setter Property="TextBlock.Text" Value="item selected"></Setter>                 </DataTrigger>             </Style.Triggers>         </Style>     </TextBlock.Style> </TextBlock> 

The first text block happily changes with SelectedItems.Count, showing 0,1,2, etc. The datatrigger on the second block never seems to fire to change the text.

Any thoughts?

Answer 1

Alternatively, you could replace your XAML with this:

<TextBlock Margin="0,0,5,0" Text="{Binding ElementName=EditListBox, Path=SelectedItems.Count}"/> <TextBlock>     <TextBlock.Style>         <Style TargetType="{x:Type TextBlock}">             <Setter Property="Text" Value="items selected"/>             <Style.Triggers>                 <DataTrigger Binding="{Binding ElementName=EditListBox, Path=SelectedItems.Count}" Value="1">                     <Setter Property="Text" Value="item selected"/>                 </DataTrigger>             </Style.Triggers>         </Style>     </TextBlock.Style> </TextBlock> 

Converters can solve a lot of binding problems but having a lot of specialized converters gets very messy.

Answer 2

The DataTrigger is firing but the Text field for your second TextBlock is hard-coded as "items selected" so it won't be able to change. To see it firing, you can remove Text="items selected".

Your problem is a good candidate for using a ValueConverter instead of DataTrigger. Here's how to create and use the ValueConverter to get it to set the Text to what you want.

Create this ValueConverter:

public class CountToSelectedTextConverter : IValueConverter {     #region IValueConverter Members      public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)     {         if ((int)value == 1)             return "item selected";         else             return "items selected";     }      public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)     {         throw new NotImplementedException();     }      #endregion } 

Add the namespace reference to your the assembly the converter is located:

xmlns:local="clr-namespace:ValueConverterExample" 

Add the converter to your resources:

<Window.Resources>     <local:CountToSelectedTextConverter x:Key="CountToSelectedTextConverter"/> </Window.Resources> 

Change your second textblock to:

    <TextBlock Text="{Binding ElementName=EditListBox, Path=SelectedItems.Count, Converter={StaticResource CountToSelectedTextConverter}}"/>