DataTrigger where value is NOT null?

Question

This is a bit of a cheat but I just set a default style and then overrode it using a DataTrigger if the value is null...

  <Style> 
      <!-- Highlight for Reviewed (Default) -->
      <Setter Property="Control.Background" Value="PaleGreen" /> 
      <Style.Triggers>
        <!-- Highlight for Not Reviewed -->
        <DataTrigger Binding="{Binding Path=REVIEWEDBY}" Value="{x:Null}">
          <Setter Property="Control.Background" Value="LightIndianRed" />
        </DataTrigger>
      </Style.Triggers>
  </Style>

You can use an IValueConverter for this:

<TextBlock>
    <TextBlock.Resources>
        <conv:IsNullConverter x:Key="isNullConverter"/>
    </TextBlock.Resources>
    <TextBlock.Style>
        <Style>
            <Style.Triggers>
                <DataTrigger Binding="{Binding SomeField, Converter={StaticResource isNullConverter}}" Value="False">
                    <Setter Property="TextBlock.Text" Value="It's NOT NULL Baby!"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

Where IsNullConverter is defined elsewhere (and conv is set to reference its namespace):

public class IsNullConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (value == null);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new InvalidOperationException("IsNullConverter can only be used OneWay.");
    }
}

A more general solution would be to implement an IValueConverter that checks for equality with the ConverterParameter, so you can check against anything, and not just null.

I ran into a similar limitation with DataTriggers, and it would seem that you can only check for equality. The closest thing I've seen that might help you is a technique for doing other types of comparisons other than equality.

This blog post describes how to do comparisons such as LT, GT, etc in a DataTrigger.

This limitation of the DataTrigger can be worked around to some extent by using a Converter to massage the data into a special value you can then compare against, as suggested in Robert Macnee's answer.

Compare with null (As Michael Noonan said):

<Style>
    <Style.Triggers>
       <DataTrigger Binding="{Binding SomeProperty}" Value="{x:Null}">
           <Setter Property="Visibility" Value="Collapsed" />
        </DataTrigger>
     </Style.Triggers>
</Style>

Compare with not null (without a converter):

<Style>
    <Setter Property="Visibility" Value="Collapsed" />
    <Style.Triggers>
       <DataTrigger Binding="{Binding SomeProperty}" Value="{x:Null}">
           <Setter Property="Visibility" Value="Visible" />
        </DataTrigger>
     </Style.Triggers>
</Style>

I'm using this to only enable a button if a listview item is selected (ie not null):

<Style TargetType="{x:Type Button}">
    <Setter Property="IsEnabled" Value="True"/>
    <Style.Triggers>
        <DataTrigger Binding="{Binding ElementName=lvMyList, Path=SelectedItem}" Value="{x:Null}">
            <Setter Property="IsEnabled" Value="False"/>
        </DataTrigger>
    </Style.Triggers>
</Style>

You can use DataTrigger class in Microsoft.Expression.Interactions.dll that come with Expression Blend.

Code Sample:

<i:Interaction.Triggers>
    <i:DataTrigger Binding="{Binding YourProperty}" Value="{x:Null}" Comparison="NotEqual">
       <ie:ChangePropertyAction PropertyName="YourTargetPropertyName" Value="{Binding YourValue}"/>
    </i:DataTrigger
</i:Interaction.Triggers>

Using this method you can trigger against GreaterThan and LessThan too. In order to use this code you should reference two dll's:

System.Windows.Interactivity.dll

Microsoft.Expression.Interactions.dll