DataTrigger where value is NOT null?

Question

I know that I can make a setter that checks to see if a value is NULL and do something. Example:

<TextBlock>
  <TextBlock.Style>
    <Style>
      <Style.Triggers>
        <DataTrigger Binding="{Binding SomeField}" Value="{x:Null}">
          <Setter Property="TextBlock.Text" Value="It's NULL Baby!" />
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </TextBlock.Style>
</TextBlock>

But how can I check for a "not" value... as in "NOT NULL", or "NOT = 3"? Is that possible in XAML?

Results: Thanks for your answers... I knew I could do a value converter (which means I would have to go in code, and that would not be pure XAML as I hoped for). However, that does answer the question that effectively "no" you can't do it in pure XAML. The answer selected, however, shows probably the best way to create that kind of functionality. Good find.

Ibbie

Answer 1

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.

Enchantress

Interestingly enough the DataTrigger actually has an internal field which controls whether it tests for equality or not equality. Unfortunately you have to do a reasonable amount of reflection to get to the required field. The problem is that it may break in the next version of .net. – Tola

Answer 2

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>

Riata

This was the best solution for my scenario! Thanks for providing this answer! – Swisher

I don't think this is a hack, you need to do this a lot of time; and this is the most clean way to do this. – Aphra

Default Setter can be used without Style.Setter tag. – Reminisce

Just the ticket! I kept putting the default in the control that owns the Style, and couldn't figure out why it kept overriding my styles :-) Thank you! – Amoreta

way better answer then using a converter...simple and clean. – Susy

Thanks! It is simple & efficient. Just what I need. – Illiterate

This is not a cheat. This is the way to do it. – Coaxial

This can give binding errors in the output window if the default style/property is dependent on the data to be not null... – Goldston

Answer 3

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.

Timid

I suppose you could make the converter a bit more generic and use ConverterParameter to pass in a value to compare against (in order to support both comparing to NOT null and NOT 3. – Enchantress

This worked a treat for me - using a Multiple Trigger, it makes it nice and powerful. – Palanquin

Answer 4

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>

Tactile

This is by far the most straight forward answer. I like it! – Massive

Isn't the second case exactly what Michael Noonan's answer shows? – Deloisedelong

Answer 5

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>

Pasquil

Sometimes the most simple solution is hidden in clear view. I believe, XAML code is interpreted from top to bottom. That way, the button will first be enabled and then disabled if no element in the listview is selected. But please tell me, is the style updated once an item is selected in the listview? – Endorsee

The button is enabled when a list item is selected, yes. – Pasquil

Answer 6

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

Saleme

Answer 7

Stop! No converter! I dont want to "sell" the library of this guy, but I hated the fact of doing converter everytime I wanted to compare stuff in XAML.

So with this library : https://github.com/Alex141/CalcBinding

you can do that [and a lot more] :

First, In the declaration of the windows/userControl :

<Windows....
     xmlns:conv="clr-namespace:CalcBinding;assembly=CalcBinding"
>

then, in the textblock

<TextBlock>
      <TextBlock.Style>
          <Style.Triggers>
          <DataTrigger Binding="{conv:Binding 'MyValue==null'}" Value="false">
             <Setter Property="Background" Value="#FF80C983"></Setter>
          </DataTrigger>
        </Style.Triggers>
      </TextBlock.Style>
    </TextBlock>

The magic part is the conv:Binding 'MYValue==null'. In fact, you could set any condition you wanted [look at the doc].

note that I am not a fan of third party. but this library is Free, and little impact (just add 2 .dll to the project).

Guelph

Answer 8

<StackPanel.Style>
  <Style>
    <Setter Property="StackPanel.Visibility" Value="Visible"></Setter>
    <Style.Triggers>
      <DataTrigger  Binding="{Binding ElementName=ProfileSelectorComboBox, Path=SelectedItem.Tag}" Value="{x:Null}">
          <Setter Property="StackPanel.Visibility" Value="Collapsed"></Setter>
      </DataTrigger>
    </Style.Triggers>
  </Style>
</StackPanel.Style>

I just used the inverse logic here...setting my stackpanel to invisible when my comboitem is not populated, it works pretty well!

Afterglow

Answer 9

My solution is in the DataContext instance (or ViewModel if using MVVM). I add a property that returns true if the Not Null condition I want is met.

    Public ReadOnly Property IsSomeFieldNull() As Boolean
        Get
            Return If(SomeField is Null, True, False)
        End Get
    End Property

and bind the DataTrigger to the above property. Note: In VB.NET be sure to use the operator If and NOT the IIf function, which doesn't work with Null objects. Then the XAML is:

    <DataTrigger Binding="{Binding IsSomeFieldNull}" Value="False">
      <Setter Property="TextBlock.Text" Value="It's NOT NULL Baby!" />
    </DataTrigger>

Unsling

Answer 10

If you are looking for a solution that does not use IValueConverter, you can always go with below mechanism

       <StackPanel>
            <TextBlock Text="Border = Red when null value" />
            <Border x:Name="border_objectForNullValueTrigger" HorizontalAlignment="Stretch" Height="20"> 
                <Border.Style>
                    <Style TargetType="Border">
                        <Setter Property="Background" Value="Black" />
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding ObjectForNullValueTrigger}" Value="{x:Null}">
                                <Setter Property="Background" Value="Red" />
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </Border.Style>
            </Border>
            <TextBlock Text="Border = Green when not null value" />
            <Border HorizontalAlignment="Stretch" Height="20">
                <Border.Style>
                    <Style TargetType="Border">
                        <Setter Property="Background" Value="Green" />
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding Background, ElementName=border_objectForNullValueTrigger}" Value="Red">
                                <Setter Property="Background" Value="Black" />
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </Border.Style>
            </Border>
            <Button Content="Invert Object state" Click="Button_Click_1"/>
        </StackPanel>

Starkey

Answer 11

Converter:

public class NullableToVisibilityConverter: IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value == null ? Visibility.Collapsed : Visibility.Visible;
    }
}

Binding:

Visibility="{Binding PropertyToBind, Converter={StaticResource nullableToVisibilityConverter}}"

Site

Answer 12

You can use a converter or create new property in your ViewModel like that:

public bool CanDoIt
{
    get
    {
        return !string.IsNullOrEmpty(SomeField);
    }
}

and use it:

<DataTrigger Binding="{Binding SomeField}" Value="{Binding CanDoIt}">

Trahurn

This approach looks very strange. Is SomeField of type bool? If yes, then why don't just compare it to True or False? If not, the comparison with boolean CanDoIt will fire warnings or errors. – Stacked