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:

Answer 1

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).

Answer 2

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.

Answer 3

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>

Answer 4

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

Answer 5

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>

Answer 6

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}">