How to get DataTemplate.DataTrigger to check for greater than or less than?

Question

The following DataTemplate.DataTrigger makes the age display red if it is equal to 30.

How do I make the age display red if it is greater than 30?

<DataTemplate DataType="{x:Type local:Customer}">
    <Grid x:Name="MainGrid" Style="{StaticResource customerGridMainStyle}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100"/>
            <ColumnDefinition Width="150"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <TextBlock Grid.Column="0" Grid.Row="0" Text="First Name" Margin="5"/>
        <TextBlock Grid.Column="1" Grid.Row="0" Text="{Binding FirstName}" Margin="5"/>
        <TextBlock Grid.Column="0" Grid.Row="1" Text="Last Name" Margin="5"/>
        <TextBlock Grid.Column="1" Grid.Row="1" Text="{Binding LastName}" Margin="5"/>
        <TextBlock Grid.Column="0" Grid.Row="2" Text="Age" Margin="5"/>
        <TextBlock x:Name="Age" Grid.Column="1" Grid.Row="2" Text="{Binding Age}" Margin="5"/>
    </Grid>

    <DataTemplate.Triggers>
        <DataTrigger Binding="{Binding Path=Age}">
            <DataTrigger.Value>30</DataTrigger.Value>
            <Setter TargetName="Age" Property="Foreground" Value="Red"/> 
        </DataTrigger>
    </DataTemplate.Triggers>

</DataTemplate>

Answer 1

You could create a IValueConverter which converts an integer to a boolean based on the cutoff. Then use DataTrigger.Value of true (or false, depending on what you are returning).

WPF DataTriggers are strictly equality comparers if I remember correctly.

So something similar to:

public class CutoffConverter : IValueConverter {
    public object ConvertTo(object obj, Type type) {
        return ((int)obj) > Cutoff;
    }

    public object ConvertFrom(object obj, Type type) {
        throw new NotImplementedException();
    }

    public int Cutoff { get; set; }
}

Then use the following XAML.

<Window.Resources>
    <myNamespace:CutoffConverter x:Key="AgeConverter" Cutoff="30" />
</Window.Resources>

<DataTemplate.Triggers>
    <DataTrigger Binding="{Binding Path=Age,
                                   Converter={StaticResource AgeConverter}}">
        <DataTrigger.Value>true</DataTrigger.Value>
        <Setter TargetName="Age" Property="Foreground" Value="Red"/> 
    </DataTrigger>
</DataTemplate.Triggers>

The code above isn't unfortunately tested (And I can't remember the real IValueConverter interface). But it should point to the right direction at least.

Answer 2

I'd recommend is using a IValueConverter to bind to the Foreground element of the Age TextBlock and isolating the coloring logic there.

<TextBlock x:Name="Age" Grid.Column="1" Grid.Row="2" Text="{Binding Age}" Foreground="{Binding Path=Age, Converter={StaticResource AgeToColorConverter}}" Margin="5"/>

Then in the Code:

[ValueConversion(typeof(int), typeof(Brush))]
public class AgeToColorConverter : IValueConverer
{
   public object Convert(object value, Type target)
   {
      int age;
      int.Parse(value.ToString(), age);
      return (age >= 30 ? Brushes.Red : Brushes.Black);
   }
}