wpf how to bind to DataContext existence?

Question

I set the datacontext dynamically in code. I would like a button on screen to be enabled/disabled depending if DataContext == null or not. I can do it in code when I assign the DataContext but it would be better if I can bind like that :)

Answer 1

You should be able to use a DataTrigger on the button style to disable your button when the DataContext is null. The other option is to bind the IsEnabled property to the DataContext and use a value converter to return false if DataContext is null and true otherwise.

With trigger:

<Button>
   <Button.Style>
       <Style TargetType="{x:Type Button}">
          <Style.Triggers>
             <DataTrigger Binding="{Binding Path=DataContext, RelativeSource={RelativeSource Self}}" Value="{x:Null}">
                 <Setter Property="IsEnabled" Value="false"/>
             </DataTrigger>
          </Style.Triggers>
       </Style>
   </Button.Style>
</Button>

With converter:

Converter:

public class DataContextSetConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value != null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

And use it

<UserControl.Resources>
   <local:DataContextSetConverter x:Key="dataContextSetConverter"/>
</UserControl.Resources>

...

<Button IsEnabled="{Binding Path=DataContext, RelativeSource={RelativeSource Self}, Converter={StaticResource dataContextSetConverter}}"/>

Answer 2

This should do it:

    <Button Content="ButtonName">
        <Button.Style>
            <Style TargetType="{x:Type Button}">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding RelativeSource={RelativeSource Self}, Path=DataContext}" Value="{x:Null}">
                        <Setter Property="Visibility" Value="Collapsed" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </Button.Style>
    </Button>