Triggers collection members must be of type EventTrigger

Question

I've created a UserControl, similar to the following:

<UserControl>
    <StackPanel Orientation="Vertical">

        <StackPanel x:Name="Launch" Orientation="Horizontal" Visibility="Collapsed">
            <!-- Children here -->
        </StackPanel>

        <ToggleButton x:Name="ToggleLaunch" IsChecked="False" Content="Launch" />

    </StackPanel>
</UserControl>

I've been trying to use a DataTrigger to make the 'Launch' StackPanel become visible when the ToggleButton is checked, and remain collapsed otherwise. However, at runtime I get an error stating "Failed object initialization (ISupportInitialize.EndInit). Triggers collection members must be of type EventTrigger". I've tried adding it to the trigger collection of the UserControl and StackPanel with no success. My trigger XAML looks like the following:

<DataTrigger Binding="{Binding ElementName=ToggleLaunch, Path=IsChecked}" Value="True">
    <Setter TargetName="Launch" Property="UIElement.Visibility" Value="Visible" />
</DataTrigger>

Answer 1

Managed to figure it out. Forgot that DataTriggers are meant for Style, ControlTemplate and DataTemplate per the MSDN Docs.

Solution was to use an EventTrigger as the error message indicated. My solution was the following:

<EventTrigger RoutedEvent="ToggleButton.Checked">
    <BeginStoryboard>
        <Storyboard>
            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility"
                                           Storyboard.TargetName="LaunchButtons">
                <DiscreteObjectKeyFrame KeyTime="0:0:0"
                                        Value="{x:Static Visibility.Visible}" />
            </ObjectAnimationUsingKeyFrames>
        </Storyboard>
    </BeginStoryboard>
</EventTrigger>
<EventTrigger RoutedEvent="ToggleButton.Unchecked">
    <BeginStoryboard>
        <Storyboard>
            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility"
                                           Storyboard.TargetName="LaunchButtons">
                <DiscreteObjectKeyFrame KeyTime="0:0:0"
                                        Value="{x:Static Visibility.Collapsed}" />
            </ObjectAnimationUsingKeyFrames>
        </Storyboard>
    </BeginStoryboard>
 </EventTrigger>

Going to hold off marking this as the answer in case someone else has another solution.

Answer 2

You could also bind the Visibility in your stackpanel to the IsChecked property in the ToggleButton. You would need to use a custom ValueConverter. Here is one I found online:

/// <summary>  
/// WPF/Silverlight ValueConverter : Convert boolean to XAML Visibility
/// </summary>  
[ValueConversion(typeof(bool), typeof(Visibility))]
public class VisibilityConverter : IValueConverter
{
  public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
  {
    return (value != null && (bool)value) ? Visibility.Visible : Visibility.Collapsed;
  }

  public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
  {
    Visibility visibility = (Visibility)value;
    return (visibility == Visibility.Visible);
  }
}