WPF Behaviors and Triggers

1. Setup

1.1 Install NuGet Package

<PackageReference Include="Microsoft.Xaml.Behaviors.Wpf" Version="1.1.*" />

1.2 XAML Namespace

<Window xmlns:b="http://schemas.microsoft.com/xaml/behaviors">

2. EventTrigger

Executes actions when events occur.

2.1 InvokeCommandAction (MVVM Recommended)

<Button Content="Click Me"> <b:Interaction.Triggers> <b:EventTrigger EventName="Click"> <b:InvokeCommandAction Command="{Binding ClickCommand}"/> </b:EventTrigger> </b:Interaction.Triggers> </Button>

2.2 With Event Args

<ListBox> <b:Interaction.Triggers> <b:EventTrigger EventName="SelectionChanged"> <b:InvokeCommandAction Command="{Binding SelectionChangedCommand}" PassEventArgsToCommand="True"/> </b:EventTrigger> </b:Interaction.Triggers> </ListBox>

2.3 ChangePropertyAction

<Button Content="Toggle Visibility"> <b:Interaction.Triggers> <b:EventTrigger EventName="Click"> <b:ChangePropertyAction TargetName="MyPanel" PropertyName="Visibility" Value="Collapsed"/> </b:EventTrigger> </b:Interaction.Triggers> </Button>

<StackPanel x:Name="MyPanel"/>

3. DataTrigger

Executes actions based on data conditions.

<TextBlock Text="{Binding Status}"> <b:Interaction.Triggers> <b:DataTrigger Binding="{Binding IsLoading}" Value="True"> <b:ChangePropertyAction PropertyName="Text" Value="Loading..."/> <b:ChangePropertyAction PropertyName="Foreground" Value="Gray"/> </b:DataTrigger> <b:DataTrigger Binding="{Binding HasError}" Value="True"> <b:ChangePropertyAction PropertyName="Foreground" Value="Red"/> </b:DataTrigger> </b:Interaction.Triggers> </TextBlock>

4. Custom Behavior

Encapsulates reusable behaviors.

4.1 Basic Behavior

public sealed class FocusOnLoadBehavior : Behavior<UIElement> { protected override void OnAttached() { base.OnAttached(); AssociatedObject.Loaded += OnLoaded; }

protected override void OnDetaching()
{
    base.OnDetaching();
    AssociatedObject.Loaded -= OnLoaded;
}

private void OnLoaded(object sender, RoutedEventArgs e)
{
    AssociatedObject.Focus();
}

}

<TextBox> <b:Interaction.Behaviors> <local:FocusOnLoadBehavior/> </b:Interaction.Behaviors> </TextBox>

4.2 Behavior with DependencyProperty

public sealed class SelectAllOnFocusBehavior : Behavior<TextBox> { public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.Register( nameof(IsEnabled), typeof(bool), typeof(SelectAllOnFocusBehavior), new PropertyMetadata(true));

public bool IsEnabled
{
    get => (bool)GetValue(IsEnabledProperty);
    set => SetValue(IsEnabledProperty, value);
}

protected override void OnAttached()
{
    base.OnAttached();
    AssociatedObject.GotFocus += OnGotFocus;
}

protected override void OnDetaching()
{
    base.OnDetaching();
    AssociatedObject.GotFocus -= OnGotFocus;
}

private void OnGotFocus(object sender, RoutedEventArgs e)
{
    if (IsEnabled)
    {
        AssociatedObject.SelectAll();
    }
}

}

<TextBox Text="Select me on focus"> <b:Interaction.Behaviors> <local:SelectAllOnFocusBehavior IsEnabled="{Binding IsSelectAllEnabled}"/> </b:Interaction.Behaviors> </TextBox>

4.3 Drag and Drop Behavior

public sealed class DragDropBehavior : Behavior<UIElement> { public static readonly DependencyProperty DropCommandProperty = DependencyProperty.Register( nameof(DropCommand), typeof(ICommand), typeof(DragDropBehavior));

public ICommand? DropCommand
{
    get => (ICommand?)GetValue(DropCommandProperty);
    set => SetValue(DropCommandProperty, value);
}

protected override void OnAttached()
{
    base.OnAttached();
    AssociatedObject.AllowDrop = true;
    AssociatedObject.Drop += OnDrop;
    AssociatedObject.DragOver += OnDragOver;
}

protected override void OnDetaching()
{
    base.OnDetaching();
    AssociatedObject.Drop -= OnDrop;
    AssociatedObject.DragOver -= OnDragOver;
}

private void OnDragOver(object sender, DragEventArgs e)
{
    e.Effects = e.Data.GetDataPresent(DataFormats.FileDrop)
        ? DragDropEffects.Copy
        : DragDropEffects.None;
    e.Handled = true;
}

private void OnDrop(object sender, DragEventArgs e)
{
    if (e.Data.GetDataPresent(DataFormats.FileDrop))
    {
        var files = (string[])e.Data.GetData(DataFormats.FileDrop)!;
        DropCommand?.Execute(files);
    }
}

}

<Border Background="LightGray" MinHeight="100"> <b:Interaction.Behaviors> <local:DragDropBehavior DropCommand="{Binding FileDroppedCommand}"/> </b:Interaction.Behaviors> <TextBlock Text="Drop files here" HorizontalAlignment="Center" VerticalAlignment="Center"/> </Border>

5. Custom TriggerAction

public sealed class ShowMessageAction : TriggerAction<DependencyObject> { public static readonly DependencyProperty MessageProperty = DependencyProperty.Register( nameof(Message), typeof(string), typeof(ShowMessageAction));

public string Message
{
    get => (string)GetValue(MessageProperty);
    set => SetValue(MessageProperty, value);
}

protected override void Invoke(object parameter)
{
    MessageBox.Show(Message, "Information",
                    MessageBoxButton.OK, MessageBoxImage.Information);
}

}

<Button Content="Show Message"> <b:Interaction.Triggers> <b:EventTrigger EventName="Click"> <local:ShowMessageAction Message="Hello, World!"/> </b:EventTrigger> </b:Interaction.Triggers> </Button>

6. Common Patterns

6.1 Close Window

<Button Content="Close"> <b:Interaction.Triggers> <b:EventTrigger EventName="Click"> <b:CallMethodAction TargetObject="{Binding RelativeSource={RelativeSource AncestorType=Window}}" MethodName="Close"/> </b:EventTrigger> </b:Interaction.Triggers> </Button>

6.2 Focus Next on Enter

public sealed class MoveNextOnEnterBehavior : Behavior<UIElement> { protected override void OnAttached() { base.OnAttached(); AssociatedObject.KeyDown += OnKeyDown; }

protected override void OnDetaching()
{
    base.OnDetaching();
    AssociatedObject.KeyDown -= OnKeyDown;
}

private void OnKeyDown(object sender, KeyEventArgs e)
{
    if (e.Key == Key.Enter)
    {
        AssociatedObject.MoveFocus(
            new TraversalRequest(FocusNavigationDirection.Next));
    }
}

}

7. Best Practices

DO DON'T

✅ Implement OnDetaching in Behavior (prevent memory leaks) ❌ Write event handlers directly in code-behind

✅ Follow MVVM pattern (InvokeCommandAction) ❌ Reference ViewModel directly in Behavior

✅ Make configurable via DependencyProperty ❌ Use hardcoded values

✅ Design reusable Behaviors ❌ Behaviors that only work with specific controls

8. Related Skills

• handling-wpf-input-commands

• ICommand, RoutedCommand

• routing-wpf-events

• Routed Events

• implementing-wpf-dragdrop

• Drag and Drop