implementing-wpf-adorners

WPF Adorner Patterns

Adorner is a mechanism for overlaying decorative visual elements on top of UIElements.

1. Adorner Concept

1.1 Characteristics

• AdornerLayer: Separate rendering layer that holds Adorners
• Z-Order: Always renders above the adorned element
• Layout Independent: No effect on target element's layout
• Event Support: Can receive mouse/keyboard events

1.2 Usage Scenarios

2. Basic Adorner Implementation

2.1 Simple Adorner

namespace MyApp.Adorners;

using System.Windows;
using System.Windows.Documents;
using System.Windows.Media;

/// <summary>
/// Adorner that draws border around element
/// </summary>
public sealed class BorderAdorner : Adorner
{
    private readonly Pen _borderPen;

    public BorderAdorner(UIElement adornedElement) : base(adornedElement)
    {
        _borderPen = new Pen(Brushes.Red, 2)
        {
            DashStyle = DashStyles.Dash
        };
        _borderPen.Freeze();

        // Disable mouse events (decoration only)
        IsHitTestVisible = false;
    }

    protected override void OnRender(DrawingContext drawingContext)
    {
        var rect = new Rect(AdornedElement.RenderSize);

        // Draw border
        drawingContext.DrawRectangle(null, _borderPen, rect);
    }
}

2.2 Applying Adorner

// Get AdornerLayer
var adornerLayer = AdornerLayer.GetAdornerLayer(targetElement);

if (adornerLayer is not null)
{
    // Add Adorner
    var adorner = new BorderAdorner(targetElement);
    adornerLayer.Add(adorner);
}

2.3 Removing Adorner

// Remove all Adorners from specific element
var adornerLayer = AdornerLayer.GetAdornerLayer(targetElement);
var adorners = adornerLayer?.GetAdorners(targetElement);

if (adorners is not null)
{
    foreach (var adorner in adorners)
    {
        adornerLayer!.Remove(adorner);
    }
}