Implementing Hit Testing

WPF FrameworkElement Hit Testing

An essential pattern for receiving mouse events when rendering directly with OnRender(DrawingContext) in a class that inherits from FrameworkElement.

1. Problem Scenario

Symptoms

• Events like MouseLeftButtonDown, MouseMove don't fire on controls inheriting from FrameworkElement
• Nothing happens when clicking

Cause

WPF Hit Testing is performed based on rendered pixels. If nothing is drawn in OnRender() or there's no background, that area is considered "empty" and mouse events won't be delivered.

2. Solution

2.1 Draw Transparent Background (Required)

namespace MyApp.Controls;

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

public sealed class MyOverlay : FrameworkElement
{
    protected override void OnRender(DrawingContext dc)
    {
        base.OnRender(dc);

        // ⚠️ Required: Draw transparent background (for mouse event reception)
        dc.DrawRectangle(
            Brushes.Transparent,
            null,
            new Rect(0, 0, ActualWidth, ActualHeight));

        // Actual rendering logic follows
        DrawContent(dc);
    }

    private void DrawContent(DrawingContext dc)
    {
        // Draw actual content
    }
}

3. Why Transparent?

Transparent vs null

Transparent is an "existing" brush with Alpha channel of 0. WPF Hit Testing checks if a brush exists, so it behaves differently from null.

4. Practical Example

4.1 Measurement Tool Overlay

namespace MyApp.Controls;

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

public sealed class RulerOverlay : FrameworkElement
{
    private static readonly Pen LinePen;
    private static readonly Brush TextBrush;

    static RulerOverlay()
    {
        // Frozen resources (performance optimization)
        LinePen = new Pen(Brushes.Yellow, 2);
        LinePen.Freeze();
        TextBrush = Brushes.Yellow;
    }

    public Point StartPoint { get; set; }
    public Point EndPoint { get; set; }
    public bool IsDrawing { get; set; }

    protected override void OnRender(DrawingContext dc)
    {
        base.OnRender(dc);

        // 1. Transparent background (required for hit testing)
        dc.DrawRectangle(
            Brushes.Transparent,
            null,
            new Rect(0, 0, ActualWidth, ActualHeight));

        // 2. Draw actual measurement line
        if (IsDrawing)
        {
            dc.DrawLine(LinePen, StartPoint, EndPoint);
        }
    }

    protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
    {
        base.OnMouseLeftButtonDown(e);

        // Now events are received normally
        StartPoint = e.GetPosition(this);
        IsDrawing = true;
        CaptureMouse();
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        base.OnMouseMove(e);

        if (IsDrawing)
        {
            EndPoint = e.GetPosition(this);
            InvalidateVisual();  // Redraw
        }
    }

    protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
    {
        base.OnMouseLeftButtonUp(e);

        if (IsDrawing)
        {
            IsDrawing = false;
            ReleaseMouseCapture();
        }
    }
}

5. Connecting Events in Code-Behind

The same principle applies when connecting events in XAML:

<controls:RulerOverlay x:Name="RulerOverlay"
                       MouseLeftButtonDown="RulerOverlay_MouseLeftButtonDown"
                       MouseMove="RulerOverlay_MouseMove"
                       MouseLeftButtonUp="RulerOverlay_MouseLeftButtonUp" />

// Code-behind
private void RulerOverlay_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    if (sender is RulerOverlay overlay)
    {
        // Without transparent background, this event won't fire!
        var point = e.GetPosition(overlay);
        // ...
    }
}

6. Relationship with IsHitTestVisible Property

Caution

<!-- IsHitTestVisible="False" blocks events regardless of transparent background -->
<controls:MyOverlay IsHitTestVisible="False" />

7. Checklist

• Draw entire area background with Brushes.Transparent in OnRender()
• Draw background before other content
• Verify IsHitTestVisible is True (default)
• Apply Freeze() to Pen, Brush (performance optimization)

8. Common Mistakes

❌ Wrong: No background

protected override void OnRender(DrawingContext dc)
{
    base.OnRender(dc);

    // Draw content without background
    dc.DrawLine(LinePen, StartPoint, EndPoint);  // Hit Test succeeds only on the line
}

✅ Correct: Include transparent background

protected override void OnRender(DrawingContext dc)
{
    base.OnRender(dc);

    // 1. Transparent background first
    dc.DrawRectangle(Brushes.Transparent, null,
        new Rect(0, 0, ActualWidth, ActualHeight));

    // 2. Then content
    dc.DrawLine(LinePen, StartPoint, EndPoint);
}

9. References

• Hit Testing in the Visual Layer - Microsoft Docs
• FrameworkElement.OnRender - Microsoft Docs

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