Freezable Performance

Rules for using Freeze() on WPF Freezable objects to maximize rendering performance.

Rule: Freeze All Freezable Objects

Call Freeze() on every Freezable after it is fully configured and before it is used for rendering.

Applies to: Brush, Pen, Geometry, Transform, PathGeometry, GradientBrush, ImageBrush, DrawingGroup, and all other Freezable subclasses.

Why Freeze Matters

Frozen objects are immutable. WPF can cache and reuse them across render passes without defensive copies.

Correct vs Incorrect

// Incorrect: brush created but never frozen
protected override void OnRender(DrawingContext dc)
{
    var brush = new SolidColorBrush(Colors.CornflowerBlue);
    var pen   = new Pen(brush, 2.0);
    dc.DrawRectangle(brush, pen, new Rect(0, 0, 100, 50));
}

// Correct: freeze immediately after configuration
protected override void OnRender(DrawingContext dc)
{
    var brush = new SolidColorBrush(Colors.CornflowerBlue);
    brush.Freeze();

    var pen = new Pen(brush, 2.0);
    pen.Freeze();

    dc.DrawRectangle(brush, pen, new Rect(0, 0, 100, 50));
}

OnRender Pattern: Create in Constructor, Reuse in OnRender

Creating Freezable objects inside OnRender allocates on every paint cycle. Create them once in the constructor (or a lazy initializer), freeze them, then reuse the frozen instances in every OnRender call.

public class MyVisual : FrameworkElement
{
    private readonly SolidColorBrush _fillBrush;
    private readonly Pen             _borderPen;

    public MyVisual()
    {
        _fillBrush = new SolidColorBrush(Colors.CornflowerBlue);
        _fillBrush.Freeze();

        _borderPen = new Pen(Brushes.DarkBlue, 1.5);
        _borderPen.Freeze();
    }

    protected override void OnRender(DrawingContext dc)
    {
        // No allocation here — reuse frozen instances
        dc.DrawRectangle(_fillBrush, _borderPen, new Rect(0, 0, ActualWidth, ActualHeight));
    }
}

If a property changes and a new brush is needed, create and freeze the replacement, then call InvalidateVisual() once.