AvaloniaUI/Avalonia · error · InvalidOperationException

Animator has no property specified.

Error message

Animator has no property specified.

What it means

Thrown by GradientBrushAnimator.BindAnimation when Property is null. The gradient animator overrides BindAnimation to bind an IGradientBrush observable to the target control's property, and needs a non-null AvaloniaProperty to do so.

Source

Thrown at src/Avalonia.Base/Animation/Animators/GradientBrushAnimator.cs:73

                    return new ImmutableLinearGradientBrush(
                        InterpolateStops(progress, oldValue.GradientStops, newValue.GradientStops),
                        s_doubleAnimator.Interpolate(progress, oldValue.Opacity, newValue.Opacity),
                        InterpolateTransform(progress, oldValue.Transform, newValue.Transform),
                        s_relativePointAnimator.Interpolate(progress, oldValue.TransformOrigin, newValue.TransformOrigin),
                        oldValue.SpreadMethod,
                        s_relativePointAnimator.Interpolate(progress, oldLinear.StartPoint, newLinear.StartPoint),
                        s_relativePointAnimator.Interpolate(progress, oldLinear.EndPoint, newLinear.EndPoint));

                default:
                    return progress >= 0.5 ? newValue : oldValue;
            }
        }

        public override IDisposable BindAnimation(Animatable control, IObservable<IGradientBrush?> instance)
        {
            if (Property is null)
            {
                throw new InvalidOperationException("Animator has no property specified.");
            }

            return control.Bind((AvaloniaProperty<IBrush?>)Property, instance, BindingPriority.Animation);
        }

        private static ImmutableTransform? InterpolateTransform(double progress,
            ITransform? oldTransform, ITransform? newTransform)
        {
            if (oldTransform is TransformOperations oldTransformOperations
                && newTransform is TransformOperations newTransformOperations)
            {

                return new ImmutableTransform(TransformOperations
                    .Interpolate(oldTransformOperations, newTransformOperations, progress).Value);
            }

            if (oldTransform is not null)
            {

View on GitHub (pinned to 11c5427268)

Solutions

  1. Set the GradientBrushAnimator.Property to the brush-typed target property before playing.
  2. Confirm the target property accepts IBrush (e.g. Border.BackgroundProperty).
  3. Use the correct specialized Animator for the brush kind (SolidColorBrushAnimator vs GradientBrushAnimator).

Example fix

// before
var animator = new GradientBrushAnimator();

// after
var animator = new GradientBrushAnimator { Property = Border.BackgroundProperty };
Defensive patterns

Strategy: type-guard

Validate before calling

if (animator.Property is null)
    throw new InvalidOperationException("GradientBrushAnimator.Property must be set.");

Type guard

static bool HasProperty(GradientBrushAnimator a) => a.Property is not null;

Prevention

When it happens

Trigger: Calling Animation.Play on a GradientBrushAnimator whose Property has not been set.

Common situations: Animating a gradient property without declaring the target AvaloniaProperty; XAML Animator missing the Property attribute for a gradient brush animation.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/fa3a8e105af4b4f5. Report an issue: GitHub.