AvaloniaUI/Avalonia · error · InvalidOperationException

Animator has no property specified.

Error message

Animator has no property specified.

What it means

Thrown by SolidColorBrushAnimator.BindAnimation when Property is null. The solid-color animator binds an ISolidColorBrush observable to the target control's property and requires a non-null AvaloniaProperty.

Source

Thrown at src/Avalonia.Base/Animation/Animators/SolidColorBrushAnimator.cs:33

        private static readonly DoubleAnimator s_doubleAnimator = new DoubleAnimator();

        public override ISolidColorBrush? Interpolate(double progress, ISolidColorBrush? oldValue, ISolidColorBrush? newValue)
        {
            if (oldValue is null || newValue is null)
            {
                return progress >= 0.5 ? newValue : oldValue;
            }

            return new ImmutableSolidColorBrush(
                ColorAnimator.InterpolateCore(progress, oldValue.Color, newValue.Color),
                s_doubleAnimator.Interpolate(progress, oldValue.Opacity, newValue.Opacity));
        }

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

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

View on GitHub (pinned to 11c5427268)

Solutions

  1. Set SolidColorBrushAnimator.Property to the brush target property before playing.
  2. Ensure the target property is brush-typed (Background, Foreground, BorderBrush).
  3. Confirm the Animator is wired through the correct property in XAML.

Example fix

// before
var animator = new SolidColorBrushAnimator();

// after
var animator = new SolidColorBrushAnimator { Property = Control.BackgroundProperty };
Defensive patterns

Strategy: type-guard

Validate before calling

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

Type guard

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

Prevention

When it happens

Trigger: Calling Animation.Play with a SolidColorBrushAnimator that has no Property assigned.

Common situations: Constructing a SolidColorBrushAnimator in code without setting Property; XAML Animator element missing the Property attribute.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/b4af2cac479c40fd. Report an issue: GitHub.