AvaloniaUI/Avalonia · error · InvalidOperationException

Animator has no property specified.

Error message

Animator has no property specified.

What it means

Thrown by Animator<T>.BindAnimation when the Animator's Property is null. BindAnimation binds the animation observable to the target control's property, so it requires a concrete AvaloniaProperty to bind against.

Source

Thrown at src/Avalonia.Base/Animation/Animators/Animator`1.cs:98

                return (
                    KeyFrameInfo.FromKeyFrame(this[Count - 2], neutralValue),
                    KeyFrameInfo.FromKeyFrame(lastFrame, neutralValue));
            }

            // Past the last frame which isn't at time 1.0: interpolate between the last frame and 1.0.
            var afterValue = lastFrame.FillAfter ? GetTypedValue(lastFrame.Value, neutralValue) : neutralValue;
            return (
                KeyFrameInfo.FromKeyFrame(lastFrame, neutralValue),
                new KeyFrameInfo(1.0, afterValue, lastFrame.KeySpline));
        }

        private static T GetTypedValue(object? untypedValue, T neutralValue)
            => untypedValue is T value ? value : neutralValue;

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

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

        /// <summary>
        /// Runs the KeyFrames Animation.
        /// </summary>
        internal IDisposable Run(Animation animation, Animatable control, IClock? clock, Action? onComplete, bool shouldPauseOnInvisible)
        {
            var instance = new AnimationInstance<T>(
                animation,
                control,
                this,
                clock ?? control.Clock ?? Clock.GlobalClock,
                onComplete,
                InterpolationHandler,
                shouldPauseOnInvisible);

View on GitHub (pinned to 11c5427268)

Solutions

  1. Set Animator.Property to the target AvaloniaProperty before Play/Apply.
  2. Construct the Animator via the XAML/animation builder that requires Property.
  3. Verify the Animator wiring in code-behind at definition time.

Example fix

// before
animator.Property = null;
animation.Play(control);

// after
animator.Property = Visual.OpacityProperty;
animation.Play(control);
Defensive patterns

Strategy: type-guard

Validate before calling

if (animator.Property is null)
    throw new InvalidOperationException("Animator.Property must be set before Play/Apply.");

Type guard

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

Prevention

When it happens

Trigger: Calling Animation.Play (which calls Animator.Run -> BindAnimation) on an Animator that has no Property set.

Common situations: Programmatically building an Animator and forgetting 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/9890b1e4a0169b96. Report an issue: GitHub.