AvaloniaUI/Avalonia · error · InvalidOperationException

Animator has no property specified.

Error message

Animator has no property specified.

What it means

EffectAnimatorBase.BindAnimation throws when the Animator's Property is null — it cannot bind an animation without knowing which AvaloniaProperty to drive. The animator base class requires a Property set (typically via XAML/animation registration) before binding. A null Property indicates the animator was constructed/registered without specifying its target property.

Source

Thrown at src/Avalonia.Base/Media/Effects/EffectAnimator.cs:77

    /// </summary>
    public override IEffect? Interpolate(double progress, IEffect? oldValue, IEffect? newValue) => progress >= 0.5 ? newValue : oldValue;

    private static bool s_Registered;
    public static void EnsureRegistered()
    {
        if(s_Registered)
            return;
        s_Registered = true;
    }
}

internal abstract class EffectAnimatorBase<T> : Animator<IEffect?> where T : class, IEffect?
{
    public override IDisposable BindAnimation(Animatable control, IObservable<IEffect?> instance)
    {
        if (Property is null)
        {
            throw new InvalidOperationException("Animator has no property specified.");
        }

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

    protected abstract T Interpolate(double progress, T oldValue, T newValue);
    public override IEffect? Interpolate(double progress, IEffect? oldValue, IEffect? newValue)
    {
        var old = oldValue as T;
        var n = newValue as T;
        if (old == null || n == null)
            return progress >= 0.5 ? newValue : oldValue;
        return Interpolate(progress, old, n);
    }
}

internal class BlurEffectAnimator : EffectAnimatorBase<IBlurEffect>
{

View on GitHub (pinned to 11c5427268)

Solutions

1. Ensure the animator's Property is set to a valid AvaloniaProperty<IEffect?> before binding/playing.
2. If registering via the animation system, confirm the property is specified in the Animation/KeyFrames or registration call.
3. Add a constructor or initialization that validates Property is non-null before use.

Example fix

// before
var animator = new MyEffectAnimator();
animator.BindAnimation(control, instance); // Property is null

// after
var animator = new MyEffectAnimator { Property = Visual.EffectProperty };
animator.BindAnimation(control, instance);

if (animator.Property is null)
    throw new InvalidOperationException("Animator.Property must be set before binding.");
animator.BindAnimation(control, instance);

When it happens

Trigger: An EffectAnimator (or subclass) instance has its Property left null when BindAnimation is called by the animation system. Typically happens during animation registration/playback if the animator wasn't fully initialized.

Common situations: Custom effect animator subclass that doesn't set Property. Registering an animation where the property binding was omitted. A XAML/C# animation definition missing the target AvaloniaProperty. Misuse of the animation API where an animator is reused without resetting Property.

Related errors

• Transitions collection cannot be reset.
• No animator registered for the property {setter.Property}. A
• Object of type '{value?.GetType()}' cannot be converted to t
• Value must be less than 10.
• Transition elements have different parents.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/9861a866a6f45c51. Report an issue: GitHub.