AvaloniaUI/Avalonia · error · InvalidOperationException

No Setter property assigned.

Error message

No Setter property assigned.

What it means

Thrown during Animation.InterpretKeyframes when a Setter inside a KeyFrame has a null Property. Each setter must target a specific AvaloniaProperty; a null Property makes the keyframe meaningless and prevents animator resolution, so Avalonia fails fast during the animation setup pass.

Solutions

1. Add the Property attribute to every Setter in the animation's keyframes (e.g. <Setter Property="Opacity" Value="1"/>).
2. If constructing Setters in code, always assign setter.Property = SomeControl.SomeProperty before adding to the keyframe.
3. Validate all Setters in a KeyFrame have a non-null Property before running/initializing the animation.
4. After renaming an AvaloniaProperty, update all animation setters referencing it.

Example fix

// before
<!-- XAML -->
<KeyFrame>
  <Setter Value="1.0"/>
</KeyFrame>

// after
<KeyFrame>
  <Setter Property="Opacity" Value="1.0"/>
</KeyFrame>

// ensure every setter has a Property before running the animation
foreach (var kf in animation.Children)
    foreach (var s in kf.Setters)
        if (s.Property is null)
            throw new InvalidOperationException($"Setter is missing a Property (value={s.Value}).");

static bool AllSettersHaveProperty(Animation a)
    => a.Children.SelectMany(k => k.Setters).All(s => s.Property is not null);

When it happens

Trigger: Defining a KeyFrame Setter in XAML or code without specifying the Property attribute/property (e.g. <Setter Value="Red"/> missing Property=...). Also reachable by programmatically adding a Setter whose Property was never assigned.

Common situations: XAML typo omitting the Property attribute on a Setter; building Setters in code and forgetting to set .Property; copy/paste of a Setter that lost its Property binding; stale XAML after renaming/removing an AvaloniaProperty without updating the setter.

Related errors

• Delay value cannot be negative.
• DelayBetweenIterations value cannot be negative.
• Duration value cannot be negative.
• Invalid KeySpline X1 value. Must be >= 0.0 and <= 1.0.
• Invalid KeySpline X2 value. Must be >= 0.0 and <= 1.0.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/411abb5b14adfe3c. Report an issue: GitHub.

Appendix: source

Thrown at src/Avalonia.Base/Animation/Animation.cs:231

                return type;
            }
            return null;
        }

        private (IList<IAnimator> Animators, IList<IDisposable> subscriptions, bool animatesVisibility) InterpretKeyframes(Animatable control)
        {
            var handlerList = new Dictionary<(Type type, AvaloniaProperty Property), Func<IAnimator>>();
            var animatorKeyFrames = new List<AnimatorKeyFrame>();
            var subscriptions = new List<IDisposable>();
            var animatesVisibility = false;

            foreach (var keyframe in Children)
            {
                foreach (var setter in keyframe.Setters)
                {
                    if (setter.Property is null)
                    {
                        throw new InvalidOperationException("No Setter property assigned.");
                    }

                    if (setter.Property == Visual.IsVisibleProperty)
                        animatesVisibility = true;

                    var handler = Animation.GetAnimator(setter) ?? GetAnimatorType(setter.Property);

                    if (handler == null)
                    {
                        throw new InvalidOperationException($"No animator registered for the property {setter.Property}. Add an animator to the Animation.Animators collection that matches this property to animate it.");
                    }

                    var (type, factory) = handler.Value;

                    if (!handlerList.ContainsKey((type, setter.Property)))
                        handlerList[(type, setter.Property)] = factory;

                    var cue = keyframe.Cue;

View on GitHub (pinned to 11c5427268)