AvaloniaUI/Avalonia · error · InvalidOperationException

{item.DebugDisplay} must have a value of type {typeof(Transf

Error message

{item.DebugDisplay} must have a value of type {typeof(TransformOperations)}.

What it means

Thrown by TransformOperationsAnimator's list validator (IAvaloniaListItemValidator<AnimatorKeyFrame>.Validate) when an added AnimatorKeyFrame's Value is not a TransformOperations instance. This animator exclusively interpolates TransformOperations, so any other value type in a key frame is rejected at insertion time.

Source

Thrown at src/Avalonia.Base/Animation/Animators/TransformOperationsAnimator.cs:32

        public override TransformOperations Interpolate(double progress, TransformOperations oldValue, TransformOperations newValue)
        {
            var oldTransform = EnsureOperations(oldValue);
            var newTransform = EnsureOperations(newValue);

            return TransformOperations.Interpolate(oldTransform, newTransform, progress);
        }

        internal static TransformOperations EnsureOperations(ITransform value)
        {
            return value as TransformOperations ?? TransformOperations.Identity;
        }

        void IAvaloniaListItemValidator<AnimatorKeyFrame>.Validate(AnimatorKeyFrame item)
        {
            if (item.Value is not TransformOperations)
            {
                throw new InvalidOperationException($"{item.DebugDisplay} must have a value of type {typeof(TransformOperations)}.");
            }
        }
    }
}

View on GitHub (pinned to 11c5427268)

Solutions

1. Set each key frame's Value to a TransformOperations instance.
2. Build TransformOperations via TransformOperations.Parse from a CSS-style transform string.
3. Use TransformAnimator instead if you need non-TransformOperations transform types.

Example fix

// before
kf.Value = "translate(10px)"; // string, not TransformOperations

// after
kf.Value = TransformOperations.Parse("translate(10px)");

foreach (var kf in animator)
    if (kf.Value is not TransformOperations)
        throw new InvalidOperationException($"KeyFrame value must be TransformOperations, got {kf.Value?.GetType()}.");

static bool ValueIsTransformOperations(AnimatorKeyFrame kf) => kf.Value is TransformOperations;

When it happens

Trigger: Adding an AnimatorKeyFrame whose Value is any type other than TransformOperations to a TransformOperationsAnimator's key frame collection (e.g. setting Value to a string, double, or a different ITransform).

Common situations: XAML key frame using a non-transform value or a raw Transform string instead of a TransformOperations; programmatic key frame with the wrong value type; confusing TransformAnimator with TransformOperationsAnimator.

Related errors

• KeyFrame value can't be null.
• KeyFrame value doesnt match property type.
• Animator has no property specified.
• You can only set either {nameof(KeyTime)} or {nameof(Cue)}.
• No Setter property assigned.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/69e9bdc3a2ec8933. Report an issue: GitHub.