AvaloniaUI/Avalonia · error · ArgumentNullException

propertyName

Error message

propertyName

What it means

StopAnimation guards its contract by rejecting a null property name with ArgumentNullException(nameof(propertyName)) before touching the server object. It is a pure argument-validation failure independent of any server state.

Source

Thrown at src/Avalonia.Base/Rendering/Composition/CompositionObject.cs:73

        }

        /// <summary>
        /// Connects an animation with the specified property of the object and starts the animation.
        /// </summary>
        public void StartAnimation(string propertyName, CompositionAnimation animation)
            => StartAnimation(propertyName, animation, null);
        
        internal virtual void StartAnimation(string propertyName, CompositionAnimation animation, ExpressionVariant? finalValue)
        {
            throw new ArgumentException("Unknown property " + propertyName);
        }

        /// <summary>Disconnects an animation from the specified property and stops the animation.</summary>
        /// <param name="propertyName">The name of the property to disconnect the animation from.</param>
        public void StopAnimation(string propertyName)
        {
            if (propertyName is null)
                throw new ArgumentNullException(nameof(propertyName));
            if (Server is not ServerObject srv)
                return;
            var prop = srv.GetCompositionProperty(propertyName) ?? throw new ArgumentException("Unknown property " + propertyName);
            srv.Animations?.RemoveAnimationForProperty(prop);
        }

        /// <summary>
        /// Starts an animation group.
        /// The StartAnimationGroup method on CompositionObject lets you start CompositionAnimationGroup.
        /// All the animations in the group will be started at the same time on the object.
        /// </summary>
        public void StartAnimationGroup(ICompositionAnimationBase grp)
        {
            if (grp is CompositionAnimation animation)
            {
                if(animation.Target == null)
                    throw new ArgumentException("Animation Target can't be null");
                StartAnimation(animation.Target, animation);

View on GitHub (pinned to 11c5427268)

Solutions

1. Pass a non-null property name literal or validate the string before calling StopAnimation.
2. Guard with a null check (or string.IsNullOrEmpty) and skip the call when there is nothing to stop.
3. Store the property name used at StartAnimation time and reuse the same value when stopping.

Example fix

// before
visual.StopAnimation(_maybeNullField);

// after
if (!string.IsNullOrEmpty(_maybeNullField))
    visual.StopAnimation(_maybeNullField);

if (string.IsNullOrEmpty(propertyName)) return;
visual.StopAnimation(propertyName);

static bool IsValidPropertyName(string? s) => !string.IsNullOrEmpty(s);

When it happens

Trigger: Calling StopAnimation(null) on any CompositionObject.

Common situations: Passing a null/unchecked string variable as the property name, or calling StopAnimation inside cleanup code where the property name field was never initialized.

Related errors

• obj
• No Setter property assigned.
• Unknown PlaybackBehavior value: {_playbackBehavior}
• SpeedRatio value cannot be negative.
• Delay value cannot be negative.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/c8d6e4acbeb52c80. Report an issue: GitHub.