AvaloniaUI/Avalonia · error · ArgumentNullException

obj

Error message

obj

What it means

CompositionPropertySet.Set(key, CompositionObject) rejects a null CompositionObject with ArgumentNullException(nameof(obj)). This internal setter is the backing for CompositionAnimation.SetReferenceParameter, which forwards the compositionObject straight into the property set.

Source

Thrown at src/Avalonia.Base/Rendering/Composition/CompositionPropertySet.cs:41

        private readonly Dictionary<string, CompositionObject> _objects = new Dictionary<string, CompositionObject>();
        
        internal CompositionPropertySet(Compositor compositor) : base(compositor, null)
        {
        }

        internal void Set(string key, ExpressionVariant value)
        {
            _objects.Remove(key);
            _variants[key] = value;
        }

        /*
         For INTERNAL USE by CompositionAnimation ONLY, we DON'T support expression
         paths like SomeParam.SomePropertyObject.SomeValue
        */
        internal void Set(string key, CompositionObject obj)
        {
            _objects[key] = obj ?? throw new ArgumentNullException(nameof(obj));
            _variants.Remove(key);
        }
        
        public void InsertColor(string propertyName, Avalonia.Media.Color value) => Set(propertyName, value);

        public void InsertMatrix3x2(string propertyName, Matrix3x2 value) => Set(propertyName, value);

        public void InsertMatrix4x4(string propertyName, Matrix4x4 value) => Set(propertyName, value);

        public void InsertQuaternion(string propertyName, Quaternion value) => Set(propertyName, value);

        public void InsertScalar(string propertyName, float value) => Set(propertyName, value);
        public void InsertVector2(string propertyName, Vector2 value) => Set(propertyName, value);

        public void InsertVector3(string propertyName, Vector3 value) => Set(propertyName, value);

        public void InsertVector4(string propertyName, Vector4 value) => Set(propertyName, value);

View on GitHub (pinned to 11c5427268)

Solutions

1. Never pass null to SetReferenceParameter; only register real composition objects.
2. Null-check the object before registering it as a reference parameter.
3. If a parameter is optional, omit the SetReferenceParameter call entirely instead of passing null.

Example fix

// before
anim.SetReferenceParameter("source", maybeNullVisual);

// after
if (maybeNullVisual != null)
    anim.SetReferenceParameter("source", maybeNullVisual);

if (refObject is null) return;
anim.SetReferenceParameter("key", refObject);

static bool IsRegistrable(CompositionObject? o) => o is not null && !o.IsDisposed;

When it happens

Trigger: Calling CompositionAnimation.SetReferenceParameter("key", null), which internally calls _propertySet.Set(key, null).

Common situations: Passing an uninitialized CompositionObject field, or conditionally passing a reference parameter that evaluated to null, into an animation's reference parameters.

Related errors

• propertyName
• 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/3815e1bbcda222d6. Report an issue: GitHub.