stride3d/stride · error · ArgumentNullException

ArgumentNullException: child

Error message

ArgumentNullException: child

What it means

SceneViewModel.CanBeParentOf checks whether one scene can be a parent of another. The child parameter is mandatory; passing null throws ArgumentNullException(nameof(child)) since the parentability check cannot be evaluated for a missing scene.

Solutions

1. Check the dragged/target SceneViewModel is non-null before calling CanBeParentOf
2. Resolve the SceneViewModel from the drag payload first; abort the drag if resolution fails
3. Change the call site to return early (treat null child as not parentable) instead of querying

Example fix

// before
bool ok = scene.CanBeParentOf(draggedScene, out msg);
// after
bool ok = draggedScene != null && scene.CanBeParentOf(draggedScene, out msg);

if (child == null) { message = "No target scene"; return false; }

bool CanEvaluate(SceneViewModel child) => child != null;

try { return scene.CanBeParentOf(child, out msg); }
catch (ArgumentNullException) { return false; // null child can never be parented
}

When it happens

Trigger: Calling CanBeParentOf with a null SceneViewModel child — e.g. drag-and-drop handlers computing parent validity before resolving the dragged item, or filtering scenes where a reference was not yet resolved.

Common situations: Custom editor plugins implementing drag/drop reparenting that call CanBeParentOf with a null candidate; UI bindings passing null while a drag operation is in flight.

Related errors

• ArgumentNullException: pasteResultItem
• Custom strides is not supported with packed PixelFormats
• Model property is unset.
• parent cannot be null
• Parent cannot be null

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/68633740526796d2. Report an issue: GitHub.

Appendix: source

Thrown at sources/editor/Stride.Assets.Presentation/ViewModel/SceneViewModel.cs:72

        [CanBeNull]
        public SceneViewModel Parent { get { return parent; } private set { SetValueUncancellable(ref parent, value, ParentPropertyChanged); } }

        [NotNull]
        internal ICommandBase SetAsDefaultCommand { get; }

        /// <summary>
        /// Checks whether this scene can be a parent of the provided <paramref name="child"/> scene.
        /// </summary>
        /// <param name="child">The potential child of this scene.</param>
        /// <param name="message">A message providing more details on why this scene can -or cannot- be a parent.</param>
        /// <param name="checkSameParent">
        /// <c>true</c> to also check if this scene is already a parent of the provided <paramref name="child"/> scene; otherwise, <c>false</c>.
        /// The default is <c>false</c>.
        /// </param>
        /// <returns><c>true</c> if this scene can be of parent of the provided <paramref name="child"/> scene; otherwise, <c>false</c>.</returns>
        public bool CanBeParentOf([NotNull] SceneViewModel child, [NotNull] out string message, bool checkSameParent = false)
        {
            if (child == null) throw new ArgumentNullException(nameof(child));

            if (child == this)
            {
                message = "Can't be a parent of itself";
                return false;
            }
            if (checkSameParent && child.Parent == this)
            {
                message = "Scene already has this parent";
                return false;
            }
            var scene = Parent;
            while (scene != null)
            {
                if (scene == child)
                {
                    message = "Circular scene hierarchy";
                    return false;

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