stride3d/stride · error · InvalidOperationException

Cannot change the parent of an AssetMountPointViewModel

Error message

Cannot change the parent of an AssetMountPointViewModel

What it means

MountPointViewModel represents the root mount point of a package in the asset tree; its Parent is defined as always null. The Parent setter throws InvalidOperationException because a mount point can never be attached to another directory.

Solutions

1. Never set Parent on a MountPointViewModel; treat it as a fixed root
2. Check the node type before attempting reparent operations
3. Operate on DirectoryViewModel children instead of the mount point itself

Example fix

// before
mountPoint.Parent = targetFolder;
// after
if (mountPoint is DirectoryViewModel dir) { dir.Parent = targetFolder; }

if (node is MountPointViewModel) throw new ArgumentException("Mount points cannot be reparented");

bool CanSetParent(DirectoryBaseViewModel n) => n is DirectoryViewModel;

try { node.Parent = target; } catch (InvalidOperationException) { /* mount point: parent is fixed null */ }

When it happens

Trigger: Assigning the Parent property of a MountPointViewModel, or calling DirectoryBaseViewModel.SetParent with a mount point as the target.

Common situations: Generic tree/drag-drop code that moves any DirectoryBaseViewModel node, accidentally picking a package's mount point as the drag source or drop child.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot delete a MountPointViewModel
• The name of a package reference cannot be set
• A PackageViewModel cannot contain two isntances of…
• A session is already open in this instance.
• A transaction failed to be created.

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/e68c65feac378b42. Report an issue: GitHub.

Appendix: source

Thrown at sources/editor/Stride.Core.Assets.Editor/ViewModel/MountPointViewModel.cs:20

// Distributed under the MIT license. See the LICENSE.md file in the project root for more information.
using System;
using System.Collections.Generic;
using System.Linq;
using Stride.Core.Presentation.Dirtiables;

namespace Stride.Core.Assets.Editor.ViewModel
{
    public abstract class MountPointViewModel : DirectoryBaseViewModel
    {
        protected MountPointViewModel(PackageViewModel package)
            : base(package.Session)
        {
            Package = package;
        }

        public override string Path => string.Empty;

        public override DirectoryBaseViewModel Parent { get { return null; } set { throw new InvalidOperationException("Cannot change the parent of an AssetMountPointViewModel"); } }

        public override MountPointViewModel Root => this;

        public override PackageViewModel Package { get; }

        /// <inheritdoc/>
        public override string TypeDisplayName => "Mount Point";

        /// <inheritdoc/>
        public override IEnumerable<IDirtiable> Dirtiables => base.Dirtiables.Concat(Package.Dirtiables);

        public IReadOnlyCollection<DirectoryBaseViewModel> GetDirectoryHierarchy()
        {
            var hierarchy = new List<DirectoryBaseViewModel>();
            GetDirectoryHierarchy(hierarchy);
            return hierarchy;
        }

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