stride3d/stride · error · ArgumentException

Cannot add the asset

Error message

Cannot add the asset [{0}] [{1}] to package [{2}] [{3}]: it is already in package [{4}] [{5}] in the current session

What it means

Within a Stride editing session, each asset id must exist in exactly one package. CheckCanAdd performs a session-wide scan: if another package in Package.Session.Packages already contains the id, the add is rejected with a message naming both packages, keeping session state consistent for references and undo.

Solutions

Assign a new id to the duplicated asset: item.Id = AssetId.New() before adding.
Remove the asset from the other package first if the intent is a move between packages.
Search the session for the existing owner (otherPackage.Assets[id]) and update it there instead of adding a copy.
Run the copy/move through session-level package operations rather than raw per-package Add.

Example fix

// before
packageB.Assets.Add(assetItem); // assetItem.Id still owned by packageA

// after
var copy = new AssetItem(assetItem.Location, (Asset)assetItem.Asset.Clone()) { Id = AssetId.New() };
packageB.Assets.Add(copy);

Defensive patterns

Strategy: validation

Validate before calling

foreach (var p in package.Session.Packages)
    if (p != package && p.Assets.ContainsById(item.Id))
        item.Id = AssetId.New();
package.Assets.Add(item);

Type guard

bool IdFreeInSession(Package target, AssetId id) =>
    target.Session?.Packages.All(p => p == target || !p.Assets.ContainsById(id)) ?? true;

Try / catch

try { package.Assets.Add(item); }
catch (ArgumentException e) when (e.Message.Contains("already in package"))
{ item.Id = AssetId.New(); package.Assets.Add(item); }

Prevention

Always regenerate ids when duplicating assets across packages
Perform cross-package moves through session-level operations
Run ContainsById checks across the whole session in copy tooling

When it happens

Trigger: Calling package.Assets.Add(item) while Package.Session is non-null and some other package in the session already contains an asset with item.Id (found via ContainsById). The throw uses the constructed formatted message without an paramName argument.

Common situations: Duplicating an asset into another package without regenerating its id; loading the same asset file into two packages of one session; scripted tools that copy assets between packages and forget new ids.

Understand the failure class

Background: "already exists" / EEXIST / FileAlreadyExistsException: what the 'file already exists' error means and how to fix it — this error's family across 37 libraries.

Related errors

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/9d7036a90b30e950. Report an issue: GitHub.

Appendix: source

Thrown at sources/assets/Stride.Core.Assets/PackageAssetCollection.cs:382

        {
            throw new ArgumentException("Asset location [{0}] cannot contain drive information".ToFormat(location), nameof(item));
        }

        if (location.GetDirectory()?.StartsWith("..", StringComparison.Ordinal) == true)
        {
            throw new ArgumentException("Asset location [{0}] cannot start with relative '..'".ToFormat(location), nameof(item));
        }

        // Double check that this asset is not already stored in another package for this session
        if (Package.Session != null)
        {
            foreach (var otherPackage in Package.Session.Packages)
            {
                if (otherPackage != Package)
                {
                    if (otherPackage.Assets.ContainsById(item.Id))
                    {
                        throw new ArgumentException("Cannot add the asset [{0}] [{1}] to package [{2}] [{3}]: it is already in package [{4}] [{5}] in the current session".ToFormat(item.Id, item.Location, Package.Meta.Name, Package.FullPath, otherPackage.Meta.Name, otherPackage.FullPath));
                    }
                }
            }
        }
    }

    public IEnumerator<AssetItem> GetEnumerator()
    {
        return registeredItems.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    void ICollection.CopyTo(Array array, int index)
    {

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