Unity-Technologies/UnityCsReference · error · ArgumentException

Folder relative path cannot start with a slash.

Error message

Folder relative path cannot start with a slash.

What it means

Thrown by LibraryFolderPathAttribute constructor when relativePath starts with a forward slash. The attribute prefixes the path with 'Library/' internally, so a leading slash would create a malformed path (e.g., 'Library//foo'). An empty or null path is handled separately via Debug.LogError (not an exception), but a leading slash is a hard error.

Source

Thrown at Editor/Mono/ScriptableSingletonDictionary.cs:31

{
    // Use the LibraryFolderPathAttribute when you want to have your scriptable singleton dictionary
    // to persist between unity sessions.
    // Example: [LibraryFolderPathAttribute("EditorWindows")]
    [AttributeUsage(AttributeTargets.Class)]
    class LibraryFolderPathAttribute : Attribute
    {
        public string folderPath { get; set; }
        public LibraryFolderPathAttribute(string relativePath)
        {
            if (string.IsNullOrEmpty(relativePath))
            {
                Debug.LogError("Invalid relative path! (its null or empty)");
                return;
            }

            // We do not want a slash as first char.
            if (relativePath[0] == '/')
                throw new ArgumentException("Folder relative path cannot start with a slash.");

            folderPath = "Library/" + relativePath;
        }
    }

    internal abstract partial class ScriptableSingletonDictionary<TDerived, TValue> : ScriptableObject
        where TDerived : ScriptableObject
        where TValue : ScriptableObject
    {
        [AutoStaticsCleanupOnCodeReload]
        private static TDerived s_Instance;
        static readonly string k_Extension = ".pref";

        protected string m_PreferencesFileName;

        public static TDerived instance
        {
            get

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Remove the leading slash from the relative path.
2. Use a relative path without a leading separator, e.g., [LibraryFolderPath("MyFolder")].
3. Trim leading slashes if the path is dynamically generated: path.TrimStart('/').

Example fix

// before
[LibraryFolderPath("/MyFolder")]
internal class MyData : ScriptableSingletonDictionary<MyData, MyType> { }

// after
[LibraryFolderPath("MyFolder")]
internal class MyData : ScriptableSingletonDictionary<MyData, MyType> { }

// Compile-time: ensure the path does not start with '/'.
// [LibraryFolderPath("MyFolder")]

When it happens

Trigger: Declaring [LibraryFolderPath("/MyFolder")] on a ScriptableSingletonDictionary subclass. Path constructed by concatenating a separator at the front.

Common situations: Copy-paste of absolute-style paths into the attribute. Convention mismatch where developers assume a leading slash is needed like in Unix paths.

Related errors

• Invalid relative path (it is empty)
• Incorrect file extension: {prefabAssetPath}. Must be '.prefa
• The LibraryFolderPathAttribute[] attribute is required for t
• Attempting to set an incorrect number of icons for {namedBui
• The setting label can't be null or empty.

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/c3c4e0087758057b. Report an issue: GitHub.