gum-project-versioning

name	gum-project-versioning
description	Gum's .gumx schema versioning and migration strategy. Triggers: shape changes to GumProjectSave, ElementSave, or any serialized save class — version bumps, backward-compat shims, XmlSerializer-aware properties that must round-trip across tool versions.

Gum Project Versioning Reference

The GumxVersions enum

Located in GumDataTypes/GumProjectSave.cs. The enum gains one entry per breaking schema change; the earliest entries cover the verbose→compact XML shift, later ones reserve slots for new variable surfaces. Read the enum itself for the current list and each entry's doc comment — don't rely on a copy here.

NativeVersion is the highest version this tool build supports / writes.

Ctor default vs. new-project factories — they intentionally differ

The GumProjectSave constructor does NOT default to NativeVersion. It defaults to an older version on purpose: the ctor default is the fallback for deserialized files that lack a Version element, so a legacy file reads back as the older version and the variable-grid version gate keeps hiding newer-only variables on it.

A brand-new project is the opposite case — it seeds the latest standard-element variable surface up front, so it is honestly at NativeVersion. The new-project factories (ProjectManager.CreateNewProject for the tool, ProjectCreator.Create for headless/CLI) therefore stamp Version = NativeVersion explicitly in their initializers rather than relying on the ctor default. When adding a new way to create a project, set the version there too — and bump any verbatim-copied template .gumx (e.g. the Forms/theme templates) only if its bundled standards actually contain the newer variable surface.

The three load-time behaviors

ProjectManager.LoadProject (~line 201 and ~288 per recent research) compares the file's Version against NativeVersion:

1. File version > NativeVersion → dialog: "saved with a newer version of Gum... Please update Gum." File is rejected.
2. File version < AttributeVersion → Output-tab warning with an upgrade docs link (https://docs.flatredball.com/gum/gum-tool/upgrading/upgrading-file-gumx-version). File still opens and works.
3. File version == NativeVersion → normal load.

Save doesn't auto-bump

This is unusual. Most authoring tools silently upgrade on save. Gum does not — a file loaded as v1 stays v1 after the user re-saves it. Upgrading is a user-initiated action following the docs link. Don't "fix" this without discussion.

XmlSerializer tolerance as the migration mechanism

There is no explicit migration pass for v1 → v2. Instead:

• GumFileSerializer (in GumDataTypesNet6/) builds separate serializers (GetCompactSerializer, GetLegacyInstancesCompactSerializer, GetGumProjectCompactSerializer) using XmlAttributeOverrides to handle the attribute-vs-element shape differences.
• Unknown XML elements are silently ignored by XmlSerializer, so adding a new element to a serialized class is inherently forward-compatible: old tools just skip it.

This means most schema changes don't require a version bump.

Bump-vs-don't-bump decision framework

• Adding a new optional field → don't bump. Old tools ignore it.
• Adding a new field while replacing/renaming an old one → don't bump, if you keep the old field as a backward-compat shim (an always-serialized view over the new one so old tools still read something sensible).
• Removing a field with no shim, or making a breaking semantic change → bump. Accept the forward-compat barrier (old tools refuse the file).
• Bundle multiple schema changes into one bump when possible.

The LocalizationFile → LocalizationFiles change in issue #2512 used the no-bump route: new List<string> property plus the legacy string property kept as a shim reading/writing index 0.

Writing backward-compat shims — XmlSerializer gotchas

If you add a new property and keep the old one as a shim, know these:

1. [Obsolete] alone silently drops the property from XML output. XmlSerializer skips [Obsolete] members. If you need a shim to keep serializing, use [EditorBrowsable(EditorBrowsableState.Never)] plus an explicit [XmlElement("OldName")] attribute instead of [Obsolete].

2. List<T> appends; T[] replaces. On deserialize, XmlSerializer calls Add on a List<T> property once per child element — so if the old element appears before the new one, both contribute and you end up with duplicates. Arrays are assigned via the setter, which replaces. If your deserialized shape might contain both old and new elements, serialize as T[] (with an [XmlArray] attribute) and expose a [XmlIgnore] List<T> that syncs.

3. Declaration order affects which setter wins when both old and new elements are present in XML. XmlSerializer applies properties in document order. If your shim and new property both set the same underlying state, putting the legacy property first (so the new property's setter fires last) is the intended round-trip shape.

4. Always emit the legacy element when the new one is populated. That way, an older tool opening a new file still reads something — typically the first entry of a list.

Related skills

• gum-tool-save-classes — broader overview of the save/load data model, ShouldSerialize*, [XmlIgnore], two-phase loading.
• gum-localization — localization-specific file loading, the place where LocalizationFile(s) is consumed.
• gum-tool-codegen — .codsj files have their own Version field (separate from .gumx versioning), migrated in CodeOutputProjectSettingsManager.MigrateIfNeeded on load. Don't auto-save — let the next user save persist the new version. Each version bump needs a comment explaining what it does and why.