After a save/load round trip, ChangeMetadata.CustomData values come back as JsonElement, so casting them to their original types throws InvalidCastException

[matt-edmondson]

What's wrong

#77 made a reloaded command keep its ChangeMetadata, including CustomData. The dictionary is typed IReadOnlyDictionary<string, object> (UndoRedo/Models/ChangeMetadata.cs:18), though, so System.Text.Json deserializes every value as a System.Text.Json.JsonElement. Values the app stored as int, string, DateTime and so on are no longer those types after LoadStateAsync. The keys survive, but every value changes type.

The regression test added for #77 (SerializationTests.cs:171) compares CustomData["author"].ToString(). That hides the problem, because JsonElement.ToString() happens to return the raw string.

Repro

var s = new UndoRedoService(new StackManager(), new SaveBoundaryManager(), new CommandMerger());
s.SetSerializer(new JsonUndoRedoSerializer());
s.Execute(new DelegateCommand("a", () => { }, () => { },
    customData: new Dictionary<string, object> { ["line"] = 42, ["name"] = "x" }));

await s.LoadStateAsync(await s.SaveStateAsync());

foreach (var kv in s.Commands[0].Metadata.CustomData!)
    Console.WriteLine($"{kv.Key}: {kv.Value.GetType().FullName} = {kv.Value}");
int line = (int)s.Commands[0].Metadata.CustomData["line"];

Observed:

line: System.Text.Json.JsonElement = 42
name: System.Text.Json.JsonElement = x
Unhandled: InvalidCastException: Unable to cast object of type 'System.Text.Json.JsonElement' to type 'System.Int32'.

Why it matters

CustomData is where apps keep things like a caret line or a selection id, which they read back to drive navigation or history UI. Code that casts or pattern-matches those values (is int line) works before a save and load, then throws or silently stops matching afterwards. Because the type depends on whether the history was reloaded, this is easy to miss in testing.

Suggested fix / acceptance criteria

• On deserialize, convert each JsonElement in CustomData back into a plain CLR value, for example with a custom converter on ChangeMetadata.CustomData:
  • string → string
  • number → long, or double when it isn't a whole number
  • true/false → bool
  • array → List<object>
  • object → Dictionary<string, object>
• Alternatively, store each value's type name next to it and deserialize to that type.
• Document which types survive a round trip, for example that an int comes back as a long.
• Change the SaveState/LoadState discards command metadata: timestamps reset to load time, ChangeSize to 1, CustomData lost #77 regression test to check each value's type and value, not its ToString(), and add a numeric entry.

[matt-edmondson]

Triage

• Category: Bug (type fidelity lost on round trip)
• Priority: Medium. CustomData values change type only after a save/load, so code that casts or pattern-matches them (is int line) breaks only on reloaded history, which makes the bug easy to miss. The SaveState/LoadState discards command metadata: timestamps reset to load time, ChangeSize to 1, CustomData lost #77 regression test masks it by comparing ToString().
• Area / suggested assignment: ChangeMetadata.CustomData (UndoRedo/Models/ChangeMetadata.cs) and JsonUndoRedoSerializer. A JsonElement→CLR converter is the lowest-friction option.
• Duplicates: none. It is a follow-up to SaveState/LoadState discards command metadata: timestamps reset to load time, ChangeSize to 1, CustomData lost #77.
• In progress: no PR fixes this. Open PR Keep a trimmed stack dirty at -1 through save, load, and restore [major] #122 changes JsonUndoRedoSerializer and the serialized state shape (a major bump), so land this after it or coordinate with it to avoid conflicts.

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