Rebuild a saved command whose SerializeData() returned an empty string

UndoRedo.Test/SerializationTests.cs

@@ -545,6 +545,29 @@
 		Assert.AreEqual(0, stack.SaveBoundaries[0].Position, "A failed load should keep the existing save boundary position");
 	}
 
+	[TestMethod]
+	public async Task UndoRedoService_SaveLoadState_ReconstructsCommandWithEmptyData()
+	{
+		// Arrange
+		UndoRedoService stack = CreateService();
+		stack.SetSerializer(new JsonUndoRedoSerializer());
+		stack.Execute(new EmptyDataSerializableCommand());
+
+		// Act
+		byte[] data = await stack.SaveStateAsync().ConfigureAwait(false);
+
+		UndoRedoService newStack = CreateService();
+		newStack.SetSerializer(new JsonUndoRedoSerializer());
+		bool success = await newStack.LoadStateAsync(data).ConfigureAwait(false);
+
+		// Assert: SerializeData() returning "" is a normal result for a command with no parameters,
+		// so the command is rebuilt rather than replaced with an un-undoable placeholder
+		Assert.IsTrue(success);
+		Assert.IsInstanceOfType<EmptyDataSerializableCommand>(newStack.Commands[0]);
+		Assert.IsTrue(await newStack.UndoAsync().ConfigureAwait(false));
+		Assert.AreEqual(-1, newStack.CurrentPosition);
+	}
+
 	private const string MalformedAssemblyName = "malformed assembly name";
 	private const string InvalidVersion = "invalid assembly version";
 	private const string NotACommand = "serializable type that is not a command";
@@ -650,6 +673,32 @@
 	}
 #pragma warning restore CA1812
 
+	private sealed class EmptyDataSerializableCommand : BaseCommand, ISerializableCommand
+	{
+		public EmptyDataSerializableCommand() : base(ChangeType.Modify, ["test"])
+		{
+		}
+
+		public override string Description => "Clear all";
+
+		public override void Execute()
+		{
+			// Test implementation
+		}
+
+		public override void Undo()
+		{
+			// Test implementation
+		}
+
+		public string SerializeData() => string.Empty;
+
+		public void DeserializeData(string data)
+		{
+			// Nothing to restore
+		}
+	}
+
 	private sealed class ConstructorOnlySerializableCommand(string value)
 		: BaseCommand(ChangeType.Modify, ["test"]), ISerializableCommand
 	{

UndoRedo/Services/JsonUndoRedoSerializer.cs

@@ -138,7 +138,10 @@ private static ICommand ConvertFromSerializableCommand(SerializableCommand seria
 	{
 		// This is a simplified approach - real implementations would need a factory pattern
 		// or registry to recreate commands from serialized data
-		if (string.IsNullOrEmpty(serializableCommand.Data))
+		// A null Data is what ConvertToSerializableCommand writes for a command that is not an
+		// ISerializableCommand. An empty string is not that marker: it is a normal SerializeData()
+		// result for a command with no parameters, and such a command is reconstructed below.
+		if (serializableCommand.Data is null)
 		{
 			// Return a placeholder command that can't execute but preserves metadata
 			return new PlaceholderCommand(serializableCommand.Description, serializableCommand.NavigationContext, serializableCommand.Metadata);
@@ -171,7 +174,7 @@ private static ICommand ConvertFromSerializableCommand(SerializableCommand seria
 
 			try
 			{
-				instance?.DeserializeData(serializableCommand.Data!);
+				instance?.DeserializeData(serializableCommand.Data);
 			}
 #pragma warning disable CA1031 // Do not catch general exception types
 			catch (Exception ex) when (ex is not OperationCanceledException)