.NET: Route messages are not handled polymorphically

[stephentoub]

Registration appears to require the exact message Type, as it's just looked up in a dictionary by that type. I expect that if I have a route handler for IEnumerable<T>, then anything that implements IEnumerable<T> should satisfy it, or if I have a handler for List<T>, List<T> or anything deriving from it should satisfy it, but that is not happening.

Repro:

    [Fact]
    public async Task ReproAsync()
    {
        var workflow = new WorkflowBuilder(new StartExecutor()).Build<IEnumerable<string>>();
        StreamingRun run = await InProcessExecution.StreamAsync<IEnumerable<string>>(workflow, ["Hello, World!"]);
        await run.RunToCompletionAsync();
    }

    private sealed class StartExecutor : Executor
    {
        protected override RouteBuilder ConfigureRoutes(RouteBuilder routeBuilder) =>
            routeBuilder.AddHandler<IEnumerable<string>>(async (msg, ctx) =>
            {
                Debugger.Break();
                Console.WriteLine("In StartExecutor");
                await ctx.SendMessageAsync(msg);
            });
    }

Result:

System.NotSupportedException : No handler found for message type <>z__ReadOnlySingleElementList`1 in executor StartExecutor.

[stephentoub]

Seems like there are a few issues here.

1. MessageRouter.RouteMessageAsync is using message.GetType() rather than using the declared type of the message.
2. The MessageRouter._typedHandlers dictionary is keyed on the single exact type. If that exact type is IEnumerable<T> and the message is of type List<T>, it won't find it in the dictionary, even though logically it should be handleable. This is called out in a comment in MessageRouter.CanHandle: "// For now we only support routing to handlers registered on the exact type (no base type delegation)". For me that's very unexpected.