var builder = WebApplication.CreateBuilder(args); builder.Services.AddSingleton<TraceState>(); var app = builder.Build();
if (!app.Environment.IsDevelopment()) throw new InvalidOperationException("This trace observer is development-only");
app.Use(async (context, next) => {
    if (context.Request.Path != "/order") { await next(context); return; }
    var trace = new List<string> { "outer-in" }; context.Items["trace"] = trace;
    await next(context); trace.Add("outer-out"); context.RequestServices.GetRequiredService<TraceState>().Last = string.Join(">", trace);
});
app.Use(async (context, next) => {
    if (context.Request.Path != "/order") { await next(context); return; }
    var trace = (List<string>)context.Items["trace"]!; trace.Add("inner-in"); await next(context); trace.Add("inner-out");
});
app.MapGet("/order", (HttpContext context) => { ((List<string>)context.Items["trace"]!).Add("endpoint"); return TypedResults.Text("handled"); });
app.MapGet("/trace", (TraceState state) => TypedResults.Text(state.Last));
app.Run();
public sealed class TraceState { public string Last { get; set; } = "none"; }