asp.net: UseRouting vs UseEndPoints

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

// builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle

var app = builder.Build();

// app.Use((context, next) =>
// {
//     context.Response.WriteAsync("Hello from nextflix");
// });

// terminal middleware
// https://learn.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-7.0
// app.Run(async context =>
// {
//     await context.Response.WriteAsync("Hello from netflix");
// });


// case #1
// app.MapGet("/prefix", async context =>
// {
//     var header = context.Request.Headers;
//     var customHeader = header["CustomHeader"].ToString();
//     var mullVad = header["VPN"].ToString();
//     await context.Response.WriteAsync($"Welcome to netflix : {customHeader}\r\n");
//     await context.Response.WriteAsync($"VNP client : {mullVad}");
// });

app.MapGet("/", async context =>
{
    var headers = context.Request.Headers;
    var header = context.Request.Headers["CustomHeader"].ToString();
    await context.Response.WriteAsync($"Welcome to netflix: {header}");
});

/// ===============================
// required because of UseEndpoints
// case #2
app.UseRouting(); // this is the endpoint finder (will look for the best match in the list of registered endpoints)
// use .UseEndpoints requires adding app.UseRouting()
app.UseEndpoints(routeBuilder =>
{
    // registering the endpoints to the list
    routeBuilder.MapGet("/prefix", async context =>
    {
        await context.Response.WriteAsync("hello");
    });
});
// NOTE: If this code is enabled then the it will run before the terminal-middleware
// if it's commented and the case #1 is uncommented, then the terminal middleware will run before the .MapGet()
// the reason is that since there is no app.UseRouting and app.UseEndpoint() before app.Run(async context) aka terminal-middleware
// the case #1 endpoint won't be called.

// # Definition
// app.UseRouting: Adds a EndpointRoutingMiddleware middleware to the specified IApplicationBuilder.
// A call to UseRouting(this IApplicationBuilder) must be followed by a call to UseEndpoints(this IApplicationBuilder, Action<IEndpointRouteBuilder>) for the same IApplicationBuilder instance.

// app.UseRouting: Adds a EndpointMiddleware middleware to the specified IApplicationBuilder with the
// EndpointDataSource instances built from configured IEndpointRouteBuilder. The EndpointMiddleware will execute the Endpoint associated with the current request.


// ===============================

// terminal middleware
// Run delegates don't receive a next parameter. The first Run delegate is always terminal and terminates the pipeline.
// Run is a convention. Some middleware components may expose Run[Middleware] methods that run at the end of the pipeline
// https://learn.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-7.0#create-a-middleware-pipeline-with-webapplication
app.Run(async context =>
{
    await context.Response.WriteAsync("Hello");
});
// NOTE: If the terminal-middleware is enable, your endpoint above registered with "MapGet" (case #1) won't run
// since this is the only middleware and also the terminal middleware when the request first come in it will just write and return.
// BUT if the case #2 is called before this terminal-middleware (which requires calling app.UseRouting and also app.UseEndpoints)
// then this middleware will run after out registered endpoint runs!

// START THE APP (NOT SAME AS TERMINAL MIDDLEWARE)
app.Run();


// IMPORTANT: THE MIDDLE-WARE ARE REGISTERED IN ORDER YOU CALL app.Use<....>
// e.g:
// app.UseLogging();
// app.UseDecorator();

// app.Run() (start the web-server) => listen for incoming http request
// when arrives calls => UseLogging => UseDecorator (if there are  no more middlewares return)