Attribute routing for simple ping controller with Get() not working

Question

I have simple asp.net core web api with a nudge controller that returns true for Get request. The PingController.cs looks as follows:

[Route("api/[Controller]")]
public class PingController : Controller
{
    [HttpGet]
    public IActionResult Get()
    {
        return Ok(true);
    }
}

Why is navigating to the controller (http://localhost:56103/api/Ping) returning 404 ?

I added route on top of the controller and the HttpMethod for specific action. What is it that I am missing or not understanding here ?

When I add app.UseMvcWithDefaultRoute() in Startup.cs the controller works fine. (This is also confusing me.) Startup.cs looks like the following :

public class Startup
{

            public IConfiguration Configuration { get; set; }

            public Startup()
            {
               Configuration = BuildConfiguration();
            }

            // This method gets called by the runtime. Use this method to add services to the container.
            public IServiceProvider ConfigureServices(IServiceCollection services)
            {
                ...
            }

            // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
            public void Configure(IApplicationBuilder app, IHostingEnvironment env)
            {
                if (env.IsDevelopment())
                {
                    app.UseDeveloperExceptionPage();
                }
                else
                {
                    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                    app.UseHsts();
                }
                ConfigureRoutes(app);
            }

            private static void ConfigureMvc(IServiceCollection services, Config config)
            {
                services.AddMvc()
                    .SetCompatibilityVersion(CompatibilityVersion.Version_2_2)// auto generated
                    .AddJsonOptions(options => { options.SerializerSettings.Formatting = Formatting.Indented; })
                    .AddControllersAsServices();

            }

            private static void ConfigureRoutes(IApplicationBuilder app)
            {
                //app.UseMvcWithDefaultRoute();
            }
        }

Answer 1

You need to add UseMvc() or UseMvcWithDefaultRoute() in startup to define routing.

UseMvcWithDefaultRoute adds a default route named 'default' to the request execution pipeline and equals to

app.UseMvc(routes =>
{
    routes.MapRoute(
        name: "default",
        template: "{controller=Home}/{action=Index}/{id?}");
});