namespace Core
{
    public class Startup
    {
        private readonly IConfiguration _config;

        public Startup(IConfiguration config)
        {
            _config = config;
        }

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

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app)
        {
            app.ConfigureApi(_config);
        }
    }
}

namespace Api.Extensions
{
    public static class ApiStartupExt
    {
        public static void AddApiServices(this IServiceCollection builder, IConfiguration config)
        {
        builder.AddControllers().AddApplicationPart(typeof(ApiStartupExt).Assembly);
            builder.AddSwaggerGen(c => c.SwaggerDoc("v0", new OpenApiInfo
            {
                Title = "test_service",
                Version = "v0"
            }));
        }
        public static void ConfigureApi(this IApplicationBuilder app, IConfiguration config)
        {
            if (!config["Environment"]!.Equals("Production"))
            {
                app.UseSwagger();
                app.UseSwaggerUI(c =>
                {
                    c.SwaggerEndpoint("/swagger/v1/swagger.json", "test_service v0");
                    c.RoutePrefix = string.Empty;
                });
            }

            // Map rest services to port 43000
            // In my actual application I also have GRPC endpoints on the same host
            // so I need to use the UseWhen method to avoid some issues there
            app.UseWhen(context => context.Connection.LocalPort == config.GetValue<int>("Rest:Port"),
              builder =>
              {
                  builder.UsePathBase(new PathString("/api"));
                  builder.UseRouting();
                  builder.UseEndpoints(endpoints => endpoints.MapControllers());
              }
            );
        }
    }
}

info: Microsoft.AspNetCore.Hosting.Diagnostics[1]
      Request starting HTTP/1.1 POST http://localhost:43000/api/WeatherForecast - 0
dbug: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
      Wildcard detected, all requests with hosts will be allowed.
dbug: Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware[1]
      POST requests are not supported
dbug: Microsoft.AspNetCore.Server.Kestrel.Connections[9]
      Connection id "0HMNNSN1PNNGU" completed keep alive response.
info: Microsoft.AspNetCore.Hosting.Diagnostics[2]
      Request finished HTTP/1.1 POST http://localhost:43000/api/WeatherForecast - 0 - 404 0 - 11.6784ms

   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNet.Mvc" Version="5.2.9" />
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
   </ItemGroup>