Program.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Hosting;

namespace BasicWebSite
{
    public class Program
    {
        public static void Main(string[] args) => CreateHostBuilder(args).Build().Run();

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder => 
                    webBuilder.UseStartup<Startup>());
    }

    public class Startup
    {
        public void Configure(IApplicationBuilder app)
        {
            app.Use(async (context, next) =>
            {
                if (context.Request.Path == "/")
                {
                    await context.Response.WriteAsync("Hello World!");
                    return;
                }

                await next();
            });
        }
    }
}