A minimal fully asynchronous C# ASP.NET Core 3.0 application with routing (learn more about ASP.NET Core here https://docs.microsoft.com/en-us/aspnet/core/?view=aspnetcore-3.0)

Program.cs

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

public class Program
{
    public static void Main(string[] args) =>
        Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.Configure(app =>
            {
                app.UseRouting();
                app.UseEndpoints(route =>
                {
                    route.MapGet("/", context => context.Response.WriteAsync("Hello world"));
                });
            });
        })
        .Build().Run();
}

WebApp.csproj

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.0</TargetFramework>
  </PropertyGroup>
</Project>