代码在Visual Studio中编译和运行,即使它具有未引用的类的名称

我正在测试一个ASP.NET核心web应用程序,它不应该编译或运行,但它可以同时编译或运行。问题 应该是 我用的是一个类名-- ActionResult --来自未引用的程序集。注意如何 与已解析的类名不同,它不是蓝色的,请注意,没有带下划线的编译器错误:

此外,该项目的编译和运行也很好(尽管其行为并不像预期的那样自然,也就是用上面所示的值响应对该URL的HTTP GET请求)。发生了什么事?

Program.cs:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Logging;

namespace WebApplication2
{
    public class Program
    {
        public static void Main(string[] args)
        {
            CreateWebHostBuilder(args).Build().Run();
        }

        public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .UseStartup<Startup>();
    }
}

Startup.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;

namespace WebApplication2
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "api/{Default}");
            });
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

namespace WebApplication2.wwwroot
{
    [Route("api/[controller]")]
    [ApiController]
    public class DefaultController : ControllerBase
    {
        // GET: api/Default
        [HttpGet]
        public async Task<ActionResult<IEnumerable<string>>> Get()
        {
            return Task.FromResult(new string[] { "value1", "value2" });
        }
    }
}

微软Visual Studio专业版2017
版本15.9.4
VisualStudio.15.Release/15.9.4+28307.222
微软.NETCore2.1