public void ConfigureServices(IServiceCollection services)
{
            //services.AddMvc(); core 3.0以前是这样写的，这个服务包括了    TageHelper等 WebApi不需要的东西，所有3.0以后可以不这样写
            services.AddControllers();
            
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
}

using Microsoft.AspNetCore.Mvc;
using Routine.Api.Service;
using System;
using System.Threading.Tasks;

namespace Routine.Api.Controllers
{
    [ApiController] //好处：ApiController不是强制的
                    //1.会启用使用属性路由（Attribute Routing）
                    //2.自动HTTP 400响应
                    //3.推断参数的绑定源
                    //4.Multipart/form-data 请求推断
                    //5.错误状态代码的问题详细信息
    [Route("api/companies")]    //写法一
    //[Route("api/[controller]")]   //写法二：意思是相当于刨除了Controller后缀，获取前面的 Companies C可以是小写，如果你改名了那么你路由的uri也跟着变了（不建议这样写）
    public class CompaniesController:ControllerBase
    {
        private readonly ICompanyRepository _companyRepository;
        public CompaniesController(ICompanyRepository companyRepository)
        {
            _companyRepository = companyRepository ?? 
                                 throw new ArgumentNullException(nameof(companyRepository));
        }
        [HttpGet]
        //IActionResult定义了一些合约，它可以代表ActionResult返回的结果
        public async Task<IActionResult> GetCompanies()
        {
            var companies =await _companyRepository.GetCompaniesAsync();//读取出来的是List
            return Ok(companies); 
        }

        [HttpGet("{companyId}")] // Controller标注了ApiController => uri=> api/companies/{companyId}
        public async Task<IActionResult> GetCompany(Guid companyId)
        {
            //判断该公司是否存在方法一：这种方法在处理并发请求时可能会出现错误，原因是查到之后，进行删除，进入company后也可能是404找不到了
            //var exists =await _companyRepository.CompanyExistsAsync(compamyId);
            //if (!exists)
            //{
            //    //不存在应该返回404
            //    return NotFound();
            //}
            var company = await _companyRepository.GetCompanyAsync(companyId);//读取出来的是List
            //方法二
            if (company==null)
            {
                return NotFound();
            }
            return Ok(company);
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Formatters;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Routine.Api.Data;
using Routine.Api.Service;

namespace Routine.Api
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            //services.AddMvc(); core 3.0以前是这样写的，这个服务包括了TageHelper等 WebApi不需要的东西，所有3.0以后可以不这样写
            services.AddControllers(setup =>
            {
                //setup.ReturnHttpNotAcceptable=false;//如果客户端默认为xml格式，服务器端为json，false就不会返回406
                setup.ReturnHttpNotAcceptable = true;//如果请求的类型和服务器请求的类型不一致就返回406
               
                //setup.OutputFormatters.Add(new XmlDataContractSerializerOutputFormatter());
                //setup.OutputFormatters.Insert(0, new XmlDataContractSerializerOutputFormatter());
            }).AddXmlDataContractSerializerFormatters();

            //配置接口服务:涉及到这个服务注册的生命周期这里采用AddScoped，表示每次的Http请求
            services.AddScoped<ICompanyRepository, CompanyRepository>();

            //获取配置文件中的数据库字符串连接
            var sqlConnection = Configuration.GetConnectionString("SqlServerConnection");

            //配置上下文类DbContext，因为它本身也是一套服务
            services.AddDbContext<RoutineDbContext>(options =>
            {
                options.UseSqlServer(sqlConnection);
            });
        }
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}