core api应用实例
，采用` core 3.1 + EF+mysql`
初始化项目`dotnet new webapi`，基于 `ASP.NET Core web API` 类型的项目，初始化文件如下
```C#
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Threading.Tasks;
    using Microsoft.AspNetCore.Builder;
    using Microsoft.AspNetCore.Hosting;
    using Microsoft.AspNetCore.HttpsPolicy;
    using Microsoft.AspNetCore.Mvc;
    using Microsoft.Extensions.Configuration;
    using Microsoft.Extensions.DependencyInjection;
    using Microsoft.Extensions.Hosting;
    using Microsoft.Extensions.Logging;
    namespace netCoreApiDemo
    {
        public class Startup
        {
            public Startup(IConfiguration configuration)
            {
                Configuration = configuration;
            }
            public IConfiguration Configuration { get; }
            // This method gets called by the runtime. Use this method to add services to the container.
            public void ConfigureServices(IServiceCollection services)
            {
                services.AddControllers();
            }
            // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
            public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
            {
                if (env.IsDevelopment())
                {
                    app.UseDeveloperExceptionPage();
                }
                app.UseHttpsRedirection();
                app.UseRouting();
                app.UseAuthorization();
                app.UseEndpoints(endpoints =>
                {
                    endpoints.MapControllers();
                });
            }
        }
    }
```
安装mysql相应的依赖
`dotnet add package Pomelo.EntityFrameworkCore.MySql`
安装完成之后编写`DBContext`
```C#
    using Microsoft.EntityFrameworkCore;
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Threading.Tasks;
    namespace netCoreApiDemo.Models
    {
        public class AppContext:DbContext
        {
            public AppContext(DbContextOptions<AppContext> options)
                :base(options)
            {
            }
        }
    }
```
配置`connectionString`
在`appsettings.json`文件中加入配置
`"ConnectionStrings": {
      "DefaultConnection": MysqlConnStringmysql下载app
  }`
在`startup.cs`文件的`ConfigureServices`方法中加入ConnectionString的配置。
```C#
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<AppContext>(options =>

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