EF Core 2.1-无法创建“MyAppContext”类型的对象。在项目中添加“IDesignTimeDbContextFactory<MyAppContext>”的实现,

ASP.Net Core 2.1 具有 EF 2.1 这就是我的上下文类的样子

 public class MyAppContext
    : DbContext
{
    private string _dbConnection = @"Data Source=myServer;Database=SampleApp;uid=sa;password=******";
    public MyAppContext(DbContextOptions<MyAppContext> options)
        : base(options)
    {

    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.HasDefaultSchema("app");
        base.OnModelCreating(modelBuilder);
    }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer(_dbConnection);
        base.OnConfiguring(optionsBuilder);
    }

    public DbSet<User> Users { get; set; }
    public DbSet<Phone> Phones { get; set; }
}

 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.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseMvc();
    }
}

程序.cs

 public class Program
{

    public static void Main(string[] args)
    {
        BuildWebHost(args).Build().Run();
    }

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

}}

我检查了以下链接:-

• EF Core Migration error: "Unable to create an object of type 'ApplicationContext'"
• EF Core 2.1 project Add-Migration NullReferenceException

&其他的很少。

如何修复此错误??

谢谢。