如何在asp.net-core-2.0中向项目中添加'IDesignTimeDbContextFactory<DataContext>‘的实现

Question

下面是我安装的包的列表：Installed Packages

我使用的是Entityframework核心2.0。我第一次使用实体框架代码优先迁移(add-migration和update-database命令)成功地创建了数据库。现在，当我更新我的实体并尝试运行迁移时，它给出了以下错误。

无法创建类型为“DataContext”的对象。将“IDesignTimeDbContextFactory”的实现添加到项目中，或查看https://go.microsoft.com/fwlink/?linkid=851728以了解设计时支持的其他模式。

这是我的代码。

Program.cs

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

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

Startup.cs

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)
{
    // Repositories

    services.AddMvc();
    services.AddDbContextPool<DataContext>(
        options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
        //options => options.UseSqlServer(@"Server=LAPTOP-518D8067;Database=Gyanstack;Trusted_Connection=True;MultipleActiveResultSets=true"));

    services.AddCors();
    services.AddScoped<ISectionRepository, SectionRepository>();
    services.AddScoped(typeof(IEntityBaseRepository<>), typeof(EntityBaseRepository<>));
}

DataContext.cs

public class DataContext : DbContext
{
    public DataContext(DbContextOptions<DataContext> options) 
        : base(options)
    { }

    public DbSet<Section> Section { get; set; }
    public DbSet<SubSection> SubSection { get; set; }
    public DbSet<Article> Article { get; set; }
    public DbSet<Comment> Comment { get; set; }
    public DbSet<User> User { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.AddConfiguration(new SectionMap());
        modelBuilder.AddConfiguration(new SubSectionMap());
        modelBuilder.AddConfiguration(new ArticleMap());
        modelBuilder.AddConfiguration(new CommentMap());
    }
}

Answer 1

编辑:现在已过时。Microsoft updated their migration document在9月底向您展示了如何更新，因此您不需要此解决方法。

如此issue I raised on github所示，您将DB初始化代码移动到Program main，将其放在BuildWebHost()和.Run()之间。

一旦您理解了需要在Main中使用var context = services.GetRequiredService<MyContext>();获取DB上下文，这就相对容易了，然后一切都会按预期工作。(虽然我仍然认为DB初始化是一次性初始化的事情，而不是每个程序运行的事情)

Answer 2

简单地更改了我的Program.cs

从这里开始

public class Program
{
  public static void Main(string[] args)
  {
    var host = new WebHostBuilder()
      .UseKestrel()
      .UseContentRoot(Directory.GetCurrentDirectory())
      .UseIISIntegration()
      .UseStartup<Startup>()
      .Build();

    host.Run();
  }
}

到这个

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

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

来源：https://wildermuth.com/2017/07/06/Program-cs-in-ASP-NET-Core-2-0

Answer 3

在2.0项目中，将SeedData.Initialize调用移动到Program.cs的Main方法

var host = BuildWebHost(args);

using (var scope = host.Services.CreateScope())
{
    var services = scope.ServiceProvider;

    try
    {
        SeedData.Initialize(services, "").Wait();
    }
    catch (Exception ex)
    {
        var logger = services.GetRequiredService<ILogger<Program>>();
        logger.LogError(ex, "An error occurred seeding the DB.");
    }
}

host.Run();   

参考：https://docs.microsoft.com/en-us/aspnet/core/migration/1x-to-2x/#move-database-initialization-code