已解析的数据库上下文产生null DbSet

我的项目中有3个组件:MyApp.Core, MyApp.Infrastructure, MyApp.Web。

MyApp.Core包含数据库模型/实体，MyApp.Infrastructure包含数据库上下文和迁移，MyApp.Web包含Startup类。

数据库是使用代码首先创建的EF，并且它存在(我检查过)。

我已经注册了数据库上下文在我的Startup类的ConfigureServices方法:

    public void ConfigureServices(IServiceCollection services)
    {
        // Add framework services.
        services.AddDbContext<MyAppDbContext>(options =>
            options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
        services.AddIdentity<Employee, IdentityRole>()
            .AddEntityFrameworkStores<KinderGardenDbContext>()
            .AddDefaultTokenProviders();
        ...
    }

我想用我的静态方法MyApp.Infrastructure.MyAppContextSeed.Seed()播种我的数据库。所以我在Configure方法的末尾添加了这个方法:

...
   MyAppDbContextSeed.Seed(app.ApplicationServices).Wait();
}

在上面的方法中，我解析了数据库上下文:

public static async Task Seed(IServiceProvider services)
{
    ...
    using (var context = services.GetRequiredService<MyAppDbContext>())
    using (var roleManager = services.GetRequiredService<RoleManager<IdentityRole>>())
    using (var userManager = services.GetRequiredService<UserManager<Employee>>())
    {
        if (!await context.Companies.AnyAsync())
        {
            ...
        }
    }
}

在上面的if语句中，Companies为空

还有，这是我的数据库上下文类:

namespace MyApp.Infrastructure.EntityFramework
{
    public class MyAppDbContext : IdentityDbContext<Employee>
    {
        public DbSet<Company> Companies;
        public DbSet<Person> Persons;
        public DbSet<Employee> Employees;
        public DbSet<KeyCard> KeyCards;
        public DbSet<KeyRequestForm> KeyRequestForms;
        public KinderGardenDbContext(DbContextOptions<KinderGardenDbContext> options) : base(options)
        {
        }
        protected override void OnModelCreating(ModelBuilder builder)
        {
            base.OnModelCreating(builder);
            builder.Entity<Company>().ToTable("Company");
            builder.Entity<Person>().ToTable("Person");
            builder.Entity<Employee>().ToTable("Employee");
            builder.Entity<KeyCard>().ToTable("KeyCard");
            builder.Entity<KeyRequestForm>().ToTable("KeyRequestForm");
            builder.Entity<KeyCardKeyRequestForm>().ToTable("KeyCardKeyRequestForm");
            builder.Entity<KeyCardKeyRequestForm>()
                .HasKey(c => new { c.KeyCardId, c.KeyRequestFormId });
        }
    }
}