实体框架中的一对多关系导致异常

为了向自己介绍实体框架，我创建了一个与之协同工作的控制台应用程序。

有两个实体：ClassA和ClassB。他们之间有一种一对多的关系。一个ClassA实例可以有多个ClassB实例。一个ClassB实例有0个或1个ClassA实例。

类的代码是：

public class ClassA {
    public virtual int Id {get; set;}
    public virtual string Name {get; set;}
    public virtual ICollection<ClassB> ClassBs {get; set;}
}
public class ClassB {
    public virtual int Id {get; set;}
    public virtual string Name {get; set;}
    public virtual ClassA ClassA {get; set;}
}

我的数据库上下文和初始化器的代码是：

public class Context : DbContext {
    public DbSet<ClassA> ClassAs {get; set;}
    public DbSet<ClassB> ClassBs {get; set;}
    protected override void OnModelCreating(DbModelBuilder modelBuilder) {
        modelBuilder.Entity<ClassA>()
            .HasMany(classA => classA.ClassBs)
            .WithRequired(classB => classB.ClassA);
        base.OnModelCreating(modelBuilder);
    }
}
public class Initializer : DropCreateDatabaseAlways<Context> {
    protected override void Seed(Context context) {
        base.Seed(context);
        for (int i = 1; i <= 3; i++) {
            var classA = new ClassA{
                Name = "A-" + i,
                ClassBs = new LinkedList<ClassB>()
            };
            for (int j = 1; j <= 3; j++) {
                var classB = new ClassB{
                    Name = "B-" + i + "-" + j,
                    ClassA = classA
                };
                classA.ClassBs.Add(classB);
                context.ClassBs.Add(classB);
            }
            context.ClassAs.Add(classA);
        }
        context.SaveChanges();
    }
}

我的主要方法是：

    static void Main(string[] args) {
        Database.SetInitializer(new Initializer());
        Context db = new Context();
        foreach (var classA in db.ClassAs) {
            Console.WriteLine(classA.Name);
            foreach (var classB in classA.ClassBs)
                Console.WriteLine("'t" + classB.Name);
        }
        Console.Write("'nFIN");
        Console.ReadKey();
    }

当Main方法中的第二个foreach循环启动时，会引发以下异常：

System.Data.EntityCommandExecutionException was unhandled
  Message=An error occurred while executing the command definition. See the inner exception for details.
  Source=System.Data.Entity
  InnerException: System.InvalidOperationException
       Message=There is already an open DataReader associated with this Command which must be closed first.
       Source=System.Data
       StackTrace:
            at System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand command)
            at System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String method, SqlCommand command)
            at System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async)
            at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
            at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
            at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
            at System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
            at System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior)
            at System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior)

似乎ClassA中的ClassBs属性加载不正确。然而，我确实在上下文中指定了ClassA有许多ClassB。

数据库连接似乎很好，因为ClassA的第一个实例显示在控制台上。我还可以访问VisualStudio中的数据库，并查看初始化器中生成的数据。

我使用了ASP.NET MVC的培训视频来开始使用这个控制台应用程序。

你知道出了什么问题吗？