受保护的内部类在班级内部工作，但不在外部工作

我尝试了一些事情，想知道为什么会这样.

I was trying few things and would like to know why this is happening so.

说，我在命名空间n中有一个名为A的类，并且我试图创建受保护的内部类B.

Say, I have a class called A in namespace n and I was trying to create protected internal class B.

namespace n
{
   public class A
   {
      public A()
      {
      }
   }
   protected internal class B //throwing error
   {
   }
}

但是当我这样尝试时(B作为A的子类)，它没有抛出错误，并且构建成功.你能解释一下为什么会这样吗?

But when i try like this (B as a sub class of A), its not throwing error and its built success. Could you explain me why it is so?

namespace n
{
   public class A
   {
      public A()
      {
      }
      protected internal class B // its not throwing error
      {
      }
   }      
}

理论上我有什么遗漏吗?相当混乱.

Am i missing anything theoretically? Its quite a bit confusing.