ASP.Net - 添加包含其他对象但包含的对象的新对象变为null

I am trying to add a Course object to my database and in my form I have 3 other objects that can be contained in the Course object. In my form, the other 3 objects show but when it returns to the controller, the 3 object values are null.I am trying to add a Course object to my databa