How to cast all child elements in an unlimited hierachical data ?

The question is what is the type of subjects?

is it an array or a list, or something else

you may use 'as' to cast back to the type it really is.

for example

Subject[] subjects=theObject.Subjects as Subject[]

You need to have the knowledge what type the original subjects is and what data you stored there before you can convert it to ur own class by a constructor

class MySubject
{
public MySubject(OtherSubject subject)
{
 this.Title=subject.Title;
 this.Code=subject.Id;
}
}