Question

Can I Cast IList into ArrayList?

if yes what should I do?

IList alls = RetrieveCourseStudents(cf);
ArrayList a = (ArrayList)alls;

Is that correct?

is has error:

Unable to cast object of type

Answer 1

This is all about polymorphism.
ArrayList is an implementation from the Interface IList.

 IList iList = new ArrayList();  

The static type from the variable iList is IList but it references an ArrayList Object!

There is no real casting from IList to ArrayList because you can not instantiate/create Object from Interface or abstract Class.