NP43 Up-casting and Down-casting

Question

September 23 2007 Hi Guys NP43 Up-casting and Down-casting What is meant by Up-casting and Down-casting in C#? Anyone knows please explain. Thank you

Maha · Answer

Thank you very much for your explanation Alan

Alan · Answer

Upcasting means converting a child class reference into a parent class reference. For example: Child c1 = new Child(); Child c2 = new Child(); Parent p1 = c1; // implicit upcast Parent p2 = (Parent)c2; // explicit upcast In these two cases the compiler knows that it is always safe to cast from a Child class to a parent class because a Child is a Parent - in other words Child contains all the members that Parent contains. Upcasting is no called because you're casting up the object hierarchy (i.e. towards System.Object which sits at the top). Downcasting means converting a parent class reference into a child class reference. For example: Child c1 = new Child(); Parent p1 = c1; Child c2 = (Child)p1; // downcast succeeds at runtime Parent p2 = new Parent(); Child c3 = p2; // attempted downcast fails at compile time Child c4 = (Child)p2; // attempted downcast fails at runtime The only siutation where downcasting is legal is if a Parent variable actually contains a reference to a Child object. The compiler knows that this is a possibility and so allows the two downcasts to c2 and c4. However the c4 cast fails at runtime because the parent variable p2 actually contains a reference to a Parent object not a Child object. A cast from a Parent class to a Child class doesn't work because the Parent class doesn't (or doesn't necessarily) contain all the members that the Child contains. Downcasting is no called because you're casting down the object hierarchy (i.e. away from System.Object). Notice that with reference types casting doesn't actually change the type of the object. It just enables them to be regarded as being of some other type.