NP43 Up-casting and Down-casting

Thank you very much for your explanation Alan

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.