Casting and Reference Conversions – Differentiate between Upcasting and Downcasting

An object reference can be:

• Implicitly upcast to a base class reference
• Explicitly downcast to a subclass reference

Upcasting and downcasting between compatible reference types performs reference conversions: a new reference is (logically) created that points to the same object. 

An upcast always succeeds; a downcast succeeds only if the object is suitably typed.

Upcasting

An upcast operation creates a base class reference from a subclass reference. For example:

Vechicle oVechicle = new Vechicle();

Car oCar = oVechicle; // Upcast

After the upcast, variable oCar still references the same Vechicle object as variable oVechicle. The object being referenced is not itself altered or converted:

Console.WriteLine (oCar == oVechicle); // True

Although oCar and oVechicle refer to the identical object, oCar has a more restrictive view on that object:

Console.WriteLine (oCar.Name); // OK

Console.WriteLine (oCar.VechicleCount); // Error: VechicleCount undefined

The last line generates a compile-time error because the variable oCar is of type Car, even though it refers to an object of type Vechicle. To get to its VechicleCount field, you must downcast the Vechicle to a Car.

Downcasting

A downcast operation creates a subclass reference from a base class reference. For example:

Vechicle oVechicle = new Vechicle();

Car oCar = oVechicle; // Upcast

Vechicle oVechicle1 = (Vechicle)oCar; // Downcast

Console.WriteLine (oVechicle1.VechicleCount); // <No error>

Console.WriteLine (oVechicle1 == oCar); // True

Console.WriteLine (oVechicle1 == oVechicle); // True

As with an upcast, only references are affected—not the underlying object. A downcast requires an explicit cast because it can potentially fail at runtime:

Maruti oMaruti = new Maruti();

Car oCar = oMaruti; // Upcast always succeeds

Vechicle oVechicle = (Vechicle)oCar; // Downcast fails: oCar is not a Car

If a downcast fails, an InvalidCastException is thrown. This is an example of runtime type checking.