So far we’ve referred to objects with a reference of their own type, an inherited type, or an implemented interface:

Dissertation diss = new Dissertation();
Book bdiss = diss;
IFlippable fdiss = diss;

The process is called upcasting. As we saw in the last exercise upcasting allows us to work with multiple types at once. It also lets us safely store an object without knowing its specific type. You can think of upcasting as using a reference “up” the inheritance hierarchy:

What happens if you try to downcast, or reference an object by a subclass? You’ll need to do this when you want to access the specific functionality of a subclass.

For example what happens when we refer to a Book object as a Dissertation type?

Book bk = new Book();
Dissertation dbk = bk;
// Error!

The code produces this error:

error CS0266: Cannot implicitly convert type `Book` to `Dissertation`. An explicit conversion exists (are you missing a cast?)

Not every downcast is possible in C#. In this case, Dissertation has a Define() method that is incompatible with Book. This is the computer’s way of telling you: there’s a chance that this cast won’t work!

To get around this error, we must explicitly downcast, like below. The desired type is written in parentheses:

Book bk = new Book();
Dissertation bdk = (Dissertation)bk;

This essentially tells the computer: “I know the risk I’m taking, and this might fail if I’m not careful.”

In many cases, the downcast will still fail. Here, the Dissertation type reference bdk can’t reference a Book object, so when we explicitly downcast we see that it fails with a new error message:

System.InvalidCastException: Specified cast is not valid.

There are multiple ways to deal with downcasting, including the as and is operators. We won’t get into those now, but you can learn about them in the Microsoft C# Programming Guide: Casting and type conversions if you’d like. For now, focus on these things:

• Upcasting is creating a superclass or interface reference from a subclass reference
• Downcasting is creating a subclass reference from a superclass or interface reference.
• Upcasting can be done implicitly, while downcasting cannot