why downcast fails at runtime

Question

Hi All, I want to know why below downcast fails @ run time:

case 1:

                  Object y = 10.23;
                 Console.WriteLine(y.GetType()); //System.Double
                   int z = (int)y;// fails @ runtime
                   Console.ReadKey();

case 2:

enter code here    Double y = 10.23;
                   Console.WriteLine(y.GetType());//System.Double
                   int z = (int)y;//success
                     Console.ReadKey();

In both the cases the type of y is System.Double, still why downcst fails in first case?

Answer 1

In the first example; unboxing (what you show) is different to downcasting or conversion; it is perhaps unfortunate that C# uses the same syntax for all 3.

You must unbox value-types (such as int/double) correctly. Or use Convert.ToInt32(y) which has the logic for this embedded.

In the second example, this is a conversion (not an unbox, and not a downcast). Conversions are defined either in the language spec (like in this case) or via custom static operators.

The difference is object. The box changes everything.

Answer 2

Marc is of course correct. For a longer analysis of why this sort of cast is not legal, see my article on the subject:

http://blogs.msdn.com/ericlippert/archive/2009/03/19/representation-and-identity.aspx

Answer 3

I found the longer discussion Casting – the escape from strong typing to be useful - but I have to add that casting is very messy and a clean distinction between type and represenation conversion would be a good idea and boxing makes it even worse.