C# int- or object-to-double casting error explanation

Question

The below code fails at the last assignment:

static void Main(string[] args)
{
    int a = 5;
    object b = 5;

    System.Diagnostics.Debug.Assert( a is int && b is int );

    double x = (double)a;
    double y = (double)b;
}

If both a and b are int, what is the cause of this error?

Answer 1

This is one of the rare cases where System.Convert comes in handy. You can use System.Convet.ToDouble(obj) to knock it out if you don't know before hand that it will be int.

Answer 2

Unboxing requires the exact type - you can do this instead:

double y = (double)(int)b;

Answer 3

This is an extremely frequently asked question. See https://ericlippert.com/2009/03/03/representation-and-identity/ for an explanation.

---

Snippet:

I get a fair number of questions about the C# cast operator. The most frequent question I get is:

short sss = 123;
object ooo = sss;            // Box the short.
int iii = (int) sss;         // Perfectly legal.
int jjj = (int) (short) ooo; // Perfectly legal
int kkk = (int) ooo;         // Invalid cast exception?! Why?

Why? Because a boxed T can only be unboxed to T. (*) Once it is unboxed, it’s just a value that can be cast as usual, so the double cast works just fine.

(*) Or Nullable<T>.

Answer 4

Implicit casting is a compile-time operation. It's not possible for b of type object.