Trying to cast a boxed int to byte

Question

Code to illustrate :

        int i = 5;
        object obj = i;
        byte b = (byte)obj; // X

When run, this generates a System.InvalidCastException ("Specified cast is not valid") at line "X". Doing a double cast works :

        byte b = (byte)(int)obj;

I would have thought that you ought to be able to cast a boxed int (if it has a value in the range 0..255) to a byte. Can anyone shed any light on this ?

(This is in .net 2.0, in case that matters).

Answer 1

The difference in behaviour you're seeing is the difference between identity and representation.

Unboxing is an identity cast, and a representation-preserving operation. Casting an int to a byte, however, is representation-changing (since there is a potential loss of precision).

You get an InvalidCastException when you try to unbox the int as a byte because the identity of the boxed value is not a byte, it is an int. When you write byte b = (byte)obj, you are telling the runtime, I know that what's in there is a byte, but what you really mean to say is, I think that what's in there can be converted to a byte.

In order to make the latter statement, you first have to declare the identity of the object, which is an int. Then and only then can you make a representation-changing conversion to byte.

Note that this applies even if the target type is "larger" - i.e. an Int64. All explicit conversions for which the destination type is not in the inheritance tree of the source type are considered to be representation-changing. And since all types derive from System.Object, unboxing by definition cannot change the representation.

Answer 2

MSDN explicitly says that unboxing to a different type will throw an InvalidCastException.

My understanding is that the type to which a variable is unboxed is actually a parameter to the underlying CIL assembly command. It is unbox opcode that actually throws the InvalidCastException.

InvalidCastException is thrown if the object is not boxed as valType.