C# value type casting: how it works? [duplicate]

Question

Possible Duplicate:
Why does this conversion doesn't work?

Hi, i discovered a strange behaviour of the framework. This code throws an exception:

    byte a = 1;
    object b = a;
    Console.WriteLine(b.GetType());
    Console.WriteLine((byte)b);
    Console.WriteLine((int)(byte)b);
    Console.WriteLine(Convert.ToInt32(b));
    Console.WriteLine((int)b);

The last line throws a System.InvalidCastException.
I'd like to know what are the mechanism in the framework that make this code illegal.
Is it a problem of boxing/unboxing?!

Answer 1

Yes, boxed value types can only be unboxed to the exact same type.

The variable b is a boxed byte.

• When you do (int)(byte)b you're unboxing b back to a byte and then converting that unboxed byte to an int.
• When you do (int)b you're attempting to unbox b directly to an int, which is illegal.

Edit...

As Jon mentions in his answer, there are cases where you don't have to unbox to the exact same type. Specifically:

• A boxed T can be unboxed to Nullable<T>.
• A boxed Nullable<T> can be unboxed to T, assuming that the nullable isn't actually null.
• A boxed enum with an underlying type of T can be unboxed to T.
• A boxed T can be unboxed to an enum with an underlying type of T.