Question

In Jesse Liberty’s Learning C# book, he says

"Objects of one type can be converted into objects of another type. This is called casting."

If you investigate the IL generated from the code below, you can clearly see that the casted assignment isn’t doing the same thing as the converted assignment. In the former, you can see the boxing/unboxing occurring; in the latter you can see a call to a convert method.

I know in the end it may be just a silly semantic difference–but is casting just another word for converting. I don’t mean to be snarky, but I’m not interested in anyone’s gut feeling on this–opinions don’t count here! Can anyone point to a definitive reference that confirms or denies if casting and converting are the same thing?

    object x;     int y;          x = 4;          y = ( int )x;          y = Convert.ToInt32( x ); 

Thank you

rp

Note added after Matt’s comment about explicit/implicit:

I don’t think implicit/explicit is the difference. In the code I posted, the change is explicit in both cases. An implicit conversion is what occurs when you assign a short to an int.

Note to Sklivvz:

I wanted confirmation that my suspicion of the looseness of Jesse Liberty’s (otherwise usually lucid and clear) language was correct. I thought that Jesse Liberty was being a little loose with his language. I understand that casting is routed in object hierarchy–i.e., you can’t cast from an integer to a string but you could cast from custom exception derived from System.Exception to a System.Exception.

It’s interesting, though, that when you do try to cast from an int to a string the compiler tells you that it couldn’t "convert" the value. Maybe Jesse is more correct than I thought!

Answer 1

The simple answer is: it depends.

For value types, casting will involve genuinely converting it to a different type. For instance:

float f = 1.5f; int i = (int) f; // Conversion 

When the casting expression unboxes, the result (assuming it works) is usually just a copy of what was in the box, with the same type. There are exceptions, however – you can unbox from a boxed int to an enum (with an underlying type of int) and vice versa; likewise you can unbox from a boxed int to a Nullable<int>.

When the casting expression is from one reference type to another and no user-defined conversion is involved, there’s no conversion as far as the object itself is concerned – only the type of the reference ‘changes’ – and that’s really only the way that the value is regarded, rather than the reference itself (which will be the same bits as before). For example:

object o = 'hello'; string x = (string) o; // No data is 'converted'; x and o refer to the same object 

When user-defined conversions get involved, this usually entails returning a different object/value. For example, you could define a conversion to string for your own type – and this would certainly not be the same data as your own object. (It might be an existing string referred to from your object already, of course.) In my experience user-defined conversions usually exist between value types rather than reference types, so this is rarely an issue.

All of these count as conversions in terms of the specification – but they don’t all count as converting an object into an object of a different type. I suspect this is a case of Jesse Liberty being loose with terminology – I’ve noticed that in Programming C# 3.0, which I’ve just been reading.

Does that cover everything?