How does casting work?

Question

I was wondering what C# does when you for example cast an object to an int.

object o = 10;
int i = (int) o;

Much appreciated :)!

Answer 1

In the general case, that is a tricky one ;p It depends on the exact scenari:

• (when the target is a value-type) if the source value is only known as object, it is an unbox operation, which reverses the special way in which value-types can be stored in an object reference (Unbox / Unbox_Any)
• if the source type is a Nullable<int>, then the .Value property is evaluated (which can cause an exception if the value is empty)
• if the source type is one of a few built-in types documented in the spec (uint, float, etc), then the specific opcode (which may be nothing at all) is emitted to perform the conversion directly in IL (Conv_I4)
• if the source type has a custom implicit or explicit conversion operator defined (matching the target type), then that operator is invoked as a static method (Call)
• (in the case of reference types) if it isn't obviously always untrue (different hierarchies), then a reference cast/check is performed (CastClass)
• otherwise the compiler treats it as an error

I think that is fairly complete?