Boxing and unboxing: when does it come up?

Question

So I understand what boxing and unboxing is. When's it come up in real-world code, or in what examples is it an issue? I can't imagine doing something like this example:

int i = 123;
object o = i;           // Boxing
int j = (int)o;     // Unboxing

...but that's almost certainly extremely oversimplified and I might have even done boxing/unboxing without knowing it before.

Answer 1

It's much less of an issue now than it was prior to generics. Now, for example, we can use:

List<int> x = new List<int>();
x.Add(10);
int y = x[0];

No boxing or unboxing required at all.

Previously, we'd have had:

ArrayList x = new ArrayList();
x.Add(10); // Boxing
int y = (int) x[0]; // Unboxing

That was my most common experience of boxing and unboxing, at least.

Without generics getting involved, I think I'd probably say that reflection is the most common cause of boxing in the projects I've worked on. The reflection APIs always use "object" for things like the return value for a method - because they have no other way of knowing what to use.

Another cause which could catch you out if you're not aware of it is if you use a value type which implements an interface, and pass that value to another method which has the interface type as its parameter. Again, generics make this less of a problem, but it can be a nasty surprise if you're not aware of it.

Answer 2

Boxing (in my experience) usually occurs in these cases:

• A value type is passed to a method that accepts an argument of type Object.
• A value type is added to a non-generic collection (like an ArrayList).

Other times you can see boxing and unboxing is when you use reflection as the .NET framework's reflection API makes heavy use of Object.