How does C# generics affect collections with primitives

Question

As I understand it, C#/.Net generics support some degree of reification. So, if I have the following code:

List<int> list = new List<int>();
list.Add(1);

Will the value 1 be autoboxed or will the 'list' object handle primitive ints efficiently?

Answer 1

No, it won't be boxed. At execution time, the backing array for the List<int> will genuinely be an int[]. Note that this isn't just the case with genuine primitive types - List<T> won't box values of any value type (assuming it's been declared as List<Guid> etc rather than List<object>).

Basically, generics in .NET keep a lot more of their information than they do in Java - the CLR natively understands generics and deals with them appropriately, rather than in Java where the JVM is pretty much ignorant of them.

For example, if you write:

object list = new List<string>();
Type type = list.GetType();

Then type will be equal to typeof(List<string>) - which is then different to (say) List<Guid> etc.

Answer 2

The int values will not be boxed within the list. This is one of the beauties with generics, that the compiler (more specifically the JIT-compiler, I believe) will construct a typed version of the List<> class, rather than storing the values as object. So it does not only enforce type safety through the exposed methods and properties, but it is genuinely typed in all aspects.