Home > Sample chapters

Understanding Values and References in Microsoft Visual C#

Boxing

As you have just seen, variables of type object can refer to any item of any reference type. However, variables of type object can also refer to a value type. For example, the following two statements initialize the variable i (of type int, a value type) to 42 and then initialize the variable o (of type object, a reference type) to i:

int i = 42;
object o = i;

The second statement requires a little explanation to appreciate what is actually happening. Remember that i is a value type and that it lives on the stack. If the reference inside o referred directly to i, the reference would refer to the stack. However, all references must refer to objects on the heap; creating references to items on the stack could seriously compromise the robustness of the runtime and create a potential security flaw, so it is not allowed. Therefore, the runtime allocates a piece of memory from the heap, copies the value of integer i to this piece of memory, and then refers the object o to this copy. This automatic copying of an item from the stack to the heap is called boxing. The following diagram shows the result:

08fig05.jpg