boxing and unboxing in int and string

Question

I am little bit confused in boxing and unboxing. According to its definition

Boxing is implicit conversion of ValueTypes to Reference Types (Object).
UnBoxing is explicit conversion of Reference Types (Object) to its equivalent ValueTypes.

the best example for describing this is

int i = 123; object o = i;  // boxing

and

o = 123; i = (int)o;  // unboxing 

But my question is that whether int is value type and string is reference type so

int i = 123; string s = i.ToString();

and

s = "123"; i = (int)s; 

Is this an example of boxing and unboxing or not???

Answer 1

Calling ToString is not boxing. It creates a new string that just happens to contain the textual representation of your int.

When calling (object)1 this creates a new instance on the heap that contains an int. But it's still an int. (You can verify that with o.GetType())

String can't be converted with a cast to int. So your code will not compile.

If you first cast your string to object your code will compile but fail at runtime, since your object is no boxed int. You can only unbox an value type into the exactly correct type(or the associated nullable).

Two examples:

Broken:

object o=i.ToString();// o is a string
int i2=(int)o;//Exception, since o is no int

Working:

object o=i;// o is a boxed int
int i2=(int)o;//works 

Answer 2

 int i = 2;
 string s = i.ToString();

This is NOT boxing. This is simply a method call to Int32.ToString() which returns a formatted string representing the value of the int.

 i = (int)s;

This code will not compile as there is no explicit conversion defined between System.String and System.Int32.

Think of it in the following way to understand what is and what is not boxing and unboxing:

1. Boxing: Its when you take a value type and just "stick" it in a reference variable. There is no need of any type specific conversion logic for this operation to work. The variable type will still be the same if you use GetType().

2. Unboxing: Its just the opposite operation. Take a value type stuck in a reference object and assign it to a value type variable. Again there is no need for any type specific conversion logic for this operation to work.

   So if (int)s were valid, it would simply be a explicit conversion and not a unboxing operation, becuase s.GetType() would return System.String, not System.Int32.