Boxing and Unboxing in C#

Boxing and Unboxing in C#

As part of graduation projects , each year we should have internships in summer , so we had to prepare ourselves for interviews for technical and non technical question .
I remember in one of the interviews , the last question was ,’can you explain to me what is boxing and unboxing ? ‘ and I was like :

Well , I stood like that for 5 minutes and ended up saying that I don’t know the answer , then the interviewer get up from his desk and explained it to me by writing on a blackboard .
When he was explaining I was saying in my mind : ‘Dude ,I have been doing that all my life , I didn’t know it’s called boxing and unboxing ! ‘ .

Well , that was an interview that didn’t go well , but at least I learned something from it which encouraged me to write about it now .

What is Boxing And Unboxing ?

Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type.

So when we say boxing,we say it’s the process of taking a value type ,putting it inside a new object on the heap and storing a reference to it on the stack . Unboxing is the exact opposite.

The difference between a value type and a reference type is that the value type stores its value directly,a reference type stores a reference that points to an object on the heap that contains the value.
so :
Unboxing extracts the value type from the object.
Boxing is implicit and unboxing is explicit.

Let’s start by a simple boxing example :

int i = 123456;
// The following line boxes i inside object o.
object o = i;

The object o now, can be unboxed and assigned to integer variable i

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

Let’s see one more example , so we have the following code :

class TestUnboxing
    static void Main()
        int i = 123;
        object o = i;  // implicit boxing

            int j = (short)o;  // attempt to unbox

            System.Console.WriteLine("Unboxing OK.");
        catch (System.InvalidCastException e)
            System.Console.WriteLine("{0} Error: Incorrect unboxing.", e.Message);

So the output of this code if you try to execute it is , ” Specified cast is not valid. Error: Incorrect unboxing. ” and why is that ?

As you can see , we have defined i as int , but when we tried to unbox it we used short which cause the exception here , so what we need to do is :

int j = (int) o;  

A picture that can explain all that :


In relation to simple assignments, boxing and unboxing are computationally expensive processes. When a value type is boxed, a new object must be allocated and constructed. To a lesser degree, the cast required for unboxing is also expensive computationally. For more information, see Performance in Microsoft official website .

Happy boxing && unboxing day 🙂

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