Saturday, 8 February 2014

Boxing and Unboxing in C#





Boxing
Unboxing
Definition:
Boxing is the process of converting a value type to the reference (object) type.
Unboxing is the process of converting a reference (object) type to value type.
Type of Conversion:
Implicit Conversion
Explicit Conversion
Example:
int i = 221;
object obj = i;
object obj = 213;
i = (int)obj;

Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap. Unboxing extracts the value type from the object. Boxing is implicit; unboxing is explicit. The concept of boxing and unboxing underlies the C# unified view of the type system in which a value of any type can be treated as an object.
In the following example, the integer variable i is boxed and assigned to object o.
int i = 123;
// The following line boxes i. 
object o = i; 
The object o can then be unboxed and assigned to integer variable i:

o = 123;
i = (int)o;  // unboxing
The following examples illustrate how boxing is used in C#.

// String.Concat example. 
// String.Concat has many versions. Rest the mouse pointer on  
// Concat in the following statement to verify that the version 
// that is used here takes three object arguments. Both 42 and 
// true must be boxed.
Console.WriteLine(String.Concat("Answer", 42, true));


// List example. 
// Create a list of objects to hold a heterogeneous collection  
// of elements.
List<object> mixedList = new List<object>();

// Add a string element to the list.
mixedList.Add("First Group:");

// Add some integers to the list.  
for (int j = 1; j < 5; j++)
{
    // Rest the mouse pointer over j to verify that you are adding 
    // an int to a list of objects. Each element j is boxed when  
    // you add j to mixedList.
    mixedList.Add(j);
}

// Add another string and more integers.
mixedList.Add("Second Group:");
for (int j = 5; j < 10; j++)
{
    mixedList.Add(j);
}

// Display the elements in the list. Declare the loop variable by  
// using var, so that the compiler assigns its type. 
foreach (var item in mixedList)
{
    // Rest the mouse pointer over item to verify that the elements 
    // of mixedList are objects.
    Console.WriteLine(item);
}

// The following loop sums the squares of the first group of boxed 
// integers in mixedList. The list elements are objects, and cannot 
// be multiplied or added to the sum until they are unboxed. The 
// unboxing must be done explicitly. 
var sum = 0;
for (var j = 1; j < 5; j++)
{
    // The following statement causes a compiler error: Operator  
    // '*' cannot be applied to operands of type 'object' and 
    // 'object'.  
    //sum += mixedList[j] * mixedList[j]); 

    // After the list elements are unboxed, the computation does  
    // not cause a compiler error.
    sum += (int)mixedList[j] * (int)mixedList[j];
}

// The sum displayed is 30, the sum of 1 + 4 + 9 + 16.
Console.WriteLine("Sum: " + sum);

// Output: 
// Answer42True 
// First Group: 
// 1 
// 2 
// 3 
// 4 
// Second Group: 
// 5 
// 6 
// 7 
// 8 
// 9 
// Sum: 30

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.


Boxing is used to store value types in the garbage-collected heap. Boxing is an implicit conversion of a value type to the type object or to any interface type implemented by this value type. Boxing a value type allocates an object instance on the heap and copies the value into the new object.
Consider the following declaration of a value-type variable:
int i = 123;
The following statement implicitly applies the boxing operation on the variable i:

// Boxing copies the value of i into object o. 
object o = i; 
The result of this statement is creating an object reference o, on the stack, that references a value of the type int, on the heap. This value is a copy of the value-type value assigned to the variable i. The difference between the two variables, i and o, is illustrated in the following figure.

 
It is also possible to perform the boxing explicitly as in the following example, but explicit boxing is never required:
int i = 123;
object o = (object)i;  // explicit boxing


This example converts an integer variable i to an object o by using boxing. Then, the value stored in the variable i is changed from 123 to 456. The example shows that the original value type and the boxed object use separate memory locations, and therefore can store different values.
class TestBoxing
{
    static void Main()
    {
        int i = 123;

        // Boxing copies the value of i into object o. 
        object o = i; 

        // Change the value of i.
        i = 456; 

        // The change in i does not effect the value stored in o.
        System.Console.WriteLine("The value-type value = {0}", i);
        System.Console.WriteLine("The object-type value = {0}", o);
    }
}
/* Output:
    The value-type value = 456
    The object-type value = 123
*/
The following example demonstrates a case of invalid unboxing and the resulting InvalidCastException. Using try and catch, an error message is displayed when the error occurs.
class TestUnboxing
{
    static void Main()
    {
        int i = 123;
        object o = i;  // implicit boxing 

        try
        {
            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);
        }
    }
}
This program outputs:
Specified cast is not valid. Error: Incorrect unboxing.
If you change the statement:
int j = (short) o;
to:
int j = (int) o;
the conversion will be performed, and you will get the output:
Unboxing OK.



Unboxing is an explicit conversion from the type object to a value type or from an interface type to a value type that implements the interface. An unboxing operation consists of:
  • Checking the object instance to make sure that it is a boxed value of the given value type.
  • Copying the value from the instance into the value-type variable.
The following statements demonstrate both boxing and unboxing operations:
int i = 123;      // a value type 
object o = i;     // boxing 
int j = (int)o;   // unboxing
The following figure demonstrates the result of the previous statements.


For the unboxing of value types to succeed at run time, the item being unboxed must be a reference to an object that was previously created by boxing an instance of that value type. Attempting to unbox null causes a NullReferenceException. Attempting to unbox a reference to an incompatible value type causes an InvalidCastException.



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