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Boxing and Unboxing in C#

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 mixedList = new List();

// 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);






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.