Boxing and Unboxing in .Net

Contents are from MSDN.

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

  

Performance

---

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.

Boxing

---

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.

Boxing Conversion

It 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

Description

---

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.

Example

---

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

---

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.

Unboxing Conversion

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.

Enjoy Have a nice day.

What are Static Constructors and how Static Constructors are invoked?

Static Constructors: A static constructor is used to initialize any static data, or to perform a particular action that needs performed once only. It is called automatically before the first instance is created or any static members are referenced.

Static constructors have the following properties:

• A static constructor does not take access modifiers or have parameters.
• A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
• A static constructor cannot be called directly.
• The user has no control on when the static constructor is executed in the program.
• A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
• Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary method.

Example :-

OUTPUT of above code :

NOTE : Static Constructors doesn't take any Access Modifier. This is rule. By default is private.

    class A

    {

        /// <summary>

        /// Static Constructors doesn't take any Access Modifier. This is rule.

        /// </summary>

        static A()

        {

            Console.WriteLine("A : Static Constructor.");

        }

       /// <summary>

        /// Default Constructor

        /// </summary>

        public A()

        {

            Console.WriteLine("A : Default Constructor.");

        }

    }

When we have static constructors in our class then it would be called first.

There can be several scenarios with static constructors.

1.       If we have static and default constructors in same class then first Static constructor would be called then Default.

2.       If we have three classes A,B,C have static constructors and are inherited in such a way C:B:A, then constructors would be invoked in following sequence :-

a.       Default Constructors would be called in A:B:C sequence. Rule is, first base class default constructor will call then derived class default constructor.

b.      Static Constructors would be called in C:B:A sequence. (Opposite to invocation of Default Constructors.). Rule is, first derived class static constructor will be called then base class static constructor.

Here is the example of the same.

Making object of class C all constructors would get invoked. First all static constructors then default constructors.

Hence we can say that :-

1.       For static constructor : First derived class static constructor will be called then base class static constructor.

2.       For default constructor : First base class default constructor will call then derived class default constructor.

Another example: