Not getting concept of boxing and unboxing

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.NET3.5

I am not getting the concept of boxing and unboxing.Why I need To do this and where.
It is said boxing means conversion of value to object type

C#

int i = 3;
            object o = i;
          Console.WriteLine(o);
            Console.ReadLine();

Result is :3
Unboxing means conversion of object to value type

C#

int i = 3;
           object o = i;
           int m = (int)o;
           Console.WriteLine(m);
           Console.ReadLine();

Result is :3
I can directly also do

C#

int i = 3;
          
          
           Console.WriteLine(i);
           Console.ReadLine();

Result is :3

Why i need to have this concept..Please tell me I am not able to get what this concept actually means
Thanks in advance

Posted 17-Oct-11 18:20pm

shivani 2013

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Sergey Alexandrovich Kryukov · Answer 1 · 2011-10-17T19:15:00

First, simply read about boxing and unboxing more thoroughly and think about it.
This should be clear enough: http://msdn.microsoft.com/en-us/library/yz2be5wk.aspx[^].

To add to it, I can only respond to your immediate concerns.

First, in all your code samples, conversion to object is completely pointless, only the last sample is reasonable. What can I add to your understanding. Even though boxing and unboxing is very important in .NET, immediate using boxing is pretty rare, and using unboxing is even more rare.

Let me explain just Write, or, very close in usage string.Format(string format, params object[] values). Look at the last method signature. This is the most universal way to provide passing parameters for formatting of the value of absolutely any type. Let's see how it works. If format string contains only placeholders in "free format" like {0}, {1}, etc., and as each parameter is boxed to System.Object, each of the placeholders is substituted with what is returned by System.Object.ToString. In other words, the mechanism uses the fact that all objects of all type have ToString method. Unboxing is already done in passing of the parameters (due to assignment compatibility to System.Object), and no unboxing is needed. If specific format specifier is used for some of the placeholders (this is called composite formatting), unboxing is needed for getting type-specific information. For example, if number of significant digits is specified, properties of floating-point arithimetic are used.

—SA

deepak kumar choudhary · Answer 2 · 2011-10-17T18:40:00

Boxing

CSS

All C# types, including the value types, are derived from object. Thus, a reference of type object can be used to refer to any other type, including value types.
When an object reference refers to a value type, a process known as boxing occurs.
Boxing causes the value of a value type to be stored in an object instance. Thus, a value type is "boxed" inside an object.
Boxing occurs automatically.
    For Ex.

C#

object obj;
            int a=10;
            obj=a;  // Boxing implicitly performs

Unboxing

Unboxing is the process of retrieving a value from a boxed object.
This action is performed using an explicit cast from the object reference to its corresponding value type.
Attempting to unbox an object into a different type will result in a runtime error

C#

int x;
object obj;
x = 10;
obj = x; // box x into an object
int y = (int)obj; // unbox obj into an int
Console.WriteLine(y);

Abhinav S · Answer 3 · 2011-10-17T19:11:00

Solution 4

You could try and read a book on .Net development - that should give you some idea on what this is.

Posted 17-Oct-11 19:11pm

Abhinav S

Firo Atrum Ventus · Answer 4 · 2011-10-17T18:26:00

Here's a wiki page about boxing/unboxing : http://en.wikipedia.org/wiki/Object_type_(object-oriented_programming)#Boxing[^] (ps. don't look at the code)

[Edit2]Thanks to SA for the heads up, I now understand that my answer bellow are not an example of boxing. But it should give you general idea. I want to give a correct example but SA already write one so please refer to SA's solution.[/Edit2]

Don't worry, You'll get better understanding about boxing/unboxing and its uses after learning about polymorpishm.

[Edit]
Now that you've understood about polymorphism, we can make this easy.

Imagine that I have an animal class:

C#

class Animal {
    public string talk(){
        return "";
    }
}

And I have two child of animal class:

C#

class Cat:Animal {
    public string talk(){
        return "meow";
    }
}

C#

class Dog:Animal {
    public string talk(){
        return "roof";
    }
}

Now if I want to use method talk in a method in another class, I can just set the parameter to use Animal class:

C#

class MainProgram{
    static void Main(string[] args)
    {
        Cat c = new Cat();
        Dog d = new Dog();
        Animal a = c;
        write(a);
        a = d;
        write(a);
    }
    
    void write(Animal a)//we can just write it like this instead writing two method with different parameter.
    {
        Console.WriteLine(a.talk());
    }
}

[/Edit]

Not getting concept of boxing and unboxing

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