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Boxing and UnBoxing in C#

, 26 Aug 2004
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This article explains the concepts of Boxing and UnBoxing in C#

Introduction

In this article I will explain the concepts of Boxing and UnBoxing. C# provides us with Value types and Reference Types. Value Types are stored on the stack and Reference types are stored on the heap. The conversion of value type to reference type is known as boxing and converting reference type back to the value type is known as unboxing.

Let me explain you little more about Value and Reference Types.

Value Types

Value types are primitive types that are mapped directly to the FCL. Like Int32 maps to System.Int32, double maps to System.double. All value types are stored on stack and all the value types are derived from System.ValueType. All structures and enumerated types that are derived from System.ValueType are created on stack, hence known as ValueType.

Reference Types

Reference Types are different from value types in such a way that memory is allocated to them from the heap. All the classes are of reference type. C# new operator returns the memory address of the object.

Examples

Lets see some examples to have a better understanding of Value Types and Reference Types. Since we know that all ValueTypes are derived from System.Value we can write something like this:

 System.ValueType r = 5;       

So what do you think about the above line of code. Will it compile ? Yes it will compile. But wait what type is it cause I don't remember any type which is called System.ValueType since its a base class from which all value types inherit. So is it Int32, Int64,double, decimal etc. It turns out that the type for variable 'r' is System.Int32. The Question arrises why Int32 and why not Int16. Well its because it is mapped to Int32 by default depending upon the Initial value of the variable.

You cannot write something like this since System.ValueType is not a primitive type its a base class for primitive value types and these mathematical operations can be performed on primitive types.

System.ValueType r = 10; 
r++;

In the above example I told you that variable 'r' will be a System.Int32 variable but if you don't believe me than you can find out yourself using the GetType() method:

 System.ValueType r = 5;
 Console.WriteLine(r.GetType()) // returns System.Int32; 

Here are few samples you can try on your own:

 
        
        System.ValueType r = 23.45; 
        Console.WriteLine(r.GetType()); // what does this print
        //-------------------------------------------------------
        System.ValueType r = 23.45F; 
        Console.WriteLine(r.GetType()); // What does this print
        //-------------------------------------------------------
        System.ValueType r = 2U; 
        Console.WriteLine(r.GetType()); // What does this print
        //-------------------------------------------------------
        System.ValueType r = 'c';
        Console.WriteLine(r.GetType()); // What does this print
        //-------------------------------------------------------
        System.ValueType r = 'ac';
        Console.WriteLine(r.GetType()); // tricky 
        //-------------------------------------------------------
        System.ValueType r = "Hello World"; 
        Console.WriteLine(r.GetType()); // tricky
        
        

Lets now jump to Boxing. Sometimes we need to convert ValueTypes to Reference Types also known as boxing. Lets see a small example below. You see in the example I wrote "implicit boxing" which means you don't need to tell the compiler that you are boxing Int32 to object because it takes care of this itself although you can always make explicit boxing as seen below right after implicit boxing.

 
        Int32 x = 10; 
        object o = x ;  // Implicit boxing
        Console.WriteLine("The Object o = {0}",o); // prints out 10
        //-----------------------------------------------------------
        Int32 x = 10; 
        object o = (object) x; // Explicit Boxing
        Console.WriteLine("The object o = {0}",o); // prints out 10
        
        

Lets now see UnBoxing an object type back to value type. Here is a simple code that unbox an object back to Int32 variable. First we need to box it so that we can unbox.

 
        Int32 x = 5; 
        object o = x; // Implicit Boxing
        x = o; // Implicit UnBoxing
        

So, you see how easy it is to box and how easy it is to unbox. The above example first boxs Int32 variable to an object type and than simply unbox it to x again. All the conversions are taking place implicitly. Everything seems right in this example there is just one small problem which is that the above code is will not compile. You cannot Implicitly convert a reference type to a value type. You must explicity specify that you are unboxing as shown in the code below.

 
        Int32 x = 5; 
        object o = x; // Implicit Boxing
        x = (Int32)o; // Explicit UnBoxing
        

Lets see another small example of unboxing.

 
        Int32 x = 5; // declaring Int32
        Int64 y = 0; // declaring Int64 double
        object o = x; // Implicit Boxing
        y = (Int64)o; // Explicit boxing to double
        Console.WriteLine("y={0}",y); 
        

This example will not work. It will compile successfully but at runtime It will generate an exception of System.InvalidCastException. The reason is variable x is boxed as Int32 variable so it must be unboxed to Int32 variable. So, the type the variable uses to box will remain the same when unboxing the same variable. Of course you can cast it to Int64 after unboxing it as Int32 as follows:

 
         Int32 x = 5; // declaring Int32
        Int64 y = 0; // declaring Int64 double
        object o = x; // Implicit Boxing
        y = (Int64)(Int32)o; // Unboxing and than casting to double
        Console.WriteLine("y={0}",y); 
        

I am sure that you all have grasp the basic understanding of Boxing and Unboxing. Happy Coding and practise a lot !

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Comments and Discussions

 
GeneralPlease explain this Pinmemberradhikadeo30-Mar-09 7:30 
void button1_Click(object sender, EventArgs e)
{
int i = 10;
object o = i; // boxing i value
ChangeI(o); // object o is pass as reference
 
}
void ChangeI(object s)
{
s = 30; // changing s to 30 should change object o too
}
 
I have this piece of code. I was expecting that after calling the function ChangeI(o), the value of o should be get changed to 30. becuase o is pass as reference to function ChangeI . But value of o doesn't get changed. Why ??
QuestionBut Why? PinmemberDJ_B16-Mar-08 2:17 
AnswerRe: But Why? Pinmemberjasonmdli8-Sep-08 11:20 
QuestionWhy has c# adopted boxing? PinmemberAJ1233-Jan-06 8:19 
QuestionBoxing same as casting in C? PinmemberLiamD29-Nov-05 21:54 
GeneralNice One PinmemberDandyGroove21-Jul-05 0:26 
GeneralWatch out PinmemberLorad20-Sep-04 10:16 
GeneralRe: Watch out PinsussAnonymous21-Sep-04 7:46 
GeneralError Pinmembereggie513-Sep-04 5:17 
GeneralRe: Error Pinmemberazamsharp19-Sep-04 12:57 
GeneralMistake PinmemberKinlan1-Sep-04 23:55 
GeneralRe: Mistake Pinmemberazamsharp2-Sep-04 2:25 
GeneralRe: Mistake PinmemberKinlan2-Sep-04 6:10 
GeneralMISTAKE Pinmemberazamsharp2-Sep-04 7:13 
GeneralPerformance considerations PinmemberMike.NET27-Aug-04 6:24 
GeneralRe: Performance considerations Pinmemberazamsharp27-Aug-04 7:58 
GeneralRe: Performance considerations PinmemberJeffrey Sax6-Sep-04 11:15 

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