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Posted 26 Aug 2004


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Boxing and UnBoxing in C#

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


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.


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; 

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

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

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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About the Author

Web Developer
United States United States
My name is Mohammad Azam and I have been developing iOS applications since 2010. I have worked as a lead mobile developer for VALIC, AIG, Schlumberger, Baker Hughes, and The Home Depot. I have also published tons of my own apps to the App Store and even got featured by Apple for my app, Vegetable Tree. I highly recommend that you check out my portfolio. At present I am working as a lead instructor at DigitalCrafts.

I also have a lot of Udemy courses which you can check out at the following link:
Mohammad Azam Udemy Courses

Comments and Discussions

GeneralPlease explain this Pin
onec#user30-Mar-09 8:30
Memberonec#user30-Mar-09 8:30 
QuestionBut Why? Pin
DJ_B16-Mar-08 3:17
MemberDJ_B16-Mar-08 3:17 
AnswerRe: But Why? Pin
jasonmdli8-Sep-08 12:20
Memberjasonmdli8-Sep-08 12:20 
QuestionWhy has c# adopted boxing? Pin
AJ1233-Jan-06 9:19
MemberAJ1233-Jan-06 9:19 
QuestionBoxing same as casting in C? Pin
LiamD29-Nov-05 22:54
MemberLiamD29-Nov-05 22:54 
GeneralNice One Pin
adeel221-Jul-05 1:26
Memberadeel221-Jul-05 1:26 
GeneralWatch out Pin
Jeff Lindholm20-Sep-04 11:16
MemberJeff Lindholm20-Sep-04 11:16 
GeneralRe: Watch out Pin
Anonymous21-Sep-04 8:46
MemberAnonymous21-Sep-04 8:46 
GeneralError Pin
eggie513-Sep-04 6:17
Membereggie513-Sep-04 6:17 
GeneralRe: Error Pin
azamsharp19-Sep-04 13:57
Memberazamsharp19-Sep-04 13:57 
GeneralMistake Pin
Kinlan2-Sep-04 0:55
MemberKinlan2-Sep-04 0:55 
GeneralRe: Mistake Pin
azamsharp2-Sep-04 3:25
Memberazamsharp2-Sep-04 3:25 
GeneralRe: Mistake Pin
Kinlan2-Sep-04 7:10
MemberKinlan2-Sep-04 7:10 
GeneralMISTAKE Pin
azamsharp2-Sep-04 8:13
Memberazamsharp2-Sep-04 8:13 
GeneralPerformance considerations Pin
Mike.NET27-Aug-04 7:24
MemberMike.NET27-Aug-04 7:24 
GeneralRe: Performance considerations Pin
azamsharp27-Aug-04 8:58
Memberazamsharp27-Aug-04 8:58 
GeneralRe: Performance considerations Pin
Jeffrey Sax6-Sep-04 12:15
MemberJeffrey Sax6-Sep-04 12:15 

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