Why do we need boxing and unboxing in C

Question

Why do we need boxing and unboxing in C    I know what boxing and unboxing is  but I can t comprehend the real use of it  Why and where should I use it   short s   25   object objshort   s     Boxing  short anothershort    short objshort     Unboxing

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The last place I had to unbox something was when writing some code that retrieved some data from a database (I wasn't using LINQ to SQL, just plain old ADO.NET):

int myIntValue = (int)reader["MyIntValue"];

Basically, if you're working with older APIs before generics, you'll encounter boxing. Other than that, it isn't that common.

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Why   To have a unified type system and allow value types to have a completely different representation of their underlying data from the way that reference types represent their underlying data  e g   an int is just a bucket of thirty-two bits which is completely different than a reference type    Think of it like this  You have a variable o of type object  And now you have an int and you want to put it into o  o is a reference to something somewhere  and the int is emphatically not a reference to something somewhere  after all  it s just a number   So  what you do is this  you make a new object that can store the int and then you assign a reference to that object to o  We call this process  boxing     So  if you don t care about having a unified type system  i e   reference types and value types have very different representations and you don t want a common way to  represent  the two  then you don t need boxing  If you don t care about having int represent their underlying value  i e   instead have int be reference types too and just store a reference to their underlying value  then you don t need boxing      where should I use it    For example  the old collection type ArrayList only eats objects  That is  it only stores references to somethings that live somewhere  Without boxing you cannot put an int into such a collection  But with boxing  you can   Now  in the days of generics you don t really need this and can generally go merrily along without thinking about the issue  But there are a few caveats to be aware of   This is correct   double e   2 718281828459045  int ee    int e    This is not   double e   2 718281828459045  object o   e     box int ee    int o     runtime exception   Instead you must do this   double e   2 718281828459045  object o   e     box int ee    int  double o    First we have to explicitly unbox the double   double o  and then cast that to an int   What is the result of the following   double e   2 718281828459045  double d   e  object o1   d  object o2   e  Console WriteLine d    e   Console WriteLine o1    o2     Think about it for a second before going on to the next sentence   If you said True and False great  Wait  what  That s because    on reference types uses reference-equality which checks if the references are equal  not if the underlying values are equal  This is a dangerously easy mistake to make  Perhaps even more subtle  double e   2 718281828459045  object o1   e  object o2   e  Console WriteLine o1    o2     will also print False   Better to say   Console WriteLine o1 Equals o2      which will then  thankfully  print True   One last subtlety    struct class  Point       public int x  y       public Point int x  int y            this x   x          this y   y           Point p   new Point 1  1   object o   p  p x   2  Console WriteLine   Point o  x     What is the output  It depends  If Point is a struct then the output is 1 but if Point is a class then the output is 2  A boxing conversion makes a copy of the value being boxed explaining the difference in behavior

User · Answer

In  net  every instance of Object  or any type derived therefrom  includes a data structure which contains information about its type    Real  value types in  net do not contain any such information   To allow data in value types to be manipulated by routines that expect to receive types derived from object  the system automatically defines for each value type a corresponding class type with the same members and fields   Boxing creates a new instances of this class type  copying the fields from a value type instance   Unboxing copies the fields from an instance of the class type to an instance of the value type   All of the class types which are created from value types are derived from the ironically named class ValueType  which  despite its name  is actually a reference type

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In general  you typically will want to avoid boxing your value types   However  there are rare occurances where this is useful  If you need to target the 1 1 framework  for example  you will not have access to the generic collections  Any use of the collections in  NET 1 1 would require treating your value type as a System Object  which causes boxing unboxing   There are still cases for this to be useful in  NET 2 0   Any time you want to take advantage of the fact that all types  including value types  can be treated as an object directly  you may need to use boxing unboxing  This can be handy at times  since it allows you to save any type in a collection  by using object instead of T in a generic collection   but in general  it is better to avoid this  as you re losing type safety  The one case where boxing frequently occurs  though  is when you re using Reflection - many of the calls in reflection will require boxing unboxing when working with value types  since the type is not known in advance

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Boxing is required  when we have a function that needs object as a parameter  but we have different value types that need to be passed  in that case we need to first convert value types to object data types before passing it to the function   I don t think that is true  try this instead   class Program               static void Main string   args                        int x   4              test x                      static void test object o                        Console WriteLine o ToString                       That runs just fine  I didn t use boxing unboxing   Unless the compiler does that behind the scenes

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The best way to understand this is to look at lower-level programming languages C  builds on   In the lowest-level languages like C  all variables go one place  The Stack  Each time you declare a variable it goes on the Stack  They can only be primitive values  like a bool  a byte  a 32-bit int  a 32-bit uint  etc  The Stack is both simple and fast  As variables are added they just go one on top of another  so the first you declare sits at say  0x00  the next at 0x01  the next at 0x02 in RAM  etc  In addition  variables are often pre-addressed at compile-time  so their address is known before you even run the program   In the next level up  like C    a second memory structure called the Heap is introduced  You still mostly live in the Stack  but special ints called Pointers can be added to the Stack  that store the memory address for the first byte of an Object  and that Object lives in the Heap  The Heap is kind of a mess and somewhat expensive to maintain  because unlike Stack variables they don t pile linearly up and then down as a program executes  They can come and go in no particular sequence  and they can grow and shrink   Dealing with pointers is hard  They re the cause of memory leaks  buffer overruns  and frustration  C  to the rescue   At a higher level  C   you don t need to think about pointers - the  Net framework  written in C    thinks about these for you and presents them to you as References to Objects  and for performance  lets you store simpler values like bools  bytes and ints as Value Types  Underneath the hood  Objects and stuff that instantiates a Class go on the expensive  Memory-Managed Heap  while Value Types go in that same Stack you had in low-level C - super-fast   For the sake of keeping the interaction between these 2 fundamentally different concepts of memory  and strategies for storage  simple from a coder s perspective  Value Types can be Boxed at any time  Boxing causes the value to be copied from the Stack  put in an Object  and placed on the Heap - more expensive  but  fluid interaction with the Reference world  As other answers point out  this will occur when you for example say   bool b   false     Cheap  on Stack object o   b     Legal  easy to code  but complex - Boxing  bool b2    bool o     Unboxing    A strong illustration of the advantage of Boxing is a check for null   if  b    null     Will not compile - bools can t be null if  o    null     Will compile and always return false   Our object o is technically an address in the Stack that points to a copy of our bool b  which has been copied to the Heap  We can check o for null because the bool s been Boxed and put there   In general you should avoid Boxing unless you need it  for example to pass an int bool whatever as an object to an argument  There are some basic structures in  Net that still demand passing Value Types as object  and so require Boxing   but for the most part you should never need to Box   A non-exhaustive list of historical C  structures that require Boxing  that you should avoid    The Event system turns out to have a Race Condition in naive use of it  and it doesn t support async  Add in the Boxing problem and it should probably be avoided   You could replace it for example with an async event system that uses Generics   The old Threading and Timer models forced a Box on their parameters but have been replaced by async await which are far cleaner and more efficient  The  Net 1 1 Collections relied entirely on Boxing  because they came before Generics  These are still kicking around in System Collections  In any new code you should be using the Collections from System Collections Generic  which in addition to avoiding Boxing also provide you with stronger type-safety    You should avoid declaring or passing your Value Types as objects  unless you have to deal with the above historical problems that force Boxing  and you want to avoid the performance hit of Boxing it later when you know it s going to be Boxed anyway   Per Mikael s suggestion below   Do This  using System Collections Generic   var employeeCount   5  var list   new List lt int gt  10     Not This  using System Collections   Int32 employeeCount   5  var list   new ArrayList 10     Update  This answer originally suggested Int32  Bool etc cause boxing  when in fact they are simple aliases for Value Types  That is   Net has types like Bool  Int32  String  and C  aliases them to bool  int  string  without any functional difference

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Boxing and Unboxing are specifically used to treat value-type objects as reference-type  moving their actual value to the managed heap and accessing their value by reference   Without boxing and unboxing you could never pass value-types by reference  and that means you could not pass value-types as instances of Object

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Boxing is the conversion of a value to a reference type with the data at some offset in an object on the heap  As for what boxing actually does  Here are some examples Mono C   void  mono object unbox  MonoObject  obj         MONO EXTERNAL ONLY GC UNSAFE  void   mono object unbox internal  obj         define MONO EXTERNAL ONLY GC UNSAFE t  expr        t result              MONO ENTER GC UNSAFE          result   expr             MONO EXIT GC UNSAFE           return result   static inline gpointer mono object unbox internal  MonoObject  obj           add assert for valuetypes         g assert  m class is valuetype  mono object class  obj         return mono object get data  obj      static inline gpointer mono object get data  MonoObject  o        return  guint8  o   MONO ABI SIZEOF  MonoObject       define MONO ABI SIZEOF type   MONO STRUCT SIZE  type    define MONO STRUCT SIZE struct  MONO SIZEOF     struct  define MONO SIZEOF MonoObject  2   MONO SIZEOF gpointer   typedef struct       MonoVTable  vtable      MonoThreadsSync  synchronisation    MonoObject   Unboxing an object in Mono is a process of casting a pointer at an offset of 2 gpointers in the object  e g  16 bytes   A gpointer is a void   This makes sense when looking at the definition of MonoObject as it s clearly just a header for the data  C   To box a value in C   you could do something like   include  lt iostream gt   define Object void   template lt class T gt  Object box T j     return new T j      template lt class T gt  T unbox Object j     T temp     T  j    delete j    return temp     int main       int j 2    Object o   box j     int k   unbox lt int gt  o     std  cout  lt  lt  k

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In the  NET framework  there are two species of types--value types and reference types   This is relatively common in OO languages     One of the important features of object oriented languages is the ability to handle instances in a type-agnostic manner   This is referred to as polymorphism   Since we want to take advantage of polymorphism  but we have two different species of types  there has to be some way to bring them together so we can handle one or the other the same way   Now  back in the olden days  1 0 of Microsoft NET   there weren t this newfangled generics hullabaloo   You couldn t write a method that had a single argument that could service a value type and a reference type   That s a violation of polymorphism   So boxing was adopted as a means to coerce a value type into an object   If this wasn t possible  the framework would be littered with methods and classes whose only purpose was to accept the other species of type   Not only that  but since value types don t truly share a common type ancestor  you d have to have a different method overload for each value type  bit  byte  int16  int32  etc etc etc    Boxing prevented this from happening   And that s why the British celebrate Boxing Day

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Boxing isn t really something that you use - it is something the runtime uses so that you can handle reference and value types in the same way when necessary  For example  if you used an ArrayList to hold a list of integers  the integers got boxed to fit in the object-type slots in the ArrayList   Using generic collections now  this pretty much goes away  If you create a List lt int gt   there is no boxing done - the List lt int gt  can hold the integers directly

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When a method only takes a reference type as a parameter  say a generic method constrained to be a class via the new constraint   you will not be able to pass a reference type to it and have to box it   This is also true for any methods that take object as a parameter - this will have to be a reference type

[c#] Why do we need boxing and unboxing in C#?

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