Boxing and Unboxing

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What is boxing and unboxing and what are the trade offs?

Ok I understand the basic concept of what happens when you box and unbox.

Box throws the value type (stack object) into a System.Object and stores it on the heap Unbox unpackages that object on the heap holding that value type and throws it back on the stack so it can be used.

Here is what I don't understand:

1. Why would this need to be done...specific real-world examples

2. Why is generics so efficient? They say because Generics doesn't need to unbox or box, ok..I don't get why...what's behind that in generics

3. Why is generics better than lets say other types. Lets say for example other collections?

so all in all I don't understand this in application in the real world in terms of code and then going further how it makes generics better...why it doesn't have to do any of this in the first place when using Generics.

Answer 1

1. Boxing needs to be done whenever you want to hold an int in an object variable.
2. A generic collection of ints contains an int[] instead of an object[].
3. Putting an int into the object[] behind a non-generic collection requires you to box the int.
   Putting an int into the int[] behind a generic collection does not invlove any boxing.

Answer 2

Firstly, the stack and heap are implementation details. a value type isnt defined by being on the stack. there is nothing to say that the concept of stack and heap will be used for all systems able to host the CLR: Link

That aside:

when a value type is boxed, the data in that value type is read, an object is created, and the data is copied to the new object. if you are boxing all the items in a collection, this is a lot of overhead.

if you have a collection of value types and are iterating over them, this will happen for each read, then the items are then unboxed (the reverse of the process) just to read a value!!

Generic collections are strongly typed to the type being stored in them, and therefore no boxing or unboxing needs to occur.

Answer 3

Here is a response around the unboxing/boxing portion.

I'm not sure how it is implemented in mono, but generic interfaces will help because the compiler creates a new function of the specific type for each different type used (internally, there are a few cases where it can utilize the same generated function). If a function of the specific type is generated, there is no need to box/unbox the type.

This is why the Collections.Generic library was a big hit at .NET 2.0 because collections no longer required boxing and became significantly more efficient.

In regards to why are generics better then other collections outside the boxing/unboxing scope is that they also force type. No longer can you readily toss a collection around which can hold any type. It can prevent bugs at compile time, versus seeing them at run time.

Answer 4

MSDN has a nice article: Boxing and Unboxing (C# Programming Guide)

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.

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.

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.

Check also: Exploring C# Boxing

And read Jeffrey Richter's Type fundamentals. Here Two sample chapters plus full TOC from Jeffrey Richter's "CLR via C#" (Microsoft Press, 2010) he published some time ago.

Also some notes from Jeffrey Richter's book CLR via C#:

It’s possible to convert a value type to a reference type by using a mechanism called boxing.

Internally, here’s what happens when an instance of a value type is boxed:

1. Memory is allocated from the managed heap. The amount of memory allocated is the size required by the value type’s fields plus the two additional overhead members (the type object pointer and the sync block index) required by all objects on the managed heap.

2. The value type’s fields are copied to the newly allocated heap memory.

3. The address of the object is returned. This address is now a reference to an object; the value type is now a reference type. The C# compiler automatically produces the IL code necessary to box a value type instance, but you still need to understand what’s going on internally so that you’re aware of code size and performance issues.

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Note. It should be noted that the FCL now includes a new set of generic collection classes that make the non-generic collection classes obsolete. For example, you should use the System.Collections.Generic.List class instead of the System.Collections.ArrayList class. The generic collection classes offer many improvements over the non-generic equivalents. For example, the API has been cleaned up and improved, and the performance of the collection classes has been greatly improved as well. But one of the biggest improvements is that the generic collection classes allow you to work with collections of value types without requiring that items in the collection be boxed/unboxed. This in itself greatly improves performance because far fewer objects will be created on the managed heap thereby reducing the number of garbage collections required by your application. Furthermore, you will get compile-time type safety, and your source code will be cleaner due to fewer casts. This will all be explained in further detail in Chapter 12, “Generics.”

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I don't want overquote full chapter here. Read his book and you gain some details on process and receive some answers. And BTW, answer to your question quite a few here on SO, around Web and in many books. It is fundamental knowledge you certainly have to understand.

Answer 5

Here is an interesting read from Eric Lippert (The truth about value types): Link

regarding your statement:

Box throws the value type (stack object) into a System.Object and stores it on the heap Unbox unpackages that object on the heap holding that value type and throws it back on the stack so it can be used.

Answer 6

To put it simple boxing and unboxing takes alot of time. Why - beacuse it's faster to use known type from the start then let this handle for runtime.

1. In colection of objects can contain differnt items : string, int, double, etc. and you must check every time that your operation with variable is corect.
2. Convert from one type to enother takes time.
3. Generic are much faster and encourage you to use them, old collections exist for backward compability

Answer 7

1. This needs to be done because at the IL level there are different instructions for value types than for reference types (ldfld vs ldflda , checkout the dissassembly for a method that calls someValueType.ToString() vs someReferenceType.ToString() and you'll see that the instructions are different).

   These instructions are not compatible so, when you need to pass a value type to a method as an object, that value needs to be wrapped in a reference type (boxing). This is ineficient because the runtime needs to copy the value type and then create a new boxing type in order to pass one value.

2. Generics are faster because value types can be stored as values and not references so no boxing is needed. Take ArrayList vs List<int>. If you want to put 1 into an ArrayList, the CLR needs to box the int so that it can be stored in a object[]. List<T> however, uses a T[] to store the list contents so List uses a int[] which means that 1 doesn't need to be boxed in order to put it in the array.

Answer 8

Suppose I want to store a bunch of variables of type Long in a List, but the system supported neither value-type generics nor boxing. The way to go about storing such values would be to define a new class "BoxedLong", which held a single field "Value" of type Long. Then to add a value to the list, one would create a new instance of a BoxedLong, set its Value field to the desired value, and store that in the list. To retrieve a value from the list, one would retrieve a BoxedLong object from the list, and take the value from its Value field.

When a value type is passed to something that expects an Object, the above is essentially what happens under the hood, except without the new identifier names.

When using generics with value types, the system doesn't use an value-holder class and pass it to routines which expect to work with objects. Instead, the system creates a new version of the routine that will work with the value type in question. If five different value types are passed to a generic routine, five different versions of the routine will be generated. In general, this will yield more code than would the use of a value-holder class, but the code will have to do less work every time a value is passed in or retrieved. Since most routines will have many values of each type passed in or out, the cost of generating different versions of the routine will be more than recouped by the elimination of boxing/unboxing operations.