C # Boxing and unpacking knowledge collation

Source: Internet
Author: User
1, boxing and unpacking is an abstract concept
2, boxing is to convert a value type to a reference type;

Unboxing is converting a reference type to a value type

With boxing and unboxing, you can link value types to reference types by allowing any value of a value type to be converted to a value of type Object

For example:

int val = 100; Object obj = val; Console.WriteLine ("Value of object = {0}", obj);

This is a boxed procedure, which is the process of converting a value type to a reference type

int val = 100; Object obj = val; int num = (int) obj; Console.WriteLine ("Num: {0}", num);

This is a process of unpacking, converting a value type to a reference type, and then converting from a reference type to a value type.

Note: Objects that have been loaded in the box can be removed
3,. NET, the data type is divided into a value type and a reference (not a pointer to C + +) type, corresponding to this, memory allocation is divided into two ways, one for the stack, and two for the heap (note: is the managed heap)
Value types are only allocated in the stack.
The reference type allocates memory to the managed heap.
The managed heap corresponds to garbage collection.

4: What is packing/unpacking?
Boxing: Used to store value types in the garbage collection heap. Boxing is an implicit conversion of a value type to an object type or to any interface type implemented by this value type.
Unboxing: an explicit conversion from an object type to a value type or from an interface type to a value type that implements the interface.

5: Why do I need to pack? (Why do you want to convert a value type to a reference type?) )
One of the most common scenarios is to invoke a method with a parameter of type object that can support any type for general purpose. Boxing is required when you need to pass in a value type (such as Int32).
Another usage is that a non-generic container is also defined as an object in order to guarantee common use. Therefore, boxing is required to add value type data to the container.

6: Internal operation of packing/unpacking

Assigns an object instance to a value type in the heap and copies the value to the new object. Proceed as three steps.

The newly allocated managed heap memory (size is the value type instance size plus one method table pointer and one syncblockindex).
Copies an instance field of a value type to the newly allocated memory.
Returns the address of the newly allocated object in the managed heap. This address is a reference to an object.
Some people understand this: if the Int32 is boxed, the returned address is pointing to a Int32. I don't think it's possible to understand this, but it does have a problem, as it is not comprehensive, and pointing to Int32 does not say its substance (in the managed heap).

Check the object instance to make sure it is a boxed value of the given value type. Copy the value from the instance to the value type variable.
In the book, unpacking just gets a pointer to the part of the value type in the Reference object, and the content copy is the trigger of the assignment statement. I don't think it matters. The key is to check the nature of the object instance, unpacking and boxing type must match, this point, on the IL layer, do not see the principle, my guess, perhaps called a similar gettype and other methods to remove the type to match (because of the need for strict matching).

7: Effect of packing/unpacking on execution efficiency

Obviously, it can be seen from the principle that when boxing occurs, a new reference object is generated, which can lead to a loss of time, which results in less efficiency.
So how do we do that?
First, you should try to avoid boxing.
For example, the two cases of Example 2, can be avoided, in the first case, can be avoided by overloading the function. In the second case, it can be avoided by generics.
Of course, everything is not absolute, assuming you want to modify the code for a third-party assembly, you cannot change, then you can only be boxed.
For the optimization of packing/unpacking code, because C # is implicit in boxing and unboxing, the fundamental approach is to analyze the code, and the most straightforward way to analyze it is to understand the principle of how to view the anti-compilation Il code.

For example, there may be excess boxing in the body of the loop, and you can simply use the pre-packing method to optimize it.

8: Further understanding of packing/unpacking

Packing/unpacking is not as straightforward as it says

For example: When boxing, become a reference object, will be more than a method table pointer, what is the use of it?

We can use the example to further explore.

As an example:

Struct a:icloneable{public Int32 x;public override String ToString () {return String.Format ("{0}", x);} public Object Clone () {return MemberwiseClone ();}} static void Main () {a A; a.x = 100; Console.WriteLine (A.tostring ()); Console.WriteLine (A.gettype ()); A a2 = (a) a.clone (); ICloneable C = A2; Ojbect o = C.clone (); }

A.tostring (). The compiler finds that a overrides the ToString method and calls the ToString Directive directly. Because A is a value type, the compiler does not show polymorphic behavior. Therefore, it is called directly, not boxed. (Note: ToString is the base class of a System.ValueType method)
A.gettype (), GetType is a method inherited from System.ValueType, to invoke it, a method table pointer is required, and a is boxed to generate a method table pointer that invokes the System.ValueType of the base class. (in other words, all value types are inherited from System.ValueType).
A.clone (), because a implements the Clone method, so no boxing is required.
ICloneable Transformation: When A2 is converted to an interface type, it must be boxed because the interface is a reference type.
C.clone (). No boxing is required, and the previous boxed object is called in the managed heap.
Attached: In fact, the above is based on a fundamental principle, because the unboxed value type does not have a method table pointer, so the virtual method inherited on it cannot be called by value type. In addition, the interface type is a reference type. To this, I understand that the method table pointers are similar to C + + virtual function table pointers, which is an important basis for implementing the polymorphic mechanism of referencing objects.

9: How to change the boxed object

For a boxed object, because the specified method cannot be called directly, the container must be disassembled before the method is called, but the new stack instance is generated, and the boxed object cannot be modified. A little dizzy, feel the tongue twister. Or For example: (Append the Change method in the example above)

public void Change (Int32 x) {this.x = x;}


A = new A (); a.x = 100; Object o = A; Boxed into O, below, to change the value of O ((A) o). Change (200); Did you get rid of it?
The reason for this is that o when unpacking, a temporary stack instance A is generated, so the changes are based on temporary a, and are not changed to the boxed object.

(Attached: in managed C + +, it is possible to directly change the instance reference from the first step when the unboxing is taken, but C # does not.) )
What's the best way to do that?
Well, the same effect can be achieved by means of an interface.
The implementation is as follows:

Interface Ichange {void change (Int32 x);} struct A:ichange {...}


((Ichange) o). Change (200);//Did you get rid of it? Got rid of

Why can we change now?

When O is converted to Ichange, it will not be boxed again, certainly not unpacking, because O is already a reference type, and since it is a ichange type, you can call change directly, so the field in the boxed object is changed to achieve the desired effect.

10. To convert a value type to a reference type, a boxing operation is required (boxing):

First allocates memory from the managed heap for the newly generated reference object
Then copy the data of the value type into the memory you just allocated
Returns the address of a newly allocated object in the managed heap
As you can see, the two comparisons that are performed to allocate memory and copy data in a single boxing operation that affect performance.

Converting a reference type to a value type requires a unboxing operation (unboxing):

The first step is to get the address of the part of the managed heap that belongs to the value type, which is a strict unboxing.
Copies the values in the Reference object to a value type instance that is on the thread stack.
After these 2 steps, it can be thought that the same boxing is the reciprocal operation. The strict unboxing does not affect performance, but the subsequent operation of the copied data will affect performance as well as the boxing operation.


NET all types are inherited by the base class System.Object, including the most commonly used underlying types: int, byte, Short,bool, and so on, meaning that all things are objects.

It is extremely inefficient to declare that these types are allocated memory in the heap at all times! (The reason and the difference between heap and stack will be said separately in another article!) )
. NET how to solve this problem? It is by dividing the type into values (value) and reference type (Regerencetype),

Value types and reference types defined in C #

Value types: Primitive Type (Sbyte, Byte, short, Ushort, Int, Uint, Long, Ulong, Char, Float, Double, Bool, Decimal), enumeration (enum), struct (struct)
Reference types: Classes, arrays, interfaces, delegates, strings, and so on
The value type is to allocate memory in the stack, initialize it at the same time as stated, to ensure that the data is not null;
The reference type is to allocate memory in the heap, initialize to NULL, the reference type needs garbage collection to reclaim the memory, the value type does not need, exceeds the scope, the system will automatically release!
Here is the definition of boxing and unpacking!
Boxing is the implicit conversion of a value type to a reference type object. Like what:

int i=0; Syste.object obj=i;

This process is boxing! is to pack I!
Unpacking is the conversion of a reference object into any value type! Like what:

int i=0; System.Object obj=i;int j= (int) obj;

The first 2 sentences of this process are the I boxing, the latter sentence is to remove obj box!

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