C ＃ basic 3-value type and reference type, packing and unpacking

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type

C# Is a strongly typed static language , When declaring variables, you need to determine the type of variables , The program runs after being compiled by the compiler

type To determine the ** 1. The amount of space the variable occupies in memory **, And then determined ** 2. Value range of variable **, for example sbyte It means signed One byte size ( 8 Bit binary bit ) The integer of , The value range is -128 to 127

The value range can be referred to

3. Determine the inheritance relationship of the type type 4. Types also affect program runtime , Is the variable stored in heap or stack ; What operators can variables use += -= Operators, etc.

Value type

Simple value types – Integer types 、 Character type 、 Real number type 、 Boolean type

Composite value type – Type of structure 、 Enumeration type

Value type And The difference between reference types

1. Value type variables are stored on the stack , Directly store the data corresponding to the variable in the stack ; Reference type variables are stored on the stack , The data corresponding to the variable is represented as an address in the heap , This address represents the heap space where , Is the data of the reference object .
2. Fast access to value types , Slow access of reference type .
3. Value type variables represent actual data , Reference type variables represent pointers or references to data stored in the heap .
4. Stack memory is automatically released , Heap memory consists of .NET GC Automatic release .
5. Value types inherit from System.ValueType , Reference types inherit from System.Object.

Packing and unpacking

Boxing is to convert the value type to object Type or any interface type implemented by this value type

Packing ： Convert value type variables into reference type variables

Unpacking ： Convert reference type variables into value type variables

Type conversion

1. (int)1.2f
2. int.Parse()
3. var m = o as Mammal;
4. Convert.ToString(value, baseValue)

Common container classes

ArrayList Variable length array

List<T> Generic linked list Fast access , Modification speed is slow

HashTable/Dictionary Hash table / Dictionary format High query efficiency It takes up a lot of space

Stack Stack Last in, first out

Queue queue fifo

ref and out The difference between

ref and out You can make the function have multiple returned values

Both keywords are parameters that decorate the function , It means that the function parameter is passed by reference instead of value copy , among out The passed reference only brings back the return value , There is no need to declare variables in advance

for forEach enumerator.MoveNext Memory consumption of the loop

for The loop can be traversed sequentially by index ,foreach and Enumerator.MoveNext Iterate through , Memory consumption doesn't make much difference in essence

Be careful ： Use forEach The element being traversed cannot be deleted when looping , After traversal , Delete the corresponding index or key The elements of