C# basic knowledge compilation: basic knowledge (11) value type, reference type-C#.Net Tutorial-php.cn

C# is an object-oriented language. In object-oriented thinking, there are only objects, and everything can be described by classes. So for example, these, int, bool, char, string, double, long, etc. are all classes, then things like 30, 2.5, "test" are all objects of the corresponding class.

        static void Main(string[] args)
        {
            string istring = 30.ToString();

            string dstring = 2.5.ToString();

            string sstring = "test".ToString();

            Console.WriteLine(string.Format("{0},{1},{2}", istring, dstring, sstring));

            Console.ReadLine();
        }

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Output:

It can be seen that they have the ToString() method, so they are objects.
When writing code in daily life, in addition to the above-mentioned ones, I have definitely used it to define data types:

         static void Main(string[] args)
        {
            Int32 i = 0;

            UInt32 j = 0;

            String str = "test";

            Console.ReadLine();
        }

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This is actually a mechanism of .NET. .NET is a platform. On this platform There are languages like C# and VB. Therefore, .NET defines a series of types that are mapped to different languages. Int32 is int in c#. Such data types are called primitive types. In C#, class objects must be generated using new. This part of the class can be directly represented by constants. Primitive types are defined in the .net Framework under the System namespace. Take a look at the type mapping of primitive types in the C# language.

true/false/System.Bytebyte 0 ~255System.Sbytesbyte-128 ~ 127Signed 8-bit integerSystem.Charchar0 ~ 65,535 Unsigned 16-bit integerSystem.Int16shortSigned 16-bit integerSystem.UInt16ushort0 ~ 65,535Unsigned 16 Bit integerSystem.Int32int-2,147,483,648 ~ 2,147,483,647Signed 32-bit integerSystem.Int64longSigned 64-bit integer System.UInt64ulongUnsigned 64-bit integerSystem.Singlefloat32-bit single precision floating point numberSystem.Doubledouble##±5.0 × 10System.Decimal±1.0 × 10System.StringSystem.UInt32

表中的除了string是引用类型（后面单独解释），其它都是值类型。
下面简单介绍下引用类型和值类型。
学习C语言的时候有个堆和栈的概念。
堆区——程序员分配释放，或者程序结束有OS回收，分配方式类似于链表。
栈区——由编译器自动分配释放，存放函数的参数值，变量值等。
栈内存结构可以快速的分配内存和回收内存，但栈空间有限，过多使用会“溢出”，因此栈只分配常用的，占用空间小的数据类型；堆内存结构分配内存较慢，但是利用空间大，可以存放大型数据。
在C#中，基本上所有的数据都存储在“堆”结构中，称之为“托管堆”，受.NET垃圾回收监控。但是相对于栈堆结构中内存分配效率比较低。为了正确进行垃圾回收，每次分配的堆空间比实际所需空间稍大，小型数据使用堆是不太合适的。
可以比较看一下值类型和引用类型：
C#中提供了Struct定义值类型，直接在栈上分配内存。

 /// <summary>
    /// 使用struct定义一个值类型，
    /// 值类型的只能实现接口，不能继承类
    /// </summary>
    public struct StructPositiveNumber : ICloneable  
    {
        /// <summary>
        /// 值类型字段
        /// </summary>
        private int number;

        /// <summary>
        /// 静态只读字段，作为类的初始值
        /// </summary>
        public readonly static StructPositiveNumber InitialValue = new StructPositiveNumber();

        /// <summary>
        /// 属性
        /// </summary>
        public int Number
        {
            get
            {
                return number;
            }

            set
            {
                if (value <= 0)
                {
                    throw new Exception();
                }

                this.number = value;
            }
        }
        /// <summary>
        /// 可以定义构造器，但是和类不同，这里的默认构造器依然存在
        /// </summary>
        public StructPositiveNumber(int value)
        {
            if (value <= 0)
            {
                throw new Exception();
            }

            this.number = value;
        }

        /// <summary>
        /// 实现克隆方法，返回当前对象
        /// </summary>
        /// <returns></returns>
        public object Clone()
        {
            return new StructPositiveNumber(this.number);
        }
    }

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调用

       static void Main(string[] args)
        {
            //声明变量，赋值
            StructPositiveNumber pNumber1 = StructPositiveNumber.InitialValue;

            pNumber1.Number = 1;

            //pNumber1赋给pNumber2
            StructPositiveNumber pNumber2 = pNumber1;

            //改变pNumber2的值
            pNumber2.Number = 2;

            //看打印结果，改变了pNumber2的值，但是不影响pNumber1
            Console.WriteLine(pNumber1.Number);//1

            Console.WriteLine(pNumber2.Number);//2

            //重新初始化pNumber2,通过构造器生成改变了初始值。
            pNumber2 = new StructPositiveNumber(3);

            Console.WriteLine(pNumber2.Number);//3

            //调用Clone将pNumber2复制给pNumber1
            pNumber1 = (StructPositiveNumber)pNumber2.Clone();

            Console.WriteLine(pNumber1.Number);//3

            //改变pNumber1的值，但是pNumber2值不改变
            pNumber1.Number = 4;

            Console.WriteLine(pNumber1.Number);//4

            Console.WriteLine(pNumber2.Number);//3

            Console.ReadLine();
        }

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结果

再看引用类型定义的：

 public class ClassPositiveNumber : ICloneable
    {
        private int number;

        public int Number
        {
            get
            {
                return this.number;
            }

            set
            {
                if (value <= 0)
                {
                    throw new Exception();
                }

                this.number = value;
            }
        }

        //引用类型自己可以初始化为null,无需定义初始值
        //public readonly static ClassPositiveNumber InitialValue = new ClassPositiveNumber();

        public ClassPositiveNumber(int value)
        {
            if (value <= 0)
            {
                throw new Exception();
            }

            this.number = value;
        }

        public object Clone()
        {
            return new ClassPositiveNumber(this.number);
        }
    }

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调用

      static void Main(string[] args)
        {
            ClassPositiveNumber cNumber1;//默认值为null

            cNumber1 = new ClassPositiveNumber(1);

            ClassPositiveNumber cNumber2 = cNumber1;

            cNumber2.Number = 2;

            //可以看出，两个引用引用到了相同的对象
            Console.WriteLine(cNumber1.Number);//2

            Console.WriteLine(cNumber2.Number);//2

            //重新初始化cNumber2,之前的对象已被丢弃
            cNumber2 = new ClassPositiveNumber(3);

            Console.WriteLine(cNumber2.Number);//3
            
            //复制是复制一个对象的副本，因此，是两个不同的对象
            cNumber1 = (ClassPositiveNumber)cNumber2.Clone();

            Console.WriteLine(cNumber1.Number);//3

            cNumber1.Number = 4;

            Console.WriteLine(cNumber1.Number);//4

            Console.WriteLine(cNumber2.Number);//3

            Console.ReadLine();
        }

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结果

通过例子，可以看出值类型的特点如下：
a、使用struct声明；
b、不能继承类，但是可以实现接口（当然除object类外）；
c、值类型使用值类型做为字段，但是字段无法有默认值；
c、值类型中必须有默认构造器，而且自己定义构造器后，默认的无参数的构造器依然存在。而且在构造其中只能访问类中的字段，但是不能访问属性。符号=对于值类型来说是赋值，所以赋值是值类型变量不能为空，因为值类型没有引用的概念，肯定有值。

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C#，基础知识整理，值类型和引用类型

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.NET Framework primitive type	C# type	Value range	Remarks
##System.Boolean	bool
			Unsigned 8-bit integer


		-32,768 ~ 32,767


		-9,223,372,036,854,775,808 ~ 9,223,372,036,854,775,807
		0 ~ 18,446,744,073,709,551,615
		±1.5 × 10 -45^{~ ±3.4 × 10}38 (7 significant digits)
		-324 to ±1.7 × 10³⁰⁸ (15 to 16 significant digits) 64-bit double precision floating point
decimal		-28 to ±7.9 × 10²⁸ (27 to 28 significant digits) 128-bit floating point number
string	Any string	/
uint	0 ~ 4,294,967,295	Unsigned 32-bit integer