Understanding Struct, 理解Struct

在.Net 中各种基本类型都是由struct定义，他与引用类型相有一些不同，有自己的特色，这篇BLOG由ECMA335中关于Value Type的定义开始

用Class定义的类型不一定都是引用类型，实际上，值类型就不是(例如：ValueType).用Class定义的ValueType包含了拆箱的值类型和关联的装箱类型. (ECMA335 Partition I, 8.9.7)

当调用ValueType上的非静态方法时, this 指针是指向一个托管实例的引用，他会调用关联的装箱类型的方法.
值类型本身不支持接口，但是他关联的装箱的类型会.
值类型不能继承
装箱类型的基类不应该包含任何字段.
与引用类型不一样，值类型在创建实例时不需要调用构造函数. 默认他会把所有的字段根据类型设置为null或者0.

关于Struct类型使用的一般规则:

1. 不能定义无参数默认构造函数

public struct Employee
    {
        public string Name;
        
        public Employee(string name)
        {
            this.Name = name;
        }
        
        public Employee()
        {
        }
        
        public void Run()
        {
            Console.WriteLine("HI, I am {0}", Name);
        }
    }

错误信息: Structs cannot contain explicit parameterless constructors (Structs 不能包含显示的无参构造)

2. 不能初始化

public struct Employee
    {
        public string Name = "Hello";
    }

错误信息: 'Roger.Testing.Employee.Name': cannot have instance field initializers in structs (Structs中不能包含有初始化后的字段)

3. Read-Only状态不能更改

static readonly Employee emp = new Employee("Roger");
        
        public static void Main()
        {
            emp.Name = "New Roger";
            
            Console.Read();
        }

错误信息: Fields of static readonly field 'Roger.Testing.Testing.emp' cannot be assigned to (except in a static constructor or a variable initializer) readonly类型的Struct不能被赋值, 除非在静态构造函数或者变量初始化时可以修改

例子:

static readonly Employee emp = new Employee("Roger");
        
        public static void Main()
        {            
            
            Console.Read();
        }
        
        static Testing()
        {
            emp.Name = "New Roger";
        }

4. 不能继承

public struct Manager : Employee
    {
        public string Title;
    }

错误信息: 类型'Employee' 在接口列表中但不是接口

5. Struct可以继承无数接口

public struct Manager : IComparable, ICloneable
        {
  
            #region IComparable Members
  
            public int CompareTo(object obj)
            {
                throw new NotImplementedException();
            }
  
            #endregion
  
            #region ICloneable Members
  
            public object Clone()
            {
                throw new NotImplementedException();
            }
  
            #endregion
        }

6. StructLayout

详细解释: http://www.cnblogs.com/happyhippy/articles/717028.html

7. 序列化Struct

public static byte[]
    RawSerialize(object anything)
{
    int rawsize =
        Marshal.SizeOf(anything);
    IntPtr buffer =
        Marshal.AllocHGlobal(rawsize);
    Marshal.StructureToPtr(anything,
        buffer, false);
    byte[] rawdata = new byte[rawsize];
    Marshal.Copy(buffer, rawdata,
        0, rawsize);
    Marshal.FreeHGlobal(buffer);
    return rawdata;
}

8. 调用API

详细解释: http://www.yesky.com/165/1621165.shtml

Struct类型上的方法调用

using System;
using System.Collections.Generic;
  
namespace Roger.Testing
{
    public class Testing
    {    
        public static void Main()
        {
            Employee e = new Employee("www.xwang.org");
            e.Run();
            
            Console.Read();
        }
    }
    
    public struct Employee
    {
        public string Name;
        
        public Employee(string name)
        {
            this.Name = name;
        }
        
        public void Run()
        {
            Console.WriteLine("HI, I am {0}", Name);
        }
    }
}

编译IL

.method public hidebysig static void  Main() cil managed
{
  .entrypoint
  // Code size       26 (0x1a)
  .maxstack  2
  .locals init (valuetype Roger.Testing.Employee V_0)
  IL_0000:  ldloca.s   V_0
  IL_0002:  ldstr      "www.xwang.org"
  IL_0007:  call       instance void Roger.Testing.Employee::.ctor(string)
  IL_000c:  ldloca.s   V_0
  IL_000e:  call       instance void Roger.Testing.Employee::Run()
  IL_0013:  call       int32 [mscorlib]System.Console::Read()
  IL_0018:  pop
  IL_0019:  ret
} // end of method Testing::Main