C#将结构体和指针互转的方法

1. 功能及位置
    将数据从托管对象封送到非托管内存块，属于.NET Framework 类库
    命名空间：System.Runtime.InteropServices
    程序集：mscorlib（在 mscorlib.dll 中）
                                                                                       
2. 语法                                                                                      
    C#：
        [ComVisibleAttribute(true)] public static void StructureToPtr (Object structure,IntPtr ptr,bool fDeleteOld);
    C++：
        [ComVisibleAttribute(true)]public: static void StructureToPtr (Object^ structure, IntPtr ptr, bool fDeleteOld);
                                                                                        
3. 参数说明                                          
    structure：托管对象，包含要封送的数据。该对象必须是格式化类的实例。
    ptr：指向非托管内存块的指针，必须在调用此方法之前分配该指针。
    fDeleteOld：设置为 true 可在执行Marshal.DestroyStructure方法前对 ptr 参数调用此方法。请注意，传递 false 可导致内存泄漏。
                                                                                        
4. 异常                                           
    异常类型：ArgumentException
    条件：structrue参数是泛型类型
                                                                                        
5. 备注                                           
    StructureToPtr将结构的内容复制到 ptr 参数指向的预分配内存块。如果 fDeleteOld 参数为 true，则使用嵌入指
    针上适当的删除 API 来删除最初由 ptr 指向的缓冲区，但该缓冲区必须包含有效数据。此方法为在镜像托管类中指
    定的每个引用字段执行清理工作。
                                                
    假设 ptr 指向非托管内存块。此内存块的布局由相应的托管类 structure 描述。StructureToPtr将字段值从结构封
    送到指针。假设 ptr 块包含引用字段，该字段指向当前包含“abc”的字符串缓冲区。假设托管端上相应的字段是包含“vwxyz”的字符串。如果不另行通知它，StructureToPtr将分配一个新的非托管缓冲区来保存“vwxyz”，并将它挂钩到 ptr 块。这将丢弃旧缓冲区“abc”使之漂移而不将其释放回非托管堆。最后，您将得到一个孤立的缓冲区，它表示在代码中存在内存泄漏。如果将 fDeleteOld 参数设置为真，则 StructureToPtr 在继续为“vwxyz”分配新缓冲区之前释放保存“abc”的缓冲区。
                                                                                       
6. 举例    
    定义PERSON结构，并将该结构的一个变量拷贝到非托管内存，再将该内存中的PERSON还原为PERSON对象，观察其内容的变化。  

    
源代码如下：
         
using System;
using System.Text;
using System.Runtime.InteropServices;

namespace testStructureToPtr
{
    public static class define  //define some constant
    {        
        public const int MAX_LENGTH_OF_IDENTICARDID = 20;   //maximum length of identicardid
        public const int MAX_LENGTH_OF_NAME = 50;           //maximum length of name
        public const int MAX_LENGTH_OF_COUNTRY = 50;        //maximum length of country
        public const int MAX_LENGTH_OF_NATION = 50;         //maximum length of nation
        public const int MAX_LENGTH_OF_BIRTHDAY = 8;        //maximum length of birthday
        public const int MAX_LENGTH_OF_ADDRESS = 200;       //maximum length of address
    }

    public struct PERSON    //person structure
    {
        //MarshalAs:指示如何在托管代码和非托管代码之间封送数据
        //UnmanagedType:指定如何将参数或字段封送到非托管内存块
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_IDENTICARDID)]
        public byte[] identicardid;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_NAME)]
        public byte[] name;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_COUNTRY)]
        public byte[] country;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_NATION)]
        public byte[] nation;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_BIRTHDAY)]
        public byte[] birthday;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_ADDRESS)]
        public byte[] address;
    }

    class testProgram
    {
        private static byte _fillChar = 0;      //the fill character

        //convert string to byte array in Ascii with length is len        
        public static byte[] CodeBytes(string str, int len)
        {
            if (string.IsNullOrEmpty(str))
            {
                str = string.Empty;
            }
 
            byte[] result = new byte[len];
            byte[] strBytes = Encoding.Default.GetBytes(str);

            //copy the array converted into result, and fill the remaining bytes with 0
            for (int i = 0; i < len; i++)
                result[i] = ((i < strBytes.Length) ? strBytes[i] : _fillChar);
            
            return result;
        }

        //show the person information
        public static void ShowPerson(PERSON person)
        {
            Console.WriteLine("cardid   :" + Encoding.ASCII.GetString(person.identicardid));
            Console.WriteLine("name     :" + Encoding.ASCII.GetString(person.name));
            Console.WriteLine("country  :" + Encoding.ASCII.GetString(person.country));
            Console.WriteLine("nation   :" + Encoding.ASCII.GetString(person.nation));
            Console.WriteLine("birthday :" + Encoding.ASCII.GetString(person.birthday));
            Console.WriteLine("address  :" + Encoding.ASCII.GetString(person.address));
        }

        static void Main(string[] args)
        {
            PERSON person;
            person.identicardid = CodeBytes("123456198001011111", define.MAX_LENGTH_OF_IDENTICARDID);
            person.name = CodeBytes("jackson", define.MAX_LENGTH_OF_NAME);
            person.country = CodeBytes("China", define.MAX_LENGTH_OF_COUNTRY);
            person.nation = CodeBytes("HanZu", define.MAX_LENGTH_OF_NATION);
            person.birthday = CodeBytes("19800101", define.MAX_LENGTH_OF_BIRTHDAY);
            person.address = CodeBytes("Luoshan Road, Shanghai", define.MAX_LENGTH_OF_ADDRESS);

            int nSizeOfPerson = Marshal.SizeOf(person);
            IntPtr intPtr = Marshal.AllocHGlobal(nSizeOfPerson);
            
            Console.WriteLine("The person infomation is as follows:");
            ShowPerson(person);

            try
            {
                //将数据从托管对象封送到非托管内存块,该内存块开始地址为intPtr
                Marshal.StructureToPtr(person, intPtr, true);

                //将数据从非托管内存块封送到新分配的指定类型的托管对象anotherPerson
                PERSON anotherPerson = (PERSON)Marshal.PtrToStructure(intPtr, typeof(PERSON));

                Console.WriteLine("The person after copied is as follows:");
                ShowPerson(anotherPerson);
            }
            catch (ArgumentException)
            {
                throw;
            }
            finally
            {
                Marshal.FreeHGlobal(intPtr);    //free tha memory
            }
        }
    }
}

7. 分析及扩展
    结构体：
    public struct PERSON    //person structure
    {
        //MarshalAs:指示如何在托管代码和非托管代码之间封送数据
        //UnmanagedType:指定如何将参数或字段封送到非托管内存块
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_IDENTICARDID)]
        public byte[] identicardid;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_NAME)]
        public byte[] name;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_COUNTRY)]
        public byte[] country;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_NATION)]
        public byte[] nation;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_BIRTHDAY)]
        public byte[] birthday;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = define.MAX_LENGTH_OF_ADDRESS)]
        public byte[] address;
    }
    
    A. 通过获取Person信息的函数返回的指针，再将该指针转换为结构体，并显示结构体中的内容
        a. 获取Person信息的函数
            //获取 Person信息
            [DllImport("person.dll", CallingConvention = CallingConvention.Cdecl)]
                public static extern IntPtr GetPerson(IntPtr pEngine);
            
        b. 使用指针定义一个变量，来获取一个返回值为该指针的函数    
            IntPtr Person = GetPerson(pEngine);
        
        c. 将指针变量装换为结构体，操作如下：
           PERSON anotherPerson = (PERSON)Marshal.PtrToStructure(intPtr, typeof(PERSON)); 
           
        d. 通过ShowPerson(PERSON person)函数，用来打印输出信息
           Console.WriteLine("cardid   :" + Encoding.ASCII.GetString(anotherPerson.identicardid));
           
          
    
    B. 通过定义结构体，并赋值结构体中的数据，再将结构体转为指针数据，并将指针传入到SetPerson信息的函数中
        a. 设置Person信息的函数
            //获取 Person信息
            [DllImport("person.dll", CallingConvention = CallingConvention.Cdecl)]
                public static extern int SetPerson(IntPtr pEngine);
    
        b. 定义结构体，并赋值结构体中的数据    
            PERSON person = new PERSON();
            person.identicardid = CodeBytes("123456198001011111", define.MAX_LENGTH_OF_IDENTICARDID);
            person.name = CodeBytes("jackson", define.MAX_LENGTH_OF_NAME);
            person.country = CodeBytes("China", define.MAX_LENGTH_OF_COUNTRY);
            person.nation = CodeBytes("HanZu", define.MAX_LENGTH_OF_NATION);
            person.birthday = CodeBytes("19800101", define.MAX_LENGTH_OF_BIRTHDAY);
            person.address = CodeBytes("Luoshan Road, Shanghai", define.MAX_LENGTH_OF_ADDRESS);
        
        c. 将结构体转为指针数据
        
            int nSizeOfPerson = Marshal.SizeOf(person);                 //定义指针长度
            IntPtr personX  = Marshal.AllocHGlobal(nSizeOfPerson);        //定义指针
            Marshal.StructureToPtr(person, personX, true);                //将结构体person转为personX指针
            
        d. 将指针传入到SetPerson函数中    
            SetPerson(personX);