C#將結構體和指針互轉的方法

1. 功能及位置
    將數據從託管對象封送到非託管內存塊，屬於.NET Framework 類庫
    命名空間：System.Runtime.InteropServices
    程序集：mscorlib（在 mscorlib.dll 中）
                                                                                       
2. 語法                                                                                      
    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. 參數說明                                          
    structure：託管對象，包含要封送的數據。該對象必須是格式化類的實例。
    ptr：指向非託管內存塊的指針，必須在調用此方法以前分配該指針。
    fDeleteOld：設置爲 true 可在執行Marshal.DestroyStructure方法前對 ptr 參數調用此方法。請注意，傳遞 false 可致使內存泄漏。
                                                                                        
4. 異常                                           
    異常類型：ArgumentException
    條件：structrue參數是泛型類型
                                                                                        
5. 備註                                           
    StructureToPtr將結構的內容複製到 ptr 參數指向的預分配內存塊。若是 fDeleteOld 參數爲 true，則使用嵌入指
    針上適當的刪除 API 來刪除最初由 ptr 指向的緩衝區，但該緩衝區必須包含有效數據。此方法爲在鏡像託管類中指
    定的每一個引用字段執行清理工做。
                                                
    假設 ptr 指向非託管內存塊。此內存塊的佈局由相應的託管類 structure 描述。StructureToPtr將字段值從結構封
    送到指針。假設 ptr 塊包含引用字段，該字段指向當前包含「abc」的字符串緩衝區。假設託管端上相應的字段是包含「vwxyz」的字符串。若是不另行通知它，StructureToPtr將分配一個新的非託管緩衝區來保存「vwxyz」，並將它掛鉤到 ptr 塊。這將丟棄舊緩衝區「abc」使之漂移而不將其釋放回非託管堆。最後，您將獲得一個孤立的緩衝區，它表示在代碼中存在內存泄漏。若是將 fDeleteOld 參數設置爲真，則 StructureToPtr 在繼續爲「vwxyz」分配新緩衝區以前釋放保存「abc」的緩衝區。
                                                                                       
6. 舉例    
    定義PERSON結構，並將該結構的一個變量拷貝到非託管內存，再將該內存中的PERSON還原爲PERSON對象，觀察其內容的變化。  

    
源代碼以下：
         
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:指示如何在託管代碼和非託管代碼之間封送數據
        //UnmanagedType:指定如何將參數或字段封送到非託管內存塊
        [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
            {
                //將數據從託管對象封送到非託管內存塊,該內存塊開始地址爲intPtr
                Marshal.StructureToPtr(person, intPtr, true);

                //將數據從非託管內存塊封送到新分配的指定類型的託管對象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. 分析及擴展
    結構體：
    public struct PERSON    //person structure
    {
        //MarshalAs:指示如何在託管代碼和非託管代碼之間封送數據
        //UnmanagedType:指定如何將參數或字段封送到非託管內存塊
        [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. 經過獲取Person信息的函數返回的指針，再將該指針轉換爲結構體，並顯示結構體中的內容
        a. 獲取Person信息的函數
            //獲取 Person信息
            [DllImport("person.dll", CallingConvention = CallingConvention.Cdecl)]
                public static extern IntPtr GetPerson(IntPtr pEngine);
            
        b. 使用指針定義一個變量，來獲取一個返回值爲該指針的函數    
            IntPtr Person = GetPerson(pEngine);
        
        c. 將指針變量裝換爲結構體，操做以下：
           PERSON anotherPerson = (PERSON)Marshal.PtrToStructure(intPtr, typeof(PERSON)); 
           
        d. 經過ShowPerson(PERSON person)函數，用來打印輸出信息
           Console.WriteLine("cardid   :" + Encoding.ASCII.GetString(anotherPerson.identicardid));
           
          
    
    B. 經過定義結構體，並賦值結構體中的數據，再將結構體轉爲指針數據，並將指針傳入到SetPerson信息的函數中
        a. 設置Person信息的函數
            //獲取 Person信息
            [DllImport("person.dll", CallingConvention = CallingConvention.Cdecl)]
                public static extern int SetPerson(IntPtr pEngine);
    
        b. 定義結構體，並賦值結構體中的數據    
            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. 將結構體轉爲指針數據
        
            int nSizeOfPerson = Marshal.SizeOf(person);                 //定義指針長度
            IntPtr personX  = Marshal.AllocHGlobal(nSizeOfPerson);        //定義指針
            Marshal.StructureToPtr(person, personX, true);                //將結構體person轉爲personX指針
            
        d. 將指針傳入到SetPerson函數中    
            SetPerson(personX);