结构或者类中的string进行封送时长度缺失的原因及解决方案

在数据通信或者调用C/C++的DLL时，会用到结构或类的封送(C#调用C++DLL传递结构体数组的终极解决方案)，但是当结构或者类中用到string类型时，封送的数据会出现缺失。下面是以类的封送转换来举例。代码如下

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;

namespace StringLayoutTest
{
    class Program
    {
        static void Main(string[] args)
        {
            TestStructToBytes();    
        }
    
        private static void TestStructToBytes()
        {
            TestStruct testStruct = new TestStruct();
            testStruct.name = "ABC";
            Console.WriteLine("Input={0}",testStruct.name);
            int testStructLen = Marshal.SizeOf(typeof(TestStruct));
            Console.WriteLine("Struct Len={0}",testStructLen);
            byte[] testStructBytes = structToBytes(testStruct);
            Console.WriteLine("Data Len={0},Data={1}",
                            testStructBytes.Length,
                            Encoding.UTF8.GetString(testStructBytes));
            foreach (byte item in testStructBytes)
            {
                Console.WriteLine("byte={0},char={1}", item, (char)item);
            }
            Console.ReadLine();
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
        public sealed class TestStruct
        {
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 3)]
            public string name = "";
        }
      
        public static byte[] structToBytes(object obj)
        {
            int size = Marshal.SizeOf(obj);//Get size of struct or class.            
            byte[] bytes = new byte[size];
            IntPtr structPtr = Marshal.AllocHGlobal(size);//Allocate memory space of the struct or class.            
            Marshal.StructureToPtr(obj, structPtr, false);//Copy struct or class to the memory space.            
            Marshal.Copy(structPtr, bytes, 0, size);//Copy memory space to byte array.           
            Marshal.FreeHGlobal(structPtr);//Release memory space.           
            return bytes;
        }
    }
}

运行结果见图1

图1

我们输入的是ABC，但经过封送后却变成了AB。再看封送后展开的字节，会发现第一个字节是65（A)，第二个字节是66(B)，第三个字节为0(空)，这里其实三个字节都已经封送了，只是最后一个字节变成了0，也就是结束符''，只是结束符输出时是空的。

所以封送的字节数还是3个，只是因为最后一个字节会默认是结束符''，这样真正放数据的长度就相当于少了一位，也就是只有2位，自然数据也就只有前两个字节了。

那如果从字节转换成结构或者类（相当于通过DLL调用得到了数据，然后要转换成所要的数据结构或者数据类），是不是也有同样的问题呢？为此，也作了测试，代码如下

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;

namespace StringLayoutTest
{
    class Program
    {
        static void Main(string[] args)
        {           
            TestBytesToStruct();          
        }

        private static void TestBytesToStruct()
        {
            byte[] testStructBytes = new byte[3];
            testStructBytes[0] = 65;//A
            testStructBytes[1] = 66;//B
            testStructBytes[2] = 67;//C
            Console.WriteLine("Input={0}", Encoding.UTF8.GetString(testStructBytes));
            TestStruct testStruct = (TestStruct)bytesToStruct(testStructBytes, typeof(TestStruct));
            Console.WriteLine("Len={0},Data={1}", testStruct.name.Length, testStruct.name);
            Console.ReadLine();
        } 

       

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
        public sealed class TestStruct
        {
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 3)]
            public string name = "";
        }
    

        public static object bytesToStruct(byte[] bytes, Type type, int startIndex = 0)
        {

            int size = Marshal.SizeOf(type);//Get size of the struct or class.          
            if (bytes.Length < size)
            {
                return null;
            }
            IntPtr structPtr = Marshal.AllocHGlobal(size);//Allocate memory space of the struct or class. 
            Marshal.Copy(bytes, startIndex, structPtr, size);//Copy byte array to the memory space.
            object obj = Marshal.PtrToStructure(structPtr, type);//Convert memory space to destination struct or class.         
            Marshal.FreeHGlobal(structPtr);//Release memory space.    
            return obj;
        }
    }
}

运行结果见图2

图2

我们输入的是ABC（代码中已经将字节数组testStructBytes赋值为ABC），但转换成类之后，却只变成了AB，原因还是因为最后一个字节被处理成了结束符。

那这个问题要如何解决呢？

方案一：

在结构或者中定义string的封送长度时多加1字节的长度（相当于在C/C++中定义char字符串时，需要多一个字节的结束位），然后进行封送。不过，这可能会引发另一 个问题。因为我实际字串长度就是3，而封送的时为了给结束符留1个字节就需要4字节的长度，这样一来，长度的控制上就有可能出问题。那有没有更好的方案呢？可以看方案二。

方案二：

不采用string来封送数据，而是使用byte数组。比如，上面的的问题，我们可以按下面的方式来解决。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;

namespace StringLayoutTest
{
    class Program
    {
        static void Main(string[] args)
        {
            //TestStructToBytes();
             //TestBytesToStruct();
           TestBytesToStruct2();
        }

        private static void TestBytesToStruct()
        {
            byte[] testStructBytes = new byte[3];
            testStructBytes[0] = 65;//A
            testStructBytes[1] = 66;//B
            testStructBytes[2] = 67;//C
            Console.WriteLine("Input={0}", Encoding.UTF8.GetString(testStructBytes));
            TestStruct testStruct = (TestStruct)bytesToStruct(testStructBytes, typeof(TestStruct));
            Console.WriteLine("Len={0},Data={1}", testStruct.name.Length, testStruct.name);
            Console.ReadLine();
        }

        private static void TestBytesToStruct2()
        {
            byte[] testStructBytes = new byte[3];
            testStructBytes[0] = 65;//A
            testStructBytes[1] = 66;//B
            testStructBytes[2] = 67;//C
            Console.WriteLine("Input={0}", Encoding.UTF8.GetString(testStructBytes));
            TestStruct2 testStruct = (TestStruct2)bytesToStruct(testStructBytes, typeof(TestStruct2));
            Console.WriteLine("Len={0},Data={1}", testStruct.name.Length, Encoding.UTF8.GetString(testStruct.name));
            Console.ReadLine();
        }

        private static void TestStructToBytes()
        {
            TestStruct testStruct = new TestStruct();
            testStruct.name = "ABC";
            Console.WriteLine("Input={0}",testStruct.name);
            int testStructLen = Marshal.SizeOf(typeof(TestStruct));
            Console.WriteLine("Struct Len={0}",testStructLen);
            byte[] testStructBytes = structToBytes(testStruct);
            Console.WriteLine("Data Len={0},Data={1}",
                            testStructBytes.Length,
                            Encoding.UTF8.GetString(testStructBytes));
            foreach (byte item in testStructBytes)
            {
                Console.WriteLine("byte={0},char={1}", item, (char)item);
            }
            Console.ReadLine();
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
        public sealed class TestStruct
        {
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 3)]
            public string name = "";
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
        public sealed class TestStruct2
        {
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)]
            public byte[] name;
        }

        public static byte[] structToBytes(object obj)
        {
            int size = Marshal.SizeOf(obj);//Get size of struct or class.            
            byte[] bytes = new byte[size];
            IntPtr structPtr = Marshal.AllocHGlobal(size);//Allocate memory space of the struct or class.            
            Marshal.StructureToPtr(obj, structPtr, false);//Copy struct or class to the memory space.            
            Marshal.Copy(structPtr, bytes, 0, size);//Copy memory space to byte array.           
            Marshal.FreeHGlobal(structPtr);//Release memory space.           
            return bytes;
        }

        public static object bytesToStruct(byte[] bytes, Type type, int startIndex = 0)
        {

            int size = Marshal.SizeOf(type);//Get size of the struct or class.          
            if (bytes.Length < size)
            {
                return null;
            }
            IntPtr structPtr = Marshal.AllocHGlobal(size);//Allocate memory space of the struct or class. 
            Marshal.Copy(bytes, startIndex, structPtr, size);//Copy byte array to the memory space.
            object obj = Marshal.PtrToStructure(structPtr, type);//Convert memory space to destination struct or class.         
            Marshal.FreeHGlobal(structPtr);//Release memory space.    
            return obj;
        }
    }
}

运行结果

输入和转换后的结果都ABC，达到了预期。不过这里有一个问题，因为封送的是字节数组，所以必须要转换成字符串。而在转换成字符串时会有编码的问题，一般使用UTF-8是可以的，但也不排除一些其他的情况。

结论：

在.NET中对类或者结构进行封送时，要特别注意string的封送，具体要采用byte数组来替换还是增加1字节的结束符，要看具体应用而定。