当传递给非托管C DLL函数时，C＃结构变量保持为空

问题：

我正在为非托管C DLL编写C＃包装器。DLL包含一个结构zint_symbol，其中包含一个变量char[] *bitmap。在我的C＃结构中，我拥有public byte[] bitmap;。

这个结构在外部函数传递ZBarcode_Encode_and_Buffer这是应该以呈现PNG图像和位图数据写入到bitmap在zint_symbol。

当我调用ZBarcode_Encode_and_Buffer并检查时bitmap，它仍然为null。

实际上，我的C＃结构中应该由DLL写入的每个变量都保持为null。

我尝试过的

1. 编组bitmap为具有很大常数的固定大小的数组，即

   [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U1, SizeConst = 25454)]
   public byte[] bitmap; 
   

   ...产生一个SystemAccessViolation：

   An unhandled exception of type 'System.AccessViolationException' occurred in mscorlib.dll
   
   Additional information: Attempted to read or write protected memory. This is often an indication     that other memory is corrupt.
   

2. 将类型更改ibtmap为char[]和string。这什么都没改变。

3. 将[In,Out]装饰器添加到我要传递给C＃函数的struct参数中。

4. 将类型bitmap从更改byte[]为IntPtr。

我不能做的是：

尽管有资源，但我似乎无法编译该库（该项目是开源的，但几年前已被放弃）。这意味着我无法更改C代码。

代码/工作示例

一个小的但完整的工作示例可以在这里下载。

包含我的结构的C＃类：

class ZintLib
{
    public struct zint_symbol
    {
        public int symbology;

        public int height;
        public int whitespace_width;
        public int border_width;

        public int output_options;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
        public string fgcolour;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
        public string bgcolour;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
        public string outfile;

        public float scale;
        public int option_1;
        public int option_2;
        public int option_3;
        public int show_hrt;

        public int input_mode;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
        public string text;

        public int rows;
        public int width;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
        public string primary;

        [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U1, SizeConst = 25454)]
        public byte[] encoded_data;

        [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.I4, SizeConst = 178)]
        public int[] row_height;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 100)]
        public string errtxt;

        public byte[] bitmap;

        public int bitmap_width;
        public int bitmap_height;
        public IntPtr rendered;
    }

    [DllImport("zint.dll", EntryPoint = "ZBarcode_Create", CallingConvention = CallingConvention.Cdecl)]
    public extern static IntPtr Create();

    [DllImport("zint.dll", EntryPoint = "ZBarcode_Encode_and_Buffer", CallingConvention = CallingConvention.Cdecl)]
    public extern static int EncodeAndBuffer(
      [In, Out] ref zint_symbol symbol,
     String input,
     int length,
     int rotate_angle);
}

调用EncodeAndBuffer的函数：

        // call DLL function to generate pointer to initialized struct
        ZintLib.zint_symbol s = (ZintLib.zint_symbol)

        // generate managed counterpart of struct
        Marshal.PtrToStructure(ZintLib.Create(), typeof(ZintLib.zint_symbol));

        // change some settings
        s.symbology = 71;
        s.outfile = "datamatrix.png";

        // DLL function call to generate output file using changed settings -- WORKS --
        //System.Console.WriteLine(ZintLib.EncodeAndPrint(ref s, "12345", 5, 0));

        // DLL function to generate data in s.bitmap, s.bitmapheight, s.bitmapwidth -- DOES NOT WORK managed struct is unaltered --
        System.Console.WriteLine(ZintLib.EncodeAndBuffer(ref s, (String)"12345", 5, 0));

        if (s.bitmap == null)
            Console.WriteLine("bitmap is null.");
        else
            Console.WriteLine("bitmap is not null.");

C语言中的结构和导出方法：

struct zint_symbol {
    int symbology;
    int height;
    int whitespace_width;
    int border_width;
    int output_options;
    char fgcolour[10];
    char bgcolour[10];
    char outfile[256];
    float scale;
    int option_1;
    int option_2;
    int option_3;
    int show_hrt;
    int input_mode;
    unsigned char text[128];
    int rows;
    int width;
    char primary[128];
    unsigned char encoded_data[178][143];
    int row_height[178]; /* Largest symbol is 177x177 QR Code */
    char errtxt[100];
    char *bitmap;
    int bitmap_width;
    int bitmap_height;
    struct zint_render *rendered;
};

#if defined(__WIN32__) || defined(_WIN32) || defined(WIN32) || defined(_MSC_VER)
#  if defined (DLL_EXPORT) || defined(PIC) || defined(_USRDLL)
#    define ZINT_EXTERN __declspec(dllexport)
#  elif defined(ZINT_DLL)
#    define ZINT_EXTERN __declspec(dllimport)
#  else
#    define ZINT_EXTERN extern
#  endif
#else
#  define ZINT_EXTERN extern    
#endif

ZINT_EXTERN struct zint_symbol *ZBarcode_Create(void);
ZINT_EXTERN void ZBarcode_Clear(struct zint_symbol *symbol);
ZINT_EXTERN void ZBarcode_Delete(struct zint_symbol *symbol);

ZINT_EXTERN int ZBarcode_Encode(struct zint_symbol *symbol, unsigned char *input, int length);
ZINT_EXTERN int ZBarcode_Encode_File(struct zint_symbol *symbol, char *filename);
ZINT_EXTERN int ZBarcode_Print(struct zint_symbol *symbol, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_and_Print(struct zint_symbol *symbol, unsigned char *input, int length, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_File_and_Print(struct zint_symbol *symbol, char *filename, int rotate_angle);

ZINT_EXTERN int ZBarcode_Render(struct zint_symbol *symbol, float width, float height);

ZINT_EXTERN int ZBarcode_Buffer(struct zint_symbol *symbol, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_and_Buffer(struct zint_symbol *symbol, unsigned char *input, int length, int rotate_angle);
ZINT_EXTERN int ZBarcode_Encode_File_and_Buffer(struct zint_symbol *symbol, char *filename, int rotate_angle);

ZINT_EXTERN int ZBarcode_ValidID(int symbol_id);

#ifdef __cplusplus
}
#endif /* __cplusplus */

#endif /* ZINT_H */

注意：在此线程中提出了类似的问题，但从未解决过。我已经为此努力了一个星期。有任何想法吗？