[RESOLVED] Marshal C struct containing pointer to other C struct

I have a DLL written in C that I must use in my C# application.

The C declarations for the two functions in question are as follows:

Code:

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IMGSTRUCT *LoadImg(const char *filename)
int UseImg(IMGSTRUCT *img,int flag)

The structures are defined as follows in C.

typedef struct tagBITMAPINFOHEADER{
        DWORD      biSize;
        LONG      biWidth;
        LONG      biHeight;
        WORD      biPlanes;
        WORD      biBitCount;
        DWORD      biCompression;
        DWORD      biSizeImage;
        LONG      biXPelsPerMeter;
        LONG      biYPelsPerMeter;
        DWORD      biClrUsed;
        DWORD      biClrImportant;
} BITMAPINFOHEADER, FAR *LPBITMAPINFOHEADER, *PBITMAPINFOHEADER;

typedef struct  {
  LPBITMAPINFOHEADER bi;  // bitmap info
  unsigned char * imgBits;  // image content 
}
IMGSTRUCT;

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I know how to marshal the BITMAPINFOHEADER structure.

Code:

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        [StructLayout(LayoutKind.Sequential)]
        public struct BITMAPINFOHEADER
        {
            public uint biSize;
            public int biWidth;
            public int biHeight;
            public ushort biPlanes;
            public ushort biBitCount;
            public uint biCompression;
            public uint biSizeImage;
            public int biXPelsPerMeter;
            public int biYPelsPerMeter;
            public uint biClrUsed;
            public uint biClrImportant;

            public void Init()
            {
                biSize = (uint)Marshal.SizeOf(this);
            }
        }

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How do I marshal the IMGSTRUCT structure?

It has a member of BITMAPINFOHEADER pointer and a pointer to an array of image data.

Then how would the P/Invoke look for both LoadImg() and UseImg()?

Can you explain better(with c# examples) how you would like to use them?

Anyway for the second struct this work?

Code:

---

[StructLayout(LayoutKind.Sequential)]
        public struct IMGSTRUCT
        {
            public BITMAPINFOHEADER bi;
            [MarshalAs(UnmanagedType.ByValArray,SizeConst=1024)]
            public byte[] imgBits; //you should convert a bitmap to byte[] with Marshal.Copy()
        }

---

I first tried these P/Invokes

Code:

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        [DllImport("thirdpartydll.dll", CharSet = CharSet.Auto)]
        public static extern IntPtr LoadImg([MarshalAs(UnmanagedType.LPStr)] string FileName);

        [DllImport("thirdpartydll.dll", CharSet = CharSet.Auto)]
        public static extern int UseImg([In] IntPtr Img, int flag);

        public delegate void outputDelegate (int infotype, int param);
        [DllImport("thirdpartydll.dll", CharSet = CharSet.Auto)]
        public static extern int SetOutputDelegate(outputDelegate delegate);

---

Then I called them from C# like this. The delegate is never getting called.

Code:

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        private outputDelegate _OutputDelegate = new outputDelegate(imgOutputDelegate);

        private void startToolStripMenuItem_Click(object sender, EventArgs e)
        {
            int ret = OCRSetOutputHandler(_OCROutputHandler);

            // _imgPath previously set
            if (String.IsNullOrEmpty(_imgPath) == false)
            {
                // single-page TIFF
                IntPtr imageindex = LoadImg(_imgPath);

                int iRes = UseImg(imageindex, 0);
            }
        }

        // delegate that receives events from UseImg()
        private static void imgOutputDelegate(int infotype, int param)
        {
            LogMsg("Inside imgOutputDelegate");
            string test = String.Empty;

            // additional processing
        }

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I am thinking the structure is not get marshalled correctly.

Your sample for the IMGSTRUCT won't work I think because the first member is a pointer to that structure, not the structure itself. The second member is unfortunately a pointer to a variable length array.

Any thoughts would be appreciated.

Code:

---

        [DllImport("thirdpartydll.dll", CharSet = CharSet.Auto)]
        public static extern IntPtr LoadImg([MarshalAs(UnmanagedType.LPStr)] string FileName);

        [DllImport("thirdpartydll.dll", CharSet = CharSet.Auto)]
        public static extern int UseImg([In] IntPtr Img, int flag);

        public delegate void outputDelegate (int infotype, int param);
        [DllImport("thirdpartydll.dll", CharSet = CharSet.Auto)]
        public static extern int SetOutputDelegate(outputDelegate delegate);

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Well I received a new copy of the thirdpartydll.dll and the above P/Invokes actually do work so I no longer need to attempt to marshal the IMGSTRUCT structure.