Converting SDK C++ function's struct to c#

[screamer81]

Hello,

I've a SDK functions description for a scanner and I try to convert unmanaged DLL C++ functions to c#. I converted all except one of them. This function is getting a struct parameter.

int WINAPI ImgSave(_T_Scan_Param *ScanP, unsigned char *szMicr,
    int *num_chars, int y0, int y1, int cOcrCode_option)

struct _T_Scan_Param { 
 
char file1[256]; 
char file2[256]; 
char file3[256]; 
unsigned char *imgptr1; 
unsigned char *imgptr2; 
unsigned char *imgptr3; 
int compr_type; 
int compr_quality; 
int imgptr_enable; 
};

My converted c# code is

private struct Scan_Parameters
{
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
   public string file1;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
   public string file2;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
   public string file3;
   public int imgptr1;
   public int imgptr2;
   public int imgptr3;
   public int compr_type;
   public int compr_quality;
   public int imgptr_enable;
}
 

And my declare function is;

[DllImport(@"c:\scanner.dll")]
private static extern int ImgSave(ref Scan_Parameters scanP, string szMicr, ref int num_chars, int y0, int y1, int cOcrCode_option);
 

When I try, I get an exception like "external component has thrown an exception". I've a " succesfully worked vb6.0 code" for this DLL and in this code, declared function is getting parameters with "ref" so I used these "ref" in my c# code too.
For the other functions, converting from vb6.0 code is working fine but in this function ( I think the reason is about converting struct ) it is not working.

Do u have any idea ? Firstly, is converted struct Ok ?

Thanks

[Plater]

I have seen people have luck by using the StringBuilder for these cases:
unsigned char *imgptr1;
unsigned char *imgptr2;
unsigned char *imgptr3;
(rather then just using an int like you are doing)

[screamer81]

Thanks for your help Plater.

StringBuilder solved my another problem about a function's unsigned char * parameter; but when I tried to use this in struct, I am getting the exception below:

{"Cannot marshal field 'imgptr1' of type 'Scan_Parameter s': Struct or class fields cannot be of type StringBuilder. The same effect can usually be achieved by using a String field and preinitializing it to a string with length matching the length of the appropriate buffer.":""}

After this exception, I also tried ;

for example : new string(' ', 50 ) but the same exception is occured.

my code is :

private struct Scan_Parameters
{
public string file1;
public string file2;
public string file3;
public StringBuilder imgptr1;
public StringBuilder imgptr2;
public StringBuilder imgptr3;
public int compr_type;
public int compr_quality;
public int imgptr_enable;
}

Scan_Parameters scanParam;

StringBuilder imgarg1 = new StringBuilder(" ", 50);
StringBuilder imgarg2 = new StringBuilder(" ", 50);
StringBuilder imgarg3 = new StringBuilder(" ", 50);

scanParam.file1 = "c:\\images\\im g_fr.jpg";
scanParam.file2 = "c:\\images\\im g_re.jpg";
scanParam.file3 = "";
scanParam.compr _quality = 50;
scanParam.compr _type = 0;
scanParam.imgpt r1 = imgarg1;
scanParam.imgpt r2 = imgarg2;
scanParam.imgpt r3 = imgarg3;
scanParam.imgpt r_enable = 0;

int returnCode = CXL_ScanImgSave (ref scanParam, codeLine, ref codelineLength, 0, 128, 2);

C++ syntax is

unsigned char *imgptr1; // NOT USED
unsigned char *imgptr2; // NOT USED
unsigned char *imgptr3; // NOT USED

"not used" is a commandline but seems it is used somehow.

[Plater]

Maybe try making them of type IntPtr then?

[screamer81]

I solved the problem two days ago. Thanks for your help Plater.

c# code is :

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
        private struct Scan_Parameters
        {
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
            public string file1;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
            public string file2;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
            public string file3;
            public IntPtr imgptr1;
            public IntPtr imgptr2;
            public IntPtr imgptr3;
            public int compr_type;
            public int compr_quality;
            public int imgptr_enable;
        }