DllImport Adventures

More fun with the scanner. Now we want to get the first name and last name in chinese from the certain types of passports. There is a dll written in C++ which supposedly provides this functionality. The function in this dll takes a path to an image as a parameter and returns a structure that contains first name and last name.

Here is the C++ signature of the function

bool recoCHN_P_Name(char *imgPath,RECO_DATA *o_data);

Here is the RECO_DATA struct

struct RECO_DATA{
 wchar_t FirstName[200];
 wchar_t Surname[200];
};

Now I have to call the C++ method from my C# code. The 'DllImport' thingy comes to mind. There are heaps of tutorials on that, for example

Call Unmanaged Code. Part 1 - Simple DLLImport

So I quickly come up with the first version:

This is the 'wrapper' class

public class cnOCRsdk
{
 [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
 public struct RECO_DATA{
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst=200)]
  public string FirstName;
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 200)]
  public string Surname;
  }

 [DllImport(@"cnOCRsdk.dll", EntryPoint="recoCHN_P_Name")]
 public static extern bool recoCHN_P_Name(byte[] imgPath, RECO_DATA o_data);
}

This is the call to the function in the wrapper class

cnOCRsdk.RECO_DATA recoData = new cnOCRsdk.RECO_DATA();

string path = @"C:\WINDOWS\twain_32\twainrgb.bmp";

System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
byte[] bytes = encoding.GetBytes(path);

bool res = cnOCRsdk.recoCHN_P_Name(bytes, recoData);

Unfortunately, this gives me a "Unable to find an entry point named 'recoCHN_P_Name' in DLL 'cnOCRsdk.dll'." error. My first guess was that I am converting types from C++ to C# incorrectly. I asked for some help

Unable to find an entry point named [function] in dll

I got a suggestion to use the 'dumpbin' tool

DUMPBIN

and also a link to the page that explained C++ name mangling

C++ Name Mangling/Demangling

my function had a mangled name of '?recoCHN_P_Name@CcnOCRsdk@@QAE_NPADPAURECO_DATA@@@Z', but how did that help me?

Anyway, I came across another article on calling C++ from C#

Inheriting From a Native C++ Class in C#

and noticed an interesting thing in it:

[DllImport("cppexp.dll", EntryPoint = "?M1@CSimpleClass@@QAEXXZ", CallingConvention = CallingConvention.ThisCall)]
    private static extern void _M1(__CSimpleClass* ths);

this guy is using the mangled function name as an entry point! Why shouldn't I try this approach?

Long story short, the 'unable to find entry point' error went away and after a few more tweaks I made this work. Unfortunately, the function always returns me false so far, which is probably because all the images I try are 'bad', but at least it does not break anymore.

public class cnOCRsdk
{
 [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
 public struct RECO_DATA
 {
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 200)]
  public string FirstName;
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 200)]
  public string Surname;
 }

 [DllImport(@"cnOCRsdk.dll", EntryPoint="?recoCHN_P_Name@CcnOCRsdk@@QAE_NPADPAURECO_DATA@@@Z")]
 public static extern bool recoCHN_P_Name(ref string imgPath, ref RECO_DATA o_data);
}

cnOCRsdk.RECO_DATA recoData = new cnOCRsdk.RECO_DATA();
recoData.FirstName = new string(new char[200]);
recoData.Surname = new string(new char[200]);

string path = @"C:\WINDOWS\twain_32\twainrgb.bmp";

bool res = cnOCRsdk.recoCHN_P_Name(ref path, ref recoData);

by Evgeny. Also posted on my website