"Unable to find an entry point named [function] in dll" (c++ to c# type conversion)

Question

I have a dll which comes from a third party, which was written in C++. Here is some information that comes from the dll documentation:

//start documentation

RECO_DATA{
wchar_t Surname[200];
wchar_t Firstname[200];
}

Description: Data structure for receiving the function result. All function result will be stored as Unicode (UTF-8).

Method:

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

Input:

char * imgPath

the full path of the image location for this function to recognize

RECO_DATA * o_data

data object for receiving the function result. Function return: True if Success, otherwise false will return.

//end documentation

I am trying to call the recoCHN_P_Name from my C# application. To this end, I came up with this code:

The code to import the dll:

    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);
}

The code to call the function:

            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);

And the error I'm getting is ""Unable to find an entry point named 'recoCHN_P_Name' in DLL 'cnOCRsdk.dll'." I'm suspecting that I'm having an error in converting a type from C++ to C#. But where exactly ... ?

Answer 1

First make sure the function is actually exported:

In the Visual Studio Command Prompt, use dumpbin /exports whatever.dll

Answer 2

C# doesn't support C++ name mangling and you either need to declare the C++ functions with

extern "C" {...}

(may not an option if they're from a third party), or call the mangled name directly if you can get it to work. It may be easier to get the third party to provide a non-mangled interface to the functionality.

Answer 3

Solved - at least to the point where the program does not break and actually returns me a bool value.

The key, I guess, was to specify the entry point as the 'mangled' name

    [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);

After that I got some other errors but the 'unable to find entry point' went away.

Answer 4

I solved the same problem in these steps:

step 1) If you program your custom DLL in C++ using Visual studio,then at the property page of your project set the Common Language Runtime Support (/clr)parameter to Common Language Runtime Support (/clr).

step 2) To function deceleration in .h file use __declspec(dllexport) keyword like below:

__declspec(dllexport) double Sum(int a,int b);

step 3) Build and export DLL file, then use the Dependency Walker software to get your function EntryPoint.

step4) Import DLL file In the C# project and set EntryPoint and CallingConvention variable like below:

[DllImport("custom.dll", EntryPoint = "?Sum@@YAXHHHHHHNNN@Z", CallingConvention = CallingConvention.Cdecl)]

    public static extern double Sum(int a,int b);