CODEWITHSUNDEEP
c#c++dlldllimportextern
Evgeny
Evgeny

Reputation: 3332

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

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 ... ?

Upvotes: 29

Views: 89235

Answers (10)

majed
majed

Reputation: 547

you may get this error due to string marshalling mismatch between DLL and your application . for example, one is using ANSI and the other is unicode.

you can try something like this:

 [DllImport("yourDLL.dll", CharSet = CharSet.Unicode )]
 public static extern String YourFunction(String name);

checkout HERE for a list of other possible reasons.

Upvotes: 1

mahdi
mahdi

Reputation: 77

We had this problem .we change EntityFramework.core to EntityFrameWork 6.4.4 and after that the program worked fine. you most change you're Framework Version.

Upvotes: 0

keivan kashani
keivan kashani

Reputation: 1359

We had this problem when we want to access to DB and solved it by changing EF core to EF 6.4.4 It may be you have a problem like this and need to change or downgrade your version of EF (If you used EF)

Upvotes: 1

paxdiablo
paxdiablo

Reputation: 882606

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.

Upvotes: 33

Hamid
Hamid

Reputation: 1563

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);

Upvotes: 7

uzrgm
uzrgm

Reputation: 395

Correct EntryPoint string could be found in ".lib" file that comes along with main unmanaged dll.

Upvotes: 2

i_am_jorf
i_am_jorf

Reputation: 54640

First make sure the function is actually exported:

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

Upvotes: 53

evilfred
evilfred

Reputation: 2416

You could try using the unmangled name while specifying a CallingConvention in the DllImport

Upvotes: -1

Evgeny
Evgeny

Reputation: 3332

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.

Upvotes: 19

Dan Byström
Dan Byström

Reputation: 9244

I'd write a wrapper using C++/CLI. This wrapper will be able to include the .h files and link to the .lib files you got from the third party vendor. Then it is both easy and safe to write a managed interface for your C# program.

Upvotes: 4

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