"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 ... ?

score 47 · Accepted Answer · edited Mar 20 '14 at 19:42

47

First make sure the function is actually exported:

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

edited Mar 20 '14 at 19:42

Phil

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answered Mar 17 '09 at 06:32

i_am_jorf

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I have a line in the output which says "14 D 00007B40 ?recoCHN_P_Name@CcnOCRsdk@@QAE_NPADPAURECO_DATA@@@Z" what does that tell me? – Evgeny Mar 17 '09 at 06:48
3

See http://www.kegel.com/mangle.html for a detailed explanation of how to decrypt it. Make sure you've got the datatypes correct. – i_am_jorf Mar 17 '09 at 07:06

score 32 · Answer 2 · edited Feb 16 '18 at 02:35

32

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.

edited Feb 16 '18 at 02:35

Bhargav Rao

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answered Mar 17 '09 at 07:13

paxdiablo

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That was it in my case, thanks! – Israel Unterman Feb 10 '22 at 13:54

score 19 · Answer 3 · answered Mar 17 '09 at 23:09

19

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.

answered Mar 17 '09 at 23:09

Evgeny

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This will work but you'll have to change the mangled name if anything about the function (signature) changes. It's recommended to define them in the DLL with `extern "C"` as mentioned above. – adam_0 Aug 27 '18 at 16:11
@Goku, you can find it with a program called PE Explorer (http://www.heaventools.com/overview.htm) or the program called `dumpbin.exe` which comes with Visual Studio. – Ryan Griffith Apr 30 '19 at 21:02

score 7 · Answer 4 · answered Jul 21 '17 at 18:13

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);

`__declspec(dllexport)` was key for me, [another answer](https://stackoverflow.com/a/315064/6620171) has people marking them with `extern "C"`instead, and nothing was being exported, THANKS! — fartwhif, Apr 05 '20 at 23:31
Thanks! I've seen about 10 solutions that recommend using "extern C" as well as "__declspec(dllexport)". The C++ compiler wouldn't work with extern but I was unable to work out why. This worked first time for me. — GrandMasterFlush, Jul 20 '21 at 11:16

score 4 · Answer 5 · answered Mar 17 '09 at 08:08

4

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.

answered Mar 17 '09 at 08:08

Dan Byström

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score 2 · Answer 6 · answered Sep 24 '15 at 10:12

2

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

answered Sep 24 '15 at 10:12

uzrgm

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score 1 · Answer 7 · answered Jun 14 '20 at 08:10

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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)

answered Jun 14 '20 at 08:10

keivan kashani

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score 1 · Answer 8 · answered Jul 21 '22 at 15:14

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.

score 0 · Answer 9 · answered Jun 14 '20 at 08:43

0

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.

answered Jun 14 '20 at 08:43

mahdi

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score -1 · Answer 10 · answered Mar 24 '09 at 23:00

-1

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

answered Mar 24 '09 at 23:00

evilfred

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

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