Using cli wrapper class for c/c++ mixed code - unresolved error

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C++

C++/CLI

C

first there was only c code, i added a c++ file and class and used that c code function within my c++ class.
Now i am creating cli wrapper class for my mixed c/c++ code:

my 'c' code:
header file : "Code.h"

#pragma once
extern "C"
{
 int perfomtask(int argc,char** argv);

class testclass
{
    public :
        void begintask(int argc, char **argv);
}; 
}

code file "code.c"

#include "Code.h"
int perfomtask(int argc, char **argv){...}

my cplusplus code file:

#include "code.h"

void testclass::begintask(int argc, char **argv)
{
     perfomtask(argc,argv);
}

now i have tested the working of the above with this main class

int main(int argc, char **argv)
{
    testclass obj1;
    obj1.begintask(argc,argv);
    return 0;

}

now that's about my c/c++ mixed code
now here's my cli wrapper code
header file=

using namespace System;

namespace wrapclass
{
    public ref class wrapclass
    {
    public :
        wrapclass();
        ~wrapclass();
        void mynativeclass(int argc,char**argv);
        int argc;
        char** argv;
    private:
        testclass *ptestclass;
    };
}

code file=

wrapclass::wrapclass::wrapclass()
{
    ptestclass = new testclass();
}

wrapclass::wrapclass::~wrapclass()
{
    delete ptestclass;
}
void wrapclass::wrapclass::mynativeclass(int argc,char** argv)
{
    ptestclass->begintask(argc,argv);

}

now i am getting
error :
error LNK2028: unresolved token (0A00032E) "extern "C" int __cdecl perfomtask(int,char * *)referenced in function "public: void __thiscall testclass::begintask(int,char * *)" could someone help me out on this one any explanation would be appreciated.