Linking C/C++ and Fortran, unresolved external symbols

I am trying to compile use a Fortran static library file with a C++ program but am getting unresolved external errors

I build the static library file using x64 

subroutine bar_ftn ( len_input_file, input_file ) bind( c )
    use, intrinsic :: iso_c_binding, only : c_int
    implicit none
    integer(c_int), value, intent(in) :: len_input_file
    character(len=1), intent(in) :: input_file(len_input_file)

    ! Local declarations (copy c char array into fortran character)
    character(len=len_input_file) :: infile
    integer :: i

    print *, "in bar_ftn"
    print *, len_input_file
    do i=1,len_input_file
    end do
end subroutine bar_ftn

the C++ program is 

#include "stdafx.h"
#include<iostream>
#include<fstream>

using namespace std;
extern "C" void bar_ftn ( int flag_len, char* flag );
static void
DisplayUsage(char* programName);

int main(int argc, char *argv[])
{
    char ctext[]="helloworld abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz";
    int ctext_len=sizeof(ctext);

    //Call the Fortran
    bar_ftn( ctext_len, ctext );

    return 0;
}

but when I compile the C++ program I get the following errors

Error    2    error LNK1120: 1 unresolved externals    D:\temp\ConsoleApplication1\Debug\ConsoleApplication1.exe    1    1    ConsoleApplication1
Error    1    error LNK2019: unresolved external symbol _bar_ftn referenced in function _main    D:\temp\ConsoleApplication1\ConsoleApplication1\main.obj    ConsoleApplication1
    3    IntelliSense: cannot open source file "stdafx.h"    d:\temp\main.cpp    1    1    ConsoleApplication1

could you pleat tell me what  I am doing wrong?