passing c# string argument to fortran 77 dll

Question

Need help , in passing passing C# string to FORTRAN 77 dll as argument.

FORTRAN 77 code:

*$pragma aux CHARIN "CHARIN" export parm(value)
      SUBROUTINE CHARIN(FCHAR)
C Declarations
      CHARACTER*(*)  FCHAR
C
      PRINT*,FCHAR
C
      RETURN
      END

C# code:

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;
using System.Diagnostics;

namespace ConsoleApplication1
{
    class Program
    {
        [DllImport("fdchar.dll",
            CharSet = CharSet.Auto,
            CallingConvention = CallingConvention.StdCall)]
        static extern void returnDecay(
            [MarshalAs(UnmanagedType.LPStr)]string FCHAR,
            int tint);

        static void Main(string[] args)
        {
            string ccc =  "ABCD"; 
            int tint = 4;

            CHARIN(ccc,tint);
            Console.Write("Press any key to EXIT");
            Console.ReadKey(false);
        }
    }
 }

Answer 1

From what I'm seeing with my own tests, the Fortran DLL appears to lack the hidden length parameter AND C# isn't sending the string in a format Fortran is expecting. It appears that Fortran 77 needs to be tricked into dereferencing the argument being passed along with being given the character argument length explicitly. The following works with Intel Visual Fortran 10.1 and C# 2.0.

Fortran 77:

C       10        20        30        40        50        60        70
C234567890123456789012345678901234567890123456789012345678901234567890
CDEC$ ATTRIBUTES STDCALL, DLLEXPORT::TEST
       SUBROUTINE TEST ( FCHAR, ILEN )
       CHARACTER*(ILEN) FCHAR(1)
C       
       PRINT *,LEN(FCHAR),FCHAR
C       
       RETURN
       ENDSUBROUTINE TEST

C#:

class Program
{
    [DllImport("F77CSharpDllTest.dll",
        EntryPoint = "test",
        CharSet = CharSet.Auto,
        CallingConvention = CallingConvention.StdCall)]
    static extern void Test(
        [MarshalAs(UnmanagedType.LPStr)] string FCHAR,
        int FCHAR_length);

    static void Main(string[] args)
    {
        string hello = "hello";
        Test(hello, hello.Length);
    }
}