Calling Fortran from C# in VS2019 with iFort

Question

I am trying to call Fortran code from C#. I am using Visual Studio 2019 and the Intel Fortran Compiler (iFort).

I created a fortran DLL project with the following code that compiles without issue (project set to 'release', 'x64'):

module Fortran_DLL_Lib
    implicit none
contains
    subroutine adder(a,b,x,y)
        !DEC$ ATTRIBUTES DLLEXPORT, ALIAS:'adder' :: adder
        !DEC$ ATTRIBUTES REFERENCE :: x,y
        implicit none
        integer, intent(in) :: a,b
        integer, intent(out) :: x,y
        y = a + b
        x = 2*a+3*b
    end subroutine
end module

I then created a C# console application with the following code (project also set to 'Release', 'x64'):

using System;
using System.Runtime.InteropServices;

namespace Call_Fortran_Dll_FromCS_Test
{
    class Program
    {
        [DllImport("Fortran_DLL_Lib.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern void adder(int a, int b, [Out] int x, [Out] int y);
        static void Main(string[] args)
        {
            int a = 4;
            int b = 3;
            int x = 0;
            int y = 0;
            Console.WriteLine(a);
            Console.WriteLine(b);
            Console.WriteLine(x);
            Console.WriteLine(y);

            adder(a, b, x, y); //error occurs here
            Console.WriteLine(a);
            Console.WriteLine(b);
            Console.WriteLine(x);
            Console.WriteLine(y);
            Console.ReadKey();
        }
    }
}

The program runs up until the line that calls the fortran function, then returns the error

Exception thrown: 'System.DllNotFoundException' in Call_Fortran_Dll_FromCS_Test.dll
An unhandled exception of type 'System.DllNotFoundException' occurred in Call_Fortran_Dll_FromCS_Test.dll
Unable to load DLL 'Fortran_DLL_Lib.dll' or one of its dependencies: Access is denied. (0x80070005 (E_ACCESSDENIED))

I have copied both the 'Fortran_DLL_Lib.dll' and 'Fortran_DLL_Lib.lib' files into both the folder that contains the c# project files, as well as the location where the executable project is located, neither seems to help/matter.

This is just based on example code I found trying to find ways to do this and isn't specific to what I'm doing. I'm just trying to get a 'proof of concept' together before jumping into more complex applications. The solution doesn't even necessarily be a DLL, that's just what I saw people recommending as the solution to this (in 7+ year old questions on this site). Open to any solutions that successfully call Fortran code from a C# project (eventually, a C# project with a WPF GUI, if that matters).

I am in a situation where I can't install Dependency Walker, change environment variables, or pretty much anything that requires elevated privileges.

Any help would be greatly appreciated!

Answer 1

a and b are still reference arguments, unless you add the VALUE attribute in their definition in Fortran.

So in C# you need to change your declaration to

public static extern void adder(ref int a, ref int b, out int x, out int y);

Note that the [Out] attribute is required for structure types, but not for primitive types.

Also to make sure the dll is next to the C# driver binary, add the dll into the CSharp solution. Use add-existing and then add as link

then select the link in the solution and set to copy if newer

this will place the dll in the output folder

additionally, I set the calling convention in Fortran as CVF, although I think this is optional as I specify explicitly which argument is REFERENCE and which is VALUE .

I checked the DLL export using dumpbin /exports

The following code works as indented:

Fortran Project

Note the addition of the VALUE attributes as well as the REFERENCE ones.

module mod_Fortran_dll
    use iso_c_binding
    implicit none
contains
    subroutine adder(a,b,x,y)
        !DEC$ ATTRIBUTES DLLEXPORT, alias:'adder' :: adder
        !DEC$ ATTRIBUTES VALUE :: a,b
        !DEC$ ATTRIBUTES REFERENCE :: x,y
        implicit none
        integer, intent(in) :: a,b
        integer, intent(out) :: x,y
        y = a + b
        x = 2*a+3*b
    end subroutine
end module

CSharp Project

Note the removal of the CDecl calling convention, and the specific EntryPoint specification.

static class Program
{
    #region Fortran
    [DllImport("FortranLib.dll", EntryPoint = "adder")]
    public static extern void adder(int a, int b, [Out] out int x, [Out] out int y);

    #endregion
    static void Main(string[] args)
    {
        int a = 4;
        int b = 3;
        int x = 0;
        int y = 0;

        Console.WriteLine($"a={a}, b={b}, x={x}, y={y}");
        // a = 4, b = 3, x = 0, y = 0

        adder(a, b, out x, out y);

        Console.WriteLine($"a={a}, b={b}, x={x}, y={y}");
        // a = 4, b = 3, x = 17, y = 7
    }
}