can't get value correctly from fortran dll to VB.net back

[Lu_Zhang]

Hi Everyone,
I can't make the following code working in VS2015 with IVF, though it is very simple. Value C from dll is always 0, not 6.0 as expected. Thanks so much.
//my VB code:
Public Class Form1
    Public Declare Sub DLL_ROUT Lib "D:\Debug\Dll2.dll" _
    (ByVal A As Double, ByVal B As Double, ByVal C As Double)
    Public Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim A As Double
        Dim B As Double
        Dim C As Double
        A = 2.0
        B = 3.0
        Call DLL_ROUT(A, B, C)
        Button1.Text = C
    End Sub
End Class
 
// my Fortran  dll code:
SUBROUTINE DLL_ROUT (A, B, C)
IMPLICIT NONE
! Specify that DLL_ROUT is exported to a DLL
! and that the external name is 'DLL_ROUT'
!DEC$ ATTRIBUTES DLLEXPORT, STDCALL :: DLL_ROUT
!DEC$ ATTRIBUTES ALIAS:'DLL_ROUT' :: DLL_ROUT
DOUBLE PRECISION A, B, C
C = A * B
RETURN
END

[FortranFan]

I suggest the following instead based on P/Invoke in .NET and C interoperability in Fortran:

   <DllImport("D:\Debug\Dll2.dll", CallingConvention:=CallingConvention.Cdecl)> _
   Public Shared Sub DLL_ROUT (A As Double, B As Double, ByRef C As Double)

   subroutine DLL_ROUT(A, B, C) bind(C, name="DLL_ROUT")
   !DEC$ ATTRIBUTES DLLEXPORT::DLL_ROUT

      use, intrinsic :: iso_c_binding, only : c_double

      !.. Argument list
      real(c_double), intent(in), value :: A
      real(c_double), intent(in), value :: B
      real(c_double), intent(inout)     :: C

      C = A * B

      return

   end subroutine DLL_ROUT

[Steve_Lionel]

I'm not sure if FortranFan's suggestion will work, but the reason your initial version doesn't work is that just saying STDCALL changes the arguments to pass-by-value, which makes it impossible to return anything. Add to the Fortran:

!DEC$ ATTRIBUTES REFERENCE :: C

This will match the ByRef in the VB.

[Lu_Zhang]

It works, amazing! Thanks FortranFan!

[Lu_Zhang]

This is awesome, Thanks Steve!

[Lu_Zhang]

Both work very well. Thank you guys!

[FortranFan]

Steve Lionel (Ret.) wrote:

I'm not sure if FortranFan's suggestion will work, ..

Huh?  It's been only >3.5 years here where readers have been guided to consider the better option with Microsoft .NET code to interoperate with Fortran is to employ P/Invoke: https://software.intel.com/en-us/forums/intel-visual-fortran-compiler-for-windows/topic/594478

Fdll.f90

module m

   use, intrinsic :: iso_c_binding, only : c_double

   implicit none

contains

   subroutine DLL_ROUT(A, B, C) bind(C, name="DLL_ROUT")
   !DEC$ ATTRIBUTES DLLEXPORT::DLL_ROUT

      !.. Argument list
      real(c_double), intent(in), value :: A
      real(c_double), intent(in), value :: B
      real(c_double), intent(inout)     :: C

      C = A * B

      return

   end subroutine DLL_ROUT

end module m

Test.vb file

Imports System.IO
Imports System.Text
Imports System.Runtime.InteropServices

Namespace Fortran

   Public Class Fdll

      <DllImport("c:\Debug\Fdll.dll", CallingConvention:=CallingConvention.Cdecl)> _
      Public Shared Sub DLL_ROUT (A As Double, B As Double, ByRef C As Double)
      end Sub

   End Class

   NotInheritable Class Test

      Private Sub New()
      End Sub

      Public Shared Sub Main()

         Dim a As Double
         Dim b as Double
         Dim c as Double

         ' Write header
         Console.WriteLine("*** Test Fortran Interop using VB ***" + vbLf)

         Try

            a = 2.0
            b = 3.0
            c = 0.0
            Fdll.DLL_ROUT(a, b, c)

            Console.WriteLine("c = " + c.ToString("###0.0#"))

         Catch ex As Exception

            Console.WriteLine(ex.Message)

         Finally

            Console.WriteLine("Press any key to continue..")
            Console.ReadKey()

         End Try

      End Sub

   End Class

End Namespace

C:\Debug>ifort /dll /libs:static fdll.f90
Intel(R) Visual Fortran Intel(R) 64 Compiler for applications running on IA-32, Version 19.0.3.203 Build 20190206
Copyright (C) 1985-2019 Intel Corporation.  All rights reserved.

Microsoft (R) Incremental Linker Version 14.16.27027.1
Copyright (C) Microsoft Corporation.  All rights reserved.

-out:fdll.dll
-dll
-implib:fdll.lib
fdll.obj
   Creating library fdll.lib and object fdll.exp

C:\Debug>vbc -platform:x86 test.vb
Microsoft (R) Visual Basic Compiler version 2.10.0.0 (b9fb1610)
Copyright (C) Microsoft Corporation. All rights reserved.


C:\Debug>test.exe
*** Test Fortran Interop using VB ***

c = 6.0
Press any key to continue..

C:\Debug>

C:\Debug>ifort /dll /libs:static Fdll.f90
Intel(R) Visual Fortran Intel(R) 64 Compiler for applications running on Intel(R) 64, Version 19.0.3.203 Build 20190206
Copyright (C) 1985-2019 Intel Corporation.  All rights reserved.

Microsoft (R) Incremental Linker Version 14.16.27027.1
Copyright (C) Microsoft Corporation.  All rights reserved.

-out:Fdll.dll
-dll
-implib:Fdll.lib
Fdll.obj
   Creating library Fdll.lib and object Fdll.exp

C:\Debug>vbc -platform:x64 test.vb
Microsoft (R) Visual Basic Compiler version 2.10.0.0 (b9fb1610)
Copyright (C) Microsoft Corporation. All rights reserved.


C:\Debug>test.exe
*** Test Fortran Interop using VB ***

c = 6.0
Press any key to continue..

C:\Debug>

[Steve_Lionel]

When I said "I'm not sure" I meant that literally - I thought it might, but am not familiar with the angle bracket syntax for use in VB code, so I didn't know. I didn't say it wouldn't work. But I knew my suggestion would work and was a smaller change.

[FortranFan]

Lu Zhang wrote:

It works, amazing! Thanks FortranFan!

Please see this thread from about 5 years ago, especially Quote #2 where I suggest moving away from VB6/COM based approaches with STDCALL, etc. from the bygone 1990s when it comes to working with .NET technologies:

https://software.intel.com/en-us/forums/intel-visual-fortran-compiler-for-windows/topic/509148

[Lu_Zhang]

Thank you so much, FortranFan.