interop - Calling FORTRAN dll from C# and assigning values to array of structures

Question

I can pass a C# struct into FORTRAN just fine. I can even pass an array of a C# struct as an array of TYPE() in FORTRAN. Where I run into trouble is when I tried to return values back into C#. Here is an example:

The fortran dll is:

MODULE TESTING

   TYPE VALUEREF
     INTEGER*4 :: A
   ENDTYPE VALUEREF

CONTAINS

   SUBROUTINE TEST_REF(T,N)
   !DEC$ ATTRIBUTES DLLEXPORT :: TEST_REF
   !DEC$ ATTRIBUTES ALIAS:'TEST_REF' :: TEST_REF
   !DEC$ ATTRIBUTES VALUE :: N
   IMPLICIT NONE
     INTEGER*4 :: A,I,N   
     TYPE(VALUEREF) :: T(N)      
     A = 100
     DO I=1,N
        T(I)%A = A + I
     END DO

   END SUBROUTINE
END MODULE

and the C# calling function that expects results is:

[StructLayout(LayoutKind.Sequential)]
public struct ValueRef
{
    public int a;
}

[DllImport("mathlib.dll")]
static extern void TEST_REF(ValueRef[] t, int n);

void Main()
{
    ValueRef[] T = new ValueRef[4];
    for (int i = 0; i < T.Length; i++)
    {
        T[i].a = i;
    }
    Console.WriteLine("Initialize");
    for (int i = 0; i < T.Length; i++)
    {
        Console.WriteLine("  A={0}", T[i].a);
    }
    Console.WriteLine("Call Fortran");
    TEST_REF(T, T.Length);
    for (int i = 0; i < T.Length; i++)
    {
        Console.WriteLine("  A={0}", T[i].a);
    }
}

With results:

Initialize
  A=0
  A=1
  A=2
  A=3
Call Fortran
  A=0
  A=1
  A=2
  A=3

Debugging through the FORTRAN code, I see the initial values pass from C# to FORTRAN just fine. The the values get overridden with new values and control is passed back into C# where the old values are still contained within the ValueRef instances.

Why is it that I can pass and return an array of float, or int in a similar fashion, just fine. and I can pass and return singular structures with ref keyword, and I can pass but not return and array of struct?

PS. I am using Compaq Visual Fortran 6.5 & .NET 3.5
PS2. I appreciate any comments/ideas on this. I am 95% done with my project, and now I run into this issue. The whole point of this project is to use structures as much as possible to reduce the #of arguments passed to functions and retain certain aspects of OOP design.

Answer 1

I have done this in the past using a pointer, not an array. I think that your structures are being copied for the P/Invoke call:

[DllImport("mathlib.dll")]
static extern void TEST_REF(ValueRef* t, int n);

You will need to pin your array before calling the method.

fixed (ValueRef* pointer = t)
{
  TEST_REF(pointer, n);
}

Edit: Based on the comments the solution is to declare the external as

[DllImport("mathlib.dll")]
static extern void TEST_REF([Out] ValueRef[] t, int n);

Here is a MSDN reference on Marshaling of arrays, and how they default to [In].