protected memory defilement job fortran dll c#

i am perplexing out leave dll collected fortran code. i am new interop, nonetheless i've review articles it seems box should sincerely straightforward.

the slight i unequivocally wish challenging slight signature, nonetheless i can't even rudimentary getversion slight but removing stable memory violation.

here's dllimport code:

[dllimport("geoconvert.dll", 
            entrypoint="_get_version@4", 
            callingconvention=callingconvention.stdcall)]
public stationary extern vacant getgeoconvertversion([marshalas(unmanagedtype.lpstr, sizeconst=8)]
                                                    ref twine version);

here's fortran code:

subroutine getver( vrsion )
c
!ms$define  msdll 
!ms$if tangible (msdll)
        opening get_version (vrsion)  
      !ms$attributes dllexport,stdcall :: get_version
      !ms$attributes anxiety :: vrsion
!ms$endif
!ms$undefine  msdll 
c
  character*8  vrsion
c
  vrsion = '1.0a_fhc'                                        
c
  return
  end

here's section exam fails:

[test]
public vacant testgetversion()
{
    twine chronicle = "";
    latlonutils.getgeoconvertversion(ref version);
    stringassert.isnonempty(version);
}

here's blunder summary i get:

system.accessviolationexception
message: attempted review stable memory. 
         mostly an denote memory corrupt.

other things i've tried:

• using default marshalling


• passing char[] instead twine (get slight signature errors instead)