Question

I am trying to call out to a legacy dll compiled from FORTRAN code. I am new to Interop, but I’ve read some articles on it and it seems like my case should be fairly straightforward.

The method I really want to call has a complex method signature, but I can’t even call this simple GetVersion method without getting a protected memory violation.

Here’s my DllImport code:

[DllImport('GeoConvert.dll',              EntryPoint='_get_version@4',              CallingConvention=CallingConvention.StdCall)] public static extern void GetGeoConvertVersion([MarshalAs(UnmanagedType.LPStr, SizeConst=8)]                                                     ref string version); 

Here’s the FORTRAN code:

SUBROUTINE GetVer( VRSION ) C !MS$DEFINE  MSDLL  !MS$IF DEFINED (MSDLL)         ENTRY Get_Version (VRSION)         !MS$ATTRIBUTES DLLEXPORT,STDCALL :: Get_Version       !MS$ATTRIBUTES REFERENCE :: VRSION !MS$ENDIF !MS$UNDEFINE  MSDLL  C   CHARACTER*8  VRSION C   VRSION = '1.0a_FhC'                                         C   RETURN   END 

Here’s my unit test that fails:

[Test] public void TestGetVersion() {     string version = '';     LatLonUtils.GetGeoConvertVersion(ref version);     StringAssert.IsNonEmpty(version); } 

Here’s the error message I get:

System.AccessViolationException Message: Attempted to read or write protected memory.           This is often an indication that other memory is corrupt. 

Other things I’ve tried:

• Using the default marshalling
• Passing a char[] instead of a string (get method signature errors instead)

Answer 1

OK, I got it to work, the problem was passing by ref. I’m not sure why, but this works:

[DllImport('GeoConvert.dll',                  EntryPoint='_get_version@4',                  CallingConvention=CallingConvention.StdCall)]     public static extern void GetGeoConvertVersion([MarshalAs(UnmanagedType.LPArray)]                                                     byte[] version); 

With this test:

[Test]     public void TestGetVersion()     {         //string version = '';         byte[] version = new byte[8];         LatLonUtils.GetGeoConvertVersion(version);         char[] versionChars = System.Text.Encoding.ASCII.GetChars(version);          string versionString = new string(versionChars);     }