C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

namespace PostCodeRecodetest
{
    class QAProClass
    {
        //Declarations

        //Declare Sub QAInitialise Lib "QAPUIEK.DLL" (ByVal vi1 As Long)
        [DllImport("qapuiek.dll")]
        public static extern int QAInitialise(long vi1);

        //Declare Function QAPro_Open Lib "QAPUIEK.DLL" (ByVal vs1 As String, ByVal vs2 As String) As Long
        [DllImport("qapuiek.dll")]
        public static extern int QAPro_Open(string vs1, string vs2);

        //Declare Sub QAPro_Close Lib "QAPUIEK.DLL" ()
        [DllImport("qapuiek.dll")]
        public static extern void QAPro_Close(); 

        //Declare Function QAPro_Search Lib "QAPUIEK.DLL" (ByVal vs1 As String) As Long
        [DllImport("qapuiek.dll")]
        public static extern int QAPro_Search(string vs1);

        //Declare Function QAPro_AddrLine Lib "QAPUIEK.DLL" (ByVal vl1 As Long, ByVal vi2 As Long, ByVal vs3 As String, ByVal rs4 As String, ByVal vi5 As Long) As Long
        [DllImport("qapuiek.dll")]
        public static extern int QAPro_AddrLine(int vl1, int vi2, string vs3, ref string rs4, int vi5);

        public string PostCodeRecoded(string postcodeIn)
        {
            string rsBuffer = "";
            int lOpenReturn = 0;
            int lSearchReturn = 0;
            string sSearchString = "";
            string returnedPostcode = postcodeIn;
            string iniPath = "C:\\a_DEV\\Qapro.ini";

            // Close QA first
            QAPro_Close();

            // Initialise the dll for faster performance
            QAInitialise(1);

            // Open the DLL with the specific ini file.  Hard coded for the moment
            lOpenReturn = QAPro_Open(iniPath, "");

            if (lOpenReturn != 0) return "QAPro Failed";

            //Add validation later to insure that the postcode is the correct number of characters

            sSearchString = postcodeIn; 

            lSearchReturn = QAPro_Search(sSearchString);

            if (lSearchReturn == -9811)
            {
                lSearchReturn = QAPro_AddrLine(0, 11, "0",ref rsBuffer, 9); //This is the line that fails
                returnedPostcode = rsBuffer;
            }

            return returnedPostcode;
        }
    
    }

}