C# calling Delphi 2010 DLL won;t run

[sentop123]

I have a dll library for which the call in .net is as follows.

Declare Function CopyDesign Lib "OS50.DLL" _
(ByVal I_RPath As String, ByVal I_Flags As Integer, ByVal I_Typ As String, _
ByVal I_CC As Integer, ByVal I_Ref As String, ByVal I_Q As Double, _
ByVal I_T0 As Double, ByVal I_TC As Double, ByVal I_TS As Double, _
ByVal I_TL As Double, ByVal I_TM As Long, ByVal I_TLC As Double, _
ByVal I_TOC As Integer, ByVal I_TOL As Double, ByVal I_TN As Double, _
ByVal I_OP As Integer, ByVal I_SP As Single, ByVal I_CR As Single, _
ByVal O_T1 As String, ByRef O_SzT1 As Integer, _
ByVal O_T2 As String, ByRef O_SzT2 As Integer, _
ByRef DesignData As TDesignData, _
ByRef O_Hint1 As Integer, ByRef O_Hint2 As Integer, _
ByVal O_Err As String, ByRef O_SzErr As Integer) As Integer
 
I have written the following call, but it won't work. 
public struct TDesignData
    {
        public int O_OP1;
        public int O_OP2;
        public double O_Q1;
        public double O_Q2;
        public double O_QU1;
        public double O_QU2;
        public double O_QN1;
        public double O_QN2;
        public double O_QC1;
        public double O_QC2;
        public double O_QH1;
        public double O_QH2;
        public double O_P1;
        public double O_P2;
        public double O_E1;
        public double O_E2;
        public double O_EN1;
        public double O_EN2;
        public double O_VG1;
        public double O_VG2;
        public float O_MT1;
        public float O_MT2;
        public double O_M1;
        public double O_M2;
        public double O_ME1;
        public double O_ME2;
        public double O_QS1;
        public double O_QS2;
        public double O_TE1;
        public double O_TE2;
        public double O_PE1;
        public double O_PE2;
        public double O_TL1;
        public double O_TL2;
        public double O_TH1;
        public double O_TH2;
        public double O_QA1;
        public double O_QA2;
        public double O_VOL1;
        public double O_VOL2;
        public double O_TOL1;
        public double O_TOL2;
        public double O_TJ1;
        public double O_TJ2;
        public double O_PJ1;
        public double O_PJ2;
    }
 
  [DllImport("OS50.DLL", 
                   CallingConvention = CallingConvention.StdCall)]
 
 
    public static extern int CopyDesign(
                         string I_RPath, 
                         int I_Flags,
                         string I_Typ,
                         int I_CC,
                           string I_Ref,
                           double I_Q,
                           double I_T0,
                             double I_TC,
                       double I_TS,
                      double I_TL,
                      long I_TM,
                       double I_TLC,
                       int I_TOC,
                       double I_TOL,
                       double I_TN,
                       int I_OP,
                     float I_SP,
                       float I_CR,
                      string O_T1, 
                   int O_SzT1,
                     string O_T2, 
                     int O_SzT2, 
                TDesignData DesignData, 
                    int O_Hint1, 
                    int O_Hint2,
                  string O_Err,
                   int O_SzErr);
 
 
 
        public void Run()
        {
 
 
                        I_RPath = RefFiles;
 
                        I_Flags = (CalcWithIPUnits + BIM_Mode);
                        I_Flags = (I_Flags + CalcWithSuperheat);
                        I_Flags = (I_Flags + CalcWithSubcooling);
                        I_CC = 0;
                        I_Ref = "R717";
                        I_TS = 1.8;
                        I_TL = 0;
                        I_TM = 0;
                        I_TLC = 0;
                        I_TOC = 0;
                        I_TOL = 175;
                        I_TN = 0;
                        I_OP = 1;
                        I_SP = 3500;
                        I_CR = 100;
                        I_Typ="OSNA5351";
                        I_T0=-30;
                        I_TC=75;
 
                        errorMsgNum = CopyDesign(I_RPath, 
                                                 I_Flags, 
                                                 I_Typ, 
                                                 I_CC, 
                                                 I_Ref, 
                                                 I_Q, 
                                                 I_T0, 
                                                 I_TC, 
                                                 I_TS, 
                                                 I_TL, 
                                                 I_TM, 
                                                 I_TLC, 
                                                 I_TOC, 
                                                 I_TOL, 
                                                 I_TN, 
                                                 I_OP, 
                                                 I_SP, 
                                                 I_CR, 
                                                 O_T1, 
                                                  O_SzT1, 
                                                 O_T2, 
                                                 O_SzT2, 
                                               DesignData, 
                                               O_Hint1, 
                                                 O_Hint2, 
                                                 O_Err, 
                                                  O_SzErr);
 
            Console.WriteLine(DesignData.O_Q1);
            Console.WriteLine(DesignData.O_M1);
            Console.WriteLine(DesignData.O_P1);
            Console.WriteLine(DesignData.O_VOL1);
            Console.WriteLine(DesignData.O_TOL1);

It gives me Access Violation Exception. Attended to read or write protected memory. I am new to C# and have been trying to resolve this in the last couple days. Any help will be appreciated.