C#调用Delphi dll文件问题

最近手头一个项目，需要用到C#调用Delphi写出的dll文件（调用的是modelDLL.dll，但是该dll文件还需要另外一个borlndmm.dll文件支持，调用的时候已经把这两个dll文件放到debug目录下了），直接附上相关代码：

Delphi相关源码：

type
    TCharStr=array[0..599] of char;

用Delphi写的直接调用可运行，代码如下：

procedure TMainDLLForm.PedBitBtnClick(Sender: TObject);
var
  fileName:TCharStr;
begin

        OpenDataFileDlg.InitialDir:= GetCurrentDir;
        OpenDataFileDlg.Title:='加载模型文件';
        OpenDataFileDlg.Filter := '定量模型[*.mdl]|*.mdl|定性模型[*.mdr]|*.mdr';
        if OpenDataFileDlg.Execute then
        begin
           StrPCopy(FileName,OpenDataFileDlg.FileName);
           tmpD:=NIRSAModelForPred(graphyData,dataLength,FileName,targetName);
        end;  
       if compareText(fileExt,'.MDR')=0 then
       begin
         memo1.Lines.Add('定性模型: '+ExtractFileName(FileName));
         memo1.Lines.Add(Format('预测结果: %10s：%0.0f',[targetName,tmpD]));
       end;
            memo1.Lines.Add('--------------');
            memo1.Lines.Add(trim(NIRSAPretreatInfor(FileName)));//调用该函数
            memo1.Lines.Add('--------------');
            memo1.Lines.Add(trim(NIRSAModelInfor(FileName)));
end;

而我现在用C#调用出现"尝试读取或写入受保护的内存。这通常指示其他内存已损坏"的错误，代码如下所示：

 [MarshalAs(UnmanagedType.LPStr, SizeConst = 600)]
        public string fileName;

        [DllImport(@"modelDLL.dll", CharSet = CharSet.Auto, CallingConvention = CallingConvention.StdCall)]
        [return: MarshalAs(UnmanagedType.LPStr, SizeConst = 600)]
        public static extern string NIRSAPretreatInfor(ref string fileName);

        private void preCalcButton_Click(object sender, EventArgs e)
        {
            OpenFileDialog dialog = new OpenFileDialog();
            dialog.Multiselect = false;
            if (dialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                fileName = dialog.FileName;
                string result = NIRSAPretreatInfor(ref fileName);
                modelInfoTextBox.Text = result;
            }                       
        }

该段代码主要参考Stack Overflow的一些解决方案，地址：链接，现在解决不了，没有C币了，请大神支支招，谢谢！

[MarshalAs(UnmanagedType.LPStr, SizeConst = 600)]
public string fileName;

这个换成
public StringBuilder fileName = new StringBuilder(600);

PS: Delphi 版本: 7.0

Delphi导入DLL代码:

 implementation
       function  
NIRSAModelForPred(Data:TGraphyData;dataLength:integer;ModelFileName:TCharStr;var targetName:TCharStr):double;stdcall;external 'modelDLL.dll';
       function  NIRSAModelInfor(ModelFileName:TCharStr):TCharStr;stdCall;external 'modelDLL.dll';
       function  NIRSAPretreatInfor(ModelFileName:TCharStr):TCharStr;stdCall;external 'modelDLL.dll';