framework2.0升级3.5 excel 列名被剃掉

以前用framework2.0做的导出excel：

//导出Excel
        public void readxlsfile(string modelfilepath, string newfilepath, System.Data.DataTable dt)
        {
            FileInfo fi = new FileInfo(modelfilepath);//读取模板
            fi.CopyTo(newfilepath, true);

            Excel.Application excel = new Excel.ApplicationClass();
            Excel.Workbook workbook = null;
            Excel.Worksheet worksheet = null;
            object missing = System.Reflection.Missing.Value;
            try
            {
                String templatePath = newfilepath;
                //创建一个Application对象并使其可见 
                //app = new Excel.ApplicationClass();

                //打开模板文件，得到WorkBook对象 
                workbook = excel.Workbooks.Open(templatePath, missing, missing, missing, missing, missing,
                missing, missing, missing, missing, missing, missing, missing, missing, missing);

                worksheet = (Excel.Worksheet)workbook.Sheets[1];//指定操作第一个表

                if (dt.Rows.Count >0)
                {
                    string[,] str1 = new string[dt.Rows.Count, dt.Columns.Count];

                    for (Int32 r = 0; r < dt.Rows.Count; r++)
                    {
                        for (int c = 0; c < dt.Columns.Count; c++)
                        {
                            str1[r, c] = (string)dt.Rows[r][c].ToString();
                        }
                    }
                    worksheet.get_Range("A2", worksheet.Cells[dt.Rows.Count, dt.Columns.Count]).Value2 = str1;
                    worksheet.Cells.HorizontalAlignment = Excel.XlHAlign.xlHAlignCenter;
                    worksheet.Cells.VerticalAlignment = Excel.XlVAlign.xlVAlignCenter;
                }

            }
            catch (Exception ex)
            {

                if (worksheet != null)
                {
                    System.Runtime.InteropServices.Marshal.ReleaseComObject(worksheet);
                }

                if (workbook != null)
                {
                    workbook.Close(false, null, null);
                    System.Runtime.InteropServices.Marshal.ReleaseComObject(workbook);
                    workbook = null;
                }

                if (excel != null)
                {
                    excel.Quit();
                    System.Runtime.InteropServices.Marshal.ReleaseComObject(excel);
                    excel = null;
                }
                return;
            }

            excel.Application.DisplayAlerts = false;    //不显示提示信息

            //不显示效果 
            //excel.Visible = false;
            string urlopen = Server.MapPath("~/Manager/Public/Template/职位信息表.xls");
            workbook.SaveCopyAs(urlopen);

            if (worksheet != null)
            {
                System.Runtime.InteropServices.Marshal.ReleaseComObject(worksheet);
                worksheet = null;
            }

            if (workbook != null)
            {
                workbook.Close(false, null, null);
                System.Runtime.InteropServices.Marshal.ReleaseComObject(workbook);
                workbook = null;
            }

            if (excel != null)
            {
                excel.Quit();
                System.Runtime.InteropServices.Marshal.ReleaseComObject(excel);
                excel = null;
            }


            GC.Collect();
            GC.WaitForPendingFinalizers();//挂起当前线程，直到处理终结器队列的线程清空该队列为止

            string path = urlopen;
            System.IO.FileInfo file = new System.IO.FileInfo(path);
            Response.Clear();
            Response.Charset = "GB2312";
            Response.ContentEncoding = System.Text.Encoding.UTF8;
            // 添加头信息，为"文件下载/另存为"对话框指定默认文件名 
            Response.AddHeader("Content-Disposition", "attachment; filename = " + Server.UrlEncode(file.Name));
            // 添加头信息，指定文件大小，让浏览器能够显示下载进度 
            Response.AddHeader("Content-Length", file.Length.ToString());
            // 指定返回的是一个不能被客户端读取的流，必须被下载 
            Response.ContentType = "application/ms-excel";
            // 把文件流发送到客户端 
            Response.WriteFile(file.FullName);
            // 停止页面的执行      
            Response.End();
列名还是存在的。但升级到3.5后再来导出时发现列名没有了，最后
worksheet.get_Range("A2", worksheet.Cells[dt.Rows.Count, dt.Columns.Count]).Value2 = str1;

改为
worksheet.get_Range("A2", worksheet.Cells[dt.Rows.Count+1, dt.Columns.Count]).Value2 = str1;
这里“+1”是指列名（也就是标题栏有1行，当然有时候标题栏有2行，那我们就＋2），就可以了。
很是想不通怎么回事。难道是BUG?