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?