将数据从DataGridView导入到Execel中抛出奇怪异常!! 苦苦思索，毫无进展，希望高手指点
using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace CangKuGuanli.BLL
{
public class ExportXLS
{
private string tempStr;
private string str;
public void ExportDataGridViewToExcel(DataGridView dataGridview1)
{
SaveFileDialog saveFileDialog = new SaveFileDialog();
saveFileDialog.Filter = "Execl files (*.xls)|*.xls ";
saveFileDialog.FilterIndex = 0;
saveFileDialog.RestoreDirectory = true;
saveFileDialog.CreatePrompt = false;
saveFileDialog.Title = "导出Excel文件到 ";

if (saveFileDialog.ShowDialog() == DialogResult.Cancel)
return;
Stream myStream = saveFileDialog.OpenFile();
StreamWriter sw = new StreamWriter(myStream,System.Text.Encoding.GetEncoding( "gb2312 ")); //gb2312

try
{
//写标题
for (int i = 0; i < dataGridview1.ColumnCount; i++)
{
if (i == 0) dataGridview1.Columns[i].HeaderText = "序号 ";
if (i > 0)
{
str += "\t ";

}

str += dataGridview1.Columns[i].HeaderText;
}

sw.WriteLine(str);
//写内容

1 for(int j = 0; j < dataGridview1.Rows.Count; j++)
2 {
3 tempStr = " ";
4
5 for (int k = 0; k < dataGridview1.Columns.Count; k++)
6 {
7
8 if( k == 0 )
9 dataGridview1.Rows[j].Cells[k].Value = j + 1;
10
11 if (k > 0)
12 {
13 tempStr += "\t ";
14 }

15 tempStr += dataGridview1.Rows[j].Cells [k].Value.ToString();
17 }
18 sw.WriteLine(tempStr);
19 }
20 sw.Close();
21 myStream.Close();
}
catch (Exception e)
{
MessageBox.Show(e.ToString());
}
finally
{
sw.Close();
myStream.Close();
}
}

}
}

//抛出异常：System.NullReferenceException:Objec reference not set to an instance of an object.
单步跟踪到1--18行，所有数据写入Execel文件后，不执行19，20行，却抛出以上异常。




[解决办法]
应该是你的哪个对象为空(null)，你调试看看
[解决办法]
8 if( k == 0 )
{
tempStr += "\t ";
9 dataGridview1.Rows[j].Cells[k].Value = j + 1;
}