导入excel的问题.....

        private void toExcel()
        {
      SaveFileDialog saveFileDialog = new SaveFileDialog(); 
            saveFileDialog.Filter = "Execl  files  (*.xls)|*.xls"; 
            saveFileDialog.FilterIndex = 0; 
            saveFileDialog.RestoreDirectory = true; 
            saveFileDialog.CreatePrompt = true;  //提示是否创建(*****).xls文件 
            saveFileDialog.Title = "导出Excel文件到";             // saveFileDialog.ShowDialog(); 
            if (saveFileDialog.ShowDialog() == DialogResult.OK) 
            { 
                Stream myStream; 
                myStream = saveFileDialog.OpenFile(); 
                StreamWriter sw = new StreamWriter(myStream, System.Text.Encoding.GetEncoding("gb2312"));                 string str = ""; 
                try 
                { 
                    //写标题    
                    for (int i = 0; i < dataGridView1.ColumnCount; i++) 
                    { 
                        if (i > 0) 
                        { 
                            str += "\t"; 
                        } 
                        str += dataGridView1.Columns[i].HeaderText; 
                    }                     sw.WriteLine(str); 
                    //写内容  
                    for (int j = 0; j < dataGridView1.Rows.Count - 1; j++) 
                    { 
                        string tempStr = ""; 
                        for (int k = 0; k < dataGridView1.Columns.Count; k++) 
                        { 
                            if (k > 0) 
                            { 
                                tempStr += "\t"; 
                            } 
                            tempStr += dataGridView1.Rows[j].Cells[k].Value.ToString(); 
                        } 
                        sw.WriteLine(tempStr); 
                    }                     MessageBox.Show("导出数据成功"); 
                    sw.Close(); 
                    myStream.Close();                 } 
                catch (Exception ee) 
                { 
                    MessageBox.Show(ee.Message); 
                    return; 
                } 
                finally 
                { 
                    sw.Close(); 
                    myStream.Close(); 
                } 
            }         }错误信息:未将对象的实例引用到对象  
为什么导出excel可以把标题导出去。 但是内容却没有呢。  是什么问题啊。