如果gridview导出excel文件时，这个文件名是存在且处于打开状态时怎么处理？

下面是一段excel导出的程序：
  private void buttonExcle_Click(object sender, EventArgs e)
          {
              SaveFileDialog saveFileDialog = new SaveFileDialog();
              DfWeb web = new DfWeb(new P101001Service(), this);
              P101001DataSet dset = (P101001DataSet)web.Execute("GetDayListBySearch");
              _griddata = dset.DayList;
              dfDataGridView1.DataSource = _griddata;              saveFileDialog.Filter = "Execl files (*.xls)|*.xls";              saveFileDialog.FilterIndex = 0;              saveFileDialog.RestoreDirectory = true;              saveFileDialog.CreatePrompt = true;              saveFileDialog.Title = "Export Excel File To";
             if (saveFileDialog.ShowDialog() == DialogResult.Cancel)   
             { 
                return;
             }
              Stream myStream;              myStream = saveFileDialog.OpenFile();                StreamWriter sw = new StreamWriter(myStream, System.Text.Encoding.GetEncoding("gb2312"));              StreamWriter sw = new StreamWriter(myStream, System.Text.Encoding.GetEncoding(-0));              string str = "";
              try
              {
                  for (int i = 0; i < dfDataGridView1.ColumnCount; i++)
                  {
                      if (i > 0)
                      {                          str += "\t";                      }
                      str += dfDataGridView1.Columns[i].HeaderText;                  }
                  sw.WriteLine(str);                    写内容                  for (int j = 0; j < dfDataGridView1.Rows.Count; j++)
                  {                      string tempStr = "";                      for (int k = 0; k < dfDataGridView1.Columns.Count; k++)
                      {
                          if (k > 0)
                          {
                              tempStr += "\t";
                          }
                          object value = dfDataGridView1.Rows[j].Cells[k].Value;
                          if (value == null)
                          {
                              value = string.Empty;
                          }
                          tempStr += value.ToString();                      }
                      sw.WriteLine(tempStr);
                  }                  sw.Close();
                  myStream.Close();
              }
              catch (Exception ex)
              {
                  MessageBox.Show(ex.ToString());              }
              finally
              {
                  sw.Close();
                  myStream.Close();
              }
          }
如果gridview导出excel文件时，这个文件名是存在且处于打开状态时怎么处理？如果不处理，程序将会崩溃。
求各位大侠鼎力帮助。