private void btnExit_Click(object sender, EventArgs e)
{
SaveFileDialog sf = new SaveFileDialog();
sf.Filter = "Excel Filles(*,xls)|*.xls";
sf.CheckFileExists = false;
sf.CheckPathExists = false;
sf.FilterIndex = 0;
sf.RestoreDirectory = true;
sf.CreatePrompt = true;
if (sf.ShowDialog() == DialogResult.OK)
{
Stream savestr = sf.OpenFile();
DataGridViewHelper.DataGridViewToExcel(dgvBook, savestr);
}
}