btnImport

using (OpenFileDialog ofd = new OpenFileDialog() { Filter = "Excel Workbook 97-2003|*.xls|Excel Workbook|*.xlsx", ValidateNames = true })
            {
                if (ofd.ShowDialog() == DialogResult.OK)
                {
                    FileStream fs = File.Open(ofd.FileName, FileMode.Open, FileAccess.Read);
                    IExcelDataReader reader;
                    if (ofd.FilterIndex == 1)
                        reader = ExcelReaderFactory.CreateBinaryReader(fs);//Read excel 97-2003
                    else
                        reader = ExcelReaderFactory.CreateOpenXmlReader(fs);//Read excel 2007
                    reader.IsFirstRowAsColumnNames = true;
                    result = reader.AsDataSet();
                    cboSheet.Items.Clear();
                    //Add sheet to comboxbox
                    foreach (System.Data.DataTable dt in result.Tables)
                        cboSheet.Items.Add(dt.TableName);
                    reader.Close();
                }
            }