private void btnOk_Click(object sender, EventArgs e)
 {
     if (excelReader != null)
     {
         var inds = new HashSet <int>();
         foreach (DataGridViewColumn col in dgvExcelInput.SelectedColumns)
         {
             inds.Add(col.Index);
         }
         ExcelData = ((DataTable)dgvExcelInput.DataSource).Copy();
         for (int i = ExcelData.Columns.Count - 1; i >= 0; --i)
         {
             if (!inds.Contains(i))
             {
                 ExcelData.Columns.RemoveAt(i);
             }
         }
         ExcelData.ReplaceAllOccurrences(DBNull.Value, 0);
         ExcelData.AcceptChanges();
     }
 }