/// <summary>
 /// Retrieve column position by name in first row.
 /// </summary>
 /// <param name="sheet">Page to find column</param>
 /// <param name="columnName">Name to find column in first row</param>
 /// <returns>Position of column name or 0 when not found</returns>
 /// <exception cref="ArgumentNullException">When <paramref name="columnName"/> is null o whitespace</exception>
 public static int ColumnByName(this IXLWorksheet sheet, string columnName)
 {
     if (string.IsNullOrWhiteSpace(columnName))
     {
         throw new ArgumentNullException(nameof(columnName), "Enter the column name to get position.");
     }
     if (sheet.RowCount() <= 0)
     {
         return(0);
     }
     for (var i = 1; i <= sheet.ColumnUsedCount(); i++)
     {
         if (sheet.Row(1).Cell(i).Value.ToString()?.ToUpper() == columnName.ToUpper())
         {
             return(i);
         }
     }
     return(0);
 }