private static void Main()
{
  List<String> categories;
  List<String> companies;
  ExtractCategoriesCompanies("NorthwindData.xlsx", out categories, out companies);

  // Do something with the categories and companies
}

private static void ExtractCategoriesCompanies(string northwinddataXlsx, out List<string> categories, out List<string> companies)
{
  categories = new List<string>();
  const int coCategoryId = 1;
  const int coCategoryName = 2;

  var wb = new XLWorkbook(northwinddataXlsx);
  var ws = wb.Worksheet("Data");

  // Look for the first row used
  var firstRowUsed = ws.FirstRowUsed();

  // Narrow down the row so that it only includes the used part
  var categoryRow = firstRowUsed.RowUsed();

  // Move to the next row (it now has the titles)
  categoryRow = categoryRow.RowBelow();

  // Get all categories
  while (!categoryRow.Cell(coCategoryId).IsEmpty())
  {
    String categoryName = categoryRow.Cell(coCategoryName).GetString();
    categories.Add(categoryName);

    categoryRow = categoryRow.RowBelow();
  }

  // There are many ways to get the company table.
  // Here we're using a straightforward method.
  // Another way would be to find the first row in the company table
  // by looping while row.IsEmpty()

  // First possible address of the company table:
  var firstPossibleAddress = ws.Row(categoryRow.RowNumber()).FirstCell().Address;
  // Last possible address of the company table:
  var lastPossibleAddress = ws.LastCellUsed().Address;

  // Get a range with the remainder of the worksheet data (the range used)
  var companyRange = ws.Range(firstPossibleAddress, lastPossibleAddress).RangeUsed();

  // Treat the range as a table (to be able to use the column names)
  var companyTable = companyRange.AsTable();

  // Get the list of company names
  companies = companyTable.DataRange.Rows()
    .Select(companyRow => companyRow.Field("Company Name").GetString())
    .ToList();
}