using NPOI.HSSF.UserModel;
using NPOI.SS.UserModel;
using NPOI.XSSF.UserModel;

IWorkbook hssfwb;
using (FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read))
{
    if (fileName.IndexOf(".xlsx") > 0) // 2007版本
        hssfwb = new XSSFWorkbook(fs);
    else // if (fileName.IndexOf(".xls") > 0) // 2003版本
        hssfwb = new HSSFWorkbook(fs); 
}

ISheet sheet = hssfwb.GetSheetAt(0); // 或 ISheet sheet = hssfwb.GetSheet("Arkusz1");
for (int row = 1; row <= sheet.LastRowNum; row++)
{
    if (sheet.GetRow(row) != null) //null is when the row only contains empty cells
    {
        var s = string.Format("Row {0} = {1}", row, sheet.GetRow(row).GetCell(0).StringCellValue);
    }
}