In this example, we create a new Excel file using HSSFWorkbook class and then add a new sheet with a row and a cell that contains the string "Hello World". Finally, we write the workbook to a file named "Output.xls". Example 2: Reading an Excel file using HSSFWorkbook classC# HSSFWorkbook workbook = new HSSFWorkbook(new FileStream("Input.xls", FileMode.Open)); ISheet sheet = workbook.GetSheet("Sheet1"); IRow row = sheet.GetRow(0); ICell cell = row.GetCell(0); string value = cell.StringCellValue; ``` In this example, we read an existing Excel file using HSSFWorkbook class and then retrieve the value of the cell in the first row and first column of the "Sheet1" sheet. Overall, NPOI.HSSF.UserModel is a powerful package library for working with Excel files in C# using HSSF format.