Read Excel File to String Question

Try this office API,add the following code can get the string:

            Workbook workbook=new Workbook();
            workbook.LoadFromFile("test.xlsx");
            Worksheet worksheet=workbook.Worksheets[0];
            string str = worksheet.Range["A1"].Value.ToString();
            Console.WriteLine(str);
            Console.ReadKey();

What assembly or namespace do I use?