Error: The supplied data appears to be in the Office 2007+ XML

Use WorkbookFactory as:

using NPOI.SS.UserModel;

string fileBase64 =// fileBase64 format data 
string fileName = "mySheet.xlsx"; // "mySheet.xls"
byte[] bytes = Convert.FromBase64String(fileBase64);
if (fileName.IndexOf(".xlsx") > -1 || fileName.IndexOf(".xls") > -1)
{
  using (MemoryStream ms = new MemoryStream(bytes))
  {
      IWorkbook wb = NPOI.SS.UserModel.WorkbookFactory.Create(ms);
      NPOI.SS.UserModel.ISheet sheet = wb.GetSheetAt(0);
      for (int row = 1; row <= sheet.LastRowNum; row++)
      {
        var sheetRow = sheet.GetRow(row);
        if (sheetRow != null)
        {
            var cell = sheetRow.GetCell(0);
            //Other processing on this cell
        }
      }
  }
}

The code explicitly requests HSSF, so it will only work with the older versions .xls (binary) files.

If you want, you can code POI to auto-detect which file type you have, and pick the appropriate one of HSSF or XSSF for your case. However, to do that you need to change your code slightly, and use interfaces rather than concrete classes (so your code works whether you get a HSSF or XSSF object)

Replace following line of code

NPOI.HSSF.UserModel.HSSFWorkbook hSSFWorkbook = new NPOI.HSSF.UserModel.HSSFWorkbook(ms);

with

IWorkbook hSSFWorkbook = NPOI.SS.UserModel.WorkbookFactory.Create(ms);

Thanks, it works for me.

Thanks, it works for me also.

great! works for me too.

great! thanks.

great! thank you.