NullReferenceException in creating Excel worksheet

问题:

i want to fill an excel file and so i use ExcelPackage: Office Open XML Format . but i have an error. my code:

string fileName = "DBE_BAKIM_FORMU" + ".xlsx";
FileInfo fi = new FileInfo(HttpContext.Current.Server.MapPath("~/") + fileName);

using (ExcelPackage xlPackage = new ExcelPackage(fi))
{
    ExcelWorksheet worksheet = xlPackage.Workbook.Worksheets[1];
    dbeDataContext db = new dbeDataContext();
    CAGRI c = (from x in db.CAGRIs where x.CagriID == ID select x).SingleOrDefault();
    USER u = (from x in db.USERs where x.UserID == Convert.ToInt32(Session["user"]) select x).SingleOrDefault();

    worksheet.Cell(6, 3).Value = c.TalepTarihi.ToShortDateString();
    worksheet.Cell(7, 3).Value = c.TalepTuru;
    worksheet.Cell(8, 3).Value = c.ModulAdi;
    worksheet.Cell(9, 3).Value = c.EkranRaporAdi;
    worksheet.Cell(10, 3).Value = c.VerilenSure;
    worksheet.Cell(11, 4).Value = c.USER.UserName + " " + c.USER.UserSurname;
    worksheet.Cell(12, 4).Value = Convert.ToString(c.USER.UserTel);
    worksheet.Cell(13, 3).Value = c.Aciklama;
    worksheet.Cell(16, 4).Value = u.UserName + " " + u.UserSurname;
    worksheet.Cell(18, 3).Value = Convert.ToString(c.DegerlendirmeTarih);
    worksheet.Cell(19, 3).Value = c.Degerlendirme;
    xlPackage.Save();
}

i have this error in here: xlPackage.Save();

my error says that Object reference not set to an instance of an object.

when i take out xlPackage.Save();, it works and fills in the excel file but it does not save. why am i taking this error?

thanks in advance..

回答1:

change line 562 of ExcelWorksheet.cs from:

XmlNode pageSetup = _worksheetXml.SelectSingleNode("//d:pageSetup", NameSpaceManager);

to:

XmlNode pageSetup = WorksheetXml.SelectSingleNode("//d:pageSetup", NameSpaceManager);

If the worksheet has not been accessed, calling the public accessor WorksheetXml instead of the private class variable initiallizes it correctly.

回答2:

Catching the null reference exception being thrown by the xlPackage.Save() method, and dumping the stack trace, the problem seems to be getting caused by ExcelWorksheet.cs:line 561 which corresponds to this line in the source:

XmlNode pageSetup = _worksheetXml.SelectSingleNode("//d:pageSetup", NameSpaceManager);

Interestingly enough, if you catch the exception the xlsx files seems to save ok anyway (at least for me it does, there might be some unexpected behaviour, but I couldn't produce any).

I'd try handle the exception and seeing if you file is actually saving. It might be worth raising an issue on the ExcelPackage site if this is a library you're going to be using a lot (I don't)

EDIT:

It seems that the ExcelPackage library is quite old and not maintained. I'd have a look at the EPPlus library, which is based on ExcelPackage. You'll need to tweak a bit of your code but nothing major. Saving works without problem.

http://epplus.codeplex.com/

回答3:

I'd recommend looking into the Microsoft.Office.Interop.Excel reference. I wish there was more intellisense, but it's pretty easy to figure out.

I've set my using as:

using Excel = Microsoft.Office.Interop.Excel;

I'm guessing you'll want the application to be invisible:

Excel.Application application = new Excel.Application();
application.Visible = false;

I've not used HttpContext, but if it's a path, then I'd advise using the Path class:

string filename = "DBE_BAKIM_FORMU.xlsx";
Excel.Workbook book = application.Workbooks.Open(Path.Combine(HttpContext.Current.Server.MapPath("~/"), filename));

The code should be similar, here's how to grab a sheet and fill cells:

Excel.Worksheet sheet = book.Worksheets[1];
sheet.Cells[6, 3].Value2 = "Something";

Saving and closing:

book.Save();
book.Close();

The Microsoft.Office.Interop.Excel reference is included in Visual Studio 2012, and 2010 if you're still using that version. Excel is required on the machine with the executable.

Best of luck!