Unexpected behavior on downloading generated Excel file via MemoryStream

Question

I'm currently trying to generate and download an Excel file with ASP.NET MVC 4.

The file generation works fine, but the download have a behavior that I don't understand.

When I click on my download button:

@Html.ActionLink("DOWNLOAD", "Download", "Session", new { sessionId = Model.Session.Id, clientId = Model.Client.Id }, new { @class = "my-link" })

The excel file open with the right content, but it is saved at D:\data\documents\System.IO.MemoryStream which is not my default download directory.

And a empty file is saved at D:\downloads\infos.xls, which is my default download directory.

Directory D:\downloads

18/07/2016  05:10    <DIR>          .
18/07/2016  05:10    <DIR>          ..
18/07/2016  05:10                 0 infos.xls
           1 file(s)                0 octets


Directory D:\data\documents

18/07/2016  05:10    <DIR>          .
18/07/2016  05:10    <DIR>          ..
18/07/2016  05:10             8 521 System.IO.MemoryStream
           1 file(s)                8 521 octets

Here is my controller method:

using Excel = Microsoft.Office.Interop.Excel;

[Authorize]
public class SessionController : ControllerBase
{
    [HttpGet]
    public ActionResult Download(int sessionId, int clientId)
    {
        /* [...] Retreive infos */
        
        Excel.Application excelApp = new Excel.Application();
        Excel.Workbook workbook = excelApp.Workbooks.Add(System.Reflection.Missing.Value);
        Excel.Worksheet worksheet = (Excel.Worksheet)workbook.Worksheets[1];
    
        /* [...]  Insert infos in worksheet */

        byte[] data;

        using (var stream = new System.IO.MemoryStream())
        {
            workbook.SaveAs(stream);
            stream.Position = 0;
            data = stream.ToArray();
        }

        return File(data, "application/vnd.ms-excel", "infos.xls");
    }
}

Why are there are two downloaded files?

Answer 1

It is because that the function is misused. From MSDN:

Worksheet.SaveAs Method (Excel)

Saves changes to the chart or worksheet in a different file.

The input requires a file name, but you put in a stream object.

workbook.SaveAs(stream);

That is why the strange file is created in your default user folder.

To understand why the empty file is created, check this code:

return File(data, "application/vnd.ms-excel", "infos.xls");

Here an empty stream is returned (the stream data is never set), with the name of infos.xls

To perform a file download, you can check the answers here.