Excel doesn't want to open XLSX files downloaded through ASP output

Question

I use ASP MVC3 framework, created an Excel file and outputted it using FileResult action with content type "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet".

When attempting to open it Excel just says "file is corrupt and cannot be opened."

When I open the source generated Excel file that was used to send the output it works without any problems. I also run file comparison on the bytes for both copies and the files are identical. I tried to email the corrupt file to myself and the attachment opens fine.

This leads me to believe it's a problem with headers or some sort of Excel/Windows security config.

If it is the latter, then I need a solution that won't make clients change their security settings.

EDIT - Found the setting:

I've found what setting causes this - "Enable protected view from files originated from the internet" in Excel's Trust Center / Protected View settings.

So I guess the question is - Is there a way for the file to appear trusted?

Here are the response headers:

• Cache-Control:private
• Content-Disposition:attachment;
• filename="Report - Monday, March 19, 2012.xlsx" Content-Length:20569
• Content-Type:application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

The action method that makes the output:

[HttpPost]
public virtual FileResult Export()
    {   
        try
        {
            ...
            string newFilePath = createNewFile(...);
            string downloadedFileName = "Report - " + DateTime.Now.ToString("D") + ".xlsx";
            return File(newFilePath, "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", downloadedFileName);

        }
        catch (Exception ex)
        {
            ...
        }
    }

How I create the Excel file:

I have a template XLSX file witch column names and some pivot charts in other sheets. From C# I create a copy of this template and then call SQL Server which outputs data into 1st sheet using OLEDB connector:

set @SQL='insert into OPENROWSET(''Microsoft.ACE.OLEDB.12.0'', ''Excel 12.0;Database=' +     @PreparedXLSXFilePath + ';'', ''SELECT * FROM [Data$]'')  ...

Thanks in advance for any help.

Answer 1

The file being written is basically just an HTML version of the data table (change the extension to .txt and open it and see). This is why it's showing up as an error. If you save the file as a .xls instead of .xlsx, Excel should still open it.

(I understand that this issue is over ten years old, but I just came across it myself, so this is for anyone who comes after.)

Answer 2

You would need a digital signature in your Excel file. How to do this from code is another question.

More info here:

http://www.delphifaq.com/faq/windows_user/f2751.shtml