Question

I have a controller action declared as follows:

[Authorize(Order = 0, Roles = "Requester,Controller,Installer")]
public FileStreamResult ExportJobCards()

The body of this method builds a collection of CSV lines, and attempts to return them as a file as follows:

using (var sw = new StreamWriter(new MemoryStream()))
{
    foreach (var line in lines)
    {
        sw.WriteLine(line);
    }
    return new FileStreamResult(sw.BaseStream, "text/csv");
}  

When I request this action using the following action link…

Html.ActionLink("Export to Excel", "ExportJobCards")

…the export method executes properly, i.e. all the required CSV data is present in the lines collection in the above code, but I get a File Not Found error rendered as the end result.

EDIT:
In agreement with Tommy’s observation, I moved the return out of the using, and I now get a file, but the file is empty. The new code that actually produces a file, ableit empty, is:

var sw = new StreamWriter(new MemoryStream());            
foreach (var line in lines)
{
    sw.WriteLine(line);
}
sw.Flush();
return new FileStreamResult(sw.BaseStream, "text/csv");  

Answer 1

With your current setup, the Using statement is disposing of the StringWriter before the return can complete, which is resulting in the null reference/file not found error. Remove the using statement or set the StringWriter to another variable before you exit out and you should be good to go on getting rid of the File Not Found error.

A thought on your second issue now, looking into memorystreams as filestream results, you may need to change your return to this

sw.BaseStream.seek(0, SeekOrigin.Begin)
return new FileStreamResult(sw.BaseStream, "text/csv"); 

as the pointer is still at the end of the stream when you return.