Write zip files to MemoryStream

Question

I have a controller action that creates a zip file and sends back to user for download. The problem is that the zip file gets created but it is empty. Somehow it's not writing the image files to the MemoryStream. I wonder what I am missing. If I write the zip file to the disk everything will work as expected, but I'd rather not save files to the disk if I can avoid it. This is what I have tried using dotnetzip:

public ActionResult DownloadGraphs()
    {
        var state = Session["State"];
        using (ZipFile zip = new ZipFile())
        {
            if (state == "IA")
            {
                zip.AddFile(Server.MapPath("~/Content/DataVizByState/FallGraphs/Watermarked/Fall_IA.jpg"), "");
                zip.AddFile(Server.MapPath("~/Content/DataVizByState/SpringGraphs/Watermarked/Spring_IA.jpg"), "");
            }
            MemoryStream output = new MemoryStream();
            zip.Save(output);
            output.Seek(0, SeekOrigin.Begin);
            var fileName = state + "Graphs.zip";
            return File(output, "application/zip", fileName);
        }
    }

This forces download in the view based on click of a button:

$('#graphDwnldBtn').click(function (evt) {
    window.location = '@Url.Action("DownloadGraphs", "DataSharing")';
})

Do I need to use StreamWriter or Reader or something? This is the first time I have ever attempted something like this and it's been cobbled together by reading various stackoverflow posts...

Answer 1

Dumb mistakes: Session["State"] is an object, so the state variable was coming out as object instead of a string like I need it to be for my conditional statement to evaluate correctly. I cast state to a string to fix it. Fixed code:

public ActionResult DownloadGraphs()
    {
        var state = Session["State"].ToString(); 
        using (ZipFile zip = new ZipFile())
        {
            if (state == "IA")
            {
                zip.AddFile(Server.MapPath("~/Content/DataVizByState/FallGraphs/Watermarked/Fall_IA.jpg"), "");
                zip.AddFile(Server.MapPath("~/Content/DataVizByState/SpringGraphs/Watermarked/Spring_IA.jpg"), "");
            }
            MemoryStream output = new MemoryStream();
            zip.Save(output);
            output.Seek(0, SeekOrigin.Begin);
            var fileName = state + "Graphs.zip";
            return File(output, "application/zip", fileName);
        }
    }