Using ASP.NET Web API, how can a controller return a collection of streamed images compressed using DotNetZip Library?

Question

How can I create a Web API controller that generates and returns a compressed zip file streamed from a collection of in-memory JPEG files (MemoryStream objects). I'm attempting to use DotNetZip Library. I found this example: http://www.4guysfromrolla.com/articles/092910-1.aspx#postadlink. But the Response.OutputStream is not available in Web API and so that technique doesn't quite work. Therefore I tried saving the zip file to a new MemoryStream; but it threw. Lastly, I tried using PushStreamContent. Here's my code:

    public HttpResponseMessage Get(string imageIDsList) {
        var imageIDs = imageIDsList.Split(',').Select(_ => int.Parse(_));
        var any = _dataContext.DeepZoomImages.Select(_ => _.ImageID).Where(_ => imageIDs.Contains(_)).Any();
        if (!any) {
            throw new HttpResponseException(new HttpResponseMessage(HttpStatusCode.NotFound));
        }
        var dzImages = _dataContext.DeepZoomImages.Where(_ => imageIDs.Contains(_.ImageID));
        using (var zipFile = new ZipFile()) {
            foreach (var dzImage in dzImages) {
                var bitmap = GetFullSizeBitmap(dzImage);
                var memoryStream = new MemoryStream();
                bitmap.Save(memoryStream, ImageFormat.Jpeg);
                var fileName = string.Format("{0}.jpg", dzImage.ImageName);
                zipFile.AddEntry(fileName, memoryStream);
            }
            var response = new HttpResponseMessage(HttpStatusCode.OK);
            var memStream = new MemoryStream();
            zipFile.Save(memStream); //Null Reference Exception
            response.Content = new ByteArrayContent(memStream.ToArray());
            response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/zip");
            response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment") { FileName = string.Format("{0}_images.zip", dzImages.Count()) };
            return response;
        }
    }

zipFile.Save(memStream) throws null reference. But neither zipFile nor memStream are null and there is no internal exception. So I'm not sure what's causing the null reference. I've very little experience with Web API, memory streams, and I've never used DotNetZipLibrary before. This is a follow up to this question: Want an efficient ASP.NET Web API controller that can reliably return 30 to 50 ~3MB JPEGs

Any ideas? thanks!

Answer 1

A more generic approach would work like this:

using Ionic.Zip; // from NUGET-Package "DotNetZip"

public HttpResponseMessage Zipped()
{
    using (var zipFile = new ZipFile())
    {
        // add all files you need from disk, database or memory
        // zipFile.AddEntry(...);

        return ZipContentResult(zipFile);
    }
}

protected HttpResponseMessage ZipContentResult(ZipFile zipFile)
{
    // inspired from http://stackoverflow.com/a/16171977/92756
    var pushStreamContent = new PushStreamContent((stream, content, context) =>
    {
        zipFile.Save(stream);
        stream.Close(); // After save we close the stream to signal that we are done writing.
    }, "application/zip");

    return new HttpResponseMessage(HttpStatusCode.OK) {Content = pushStreamContent};
}

The ZipContentResult method could also live in a base class and be used from any other action in any api controller.