Reliable image handling on Azure platform / Dotnet core

Question

I'm struggling a little with images on the Azure platform under dotnet core and I'm hoping someone can make a sensible suggestion. Simple enough premise: user uploads image, saved in a database as base64 (about to move to Azure storage blob, but that's irrelevant to this). Later on, site owner comes along and clicks a button to get all these images down in a ZIP file. In the old days of .net framework this was easy. Now I seem to be hitting a set of 'yes, but' comments.

Yes, there's system.drawing.image but you can't use that because it's not in dotnet core (until recently). Yes, you can use CoreCompat but it doesn't work on Azure because in Web Applications there's no support for GDI+. Yes, even if I could, I'm developing on a Mac so it won't work locally as far as I can see.

I have tried beta4 of ImageSharp without a lot of success. It's random - sometimes it works, sometimes it just throws OutOfMemoryException. I have tried SkiaSharp but similar results; sometimes it works, sometimes it spits out random error messages.

I'm not doing anything fancy in terms of processing, no resizing or anything. It should be a case of load file to byte array from Convert.FromBase64String, create Zip file entry, ultimately spit out zip file. The ZIP portion is fine, but I need something decent that can do the image work.

Here's a bit of code:

if(!string.IsNullOrEmpty(del.Headshot))
{
    var output=SKImage.FromBitmap(SKBitmap.Decode(Convert.FromBase64String(del.Headshot)));
    MemoryStream savedFile=new MemoryStream();
    output.Encode(SKEncodedImageFormat.Jpeg, 100).SaveTo(savedFile);
    string name=$"{del.FirstName} {del.LastName} - {del.Company}".Trim(Path.GetInvalidFileNameChars()) + "_Headshot.jpg";
    ZipArchiveEntry entry=zip.CreateEntry(name);
    using(Stream entryStream=entry.Open())
    {
        entryStream.Write(savedFile.ToArray(), 0, Convert.ToInt32(savedFile.Length));
    }
    output.Dispose();
    savedFile.Dispose();
}

Can anyone give me a sensible suggestion for a library that can handle images, cross-platform and on Azure, before I pull out what little hair remains!

Thanks

EDIT: The first answer is technically correct, I don't need anything else. However, I might have been a bit wrong when I said I wasn't doing any image manipulation. Because it's all base64 without a filename being stored anywhere, I've no idea what sort of file it is. I'm therefore saving each one as JPEG to ensure that I can always output that file type and extension. Users I guess could be uploading JPG / PNG or even GIF.

Answer 1

Technically you do not need any of those other imaging (unless you are doing more that just zipping the content). Convert the base64 to byte array and pass that to the zip file. No need to save to disk just to read it back again for zipping.

//...

if(!string.IsNullOrEmpty(del.Headshot)) {
    var imageBytes = Convert.FromBase64String(del.Headshot);
    string name = $"{del.FirstName} {del.LastName} - {del.Company}".Trim(Path.GetInvalidFileNameChars()) + "_Headshot.jpg";
    ZipArchiveEntry entry = zip.CreateEntry(name);
    using(Stream entryStream = entry.Open()) {
        entryStream.Write(imageBytes, 0, imageBytes.Length));
    }
}

//...

Also using a minor hack for known image types when converted to base64

public static class ImagesUtility {
    static IDictionary<string, string> mimeMap = 
        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
        {
            { "IVBOR", "png" },
            { "/9J/4", "jpg" },
            //...add others
        };

    /// <summary>
    /// Extract image file extension from base64 string.
    /// </summary>
    /// <param name="base64String">base64 string.</param>
    /// <returns>file extension from string.</returns>
    public static string GetFileExtension(string base64String) {
        var data = base64String.Substring(0, 5);        
        var extension = mimeMap[data.ToUpper()];
        return extension;
    }
}

You could try to determine the file extension from its prefix

if(!string.IsNullOrEmpty(del.Headshot)) {
    var imageBytes = Convert.FromBase64String(del.Headshot);
    var ext = ImagesUtility.GetFileExtension(del.Headshot) ?? "jpg";
    string name = $"{del.FirstName} {del.LastName} - {del.Company}".Trim(Path.GetInvalidFileNameChars()) + $"_Headshot.{ext}";
    ZipArchiveEntry entry = zip.CreateEntry(name);
    using(Stream entryStream = entry.Open()) {
        entryStream.Write(imageBytes, 0, imageBytes.Length));
    }
}

Now ideally, if you are able to control the type of image uploaded, then you should also validate and do what ever image processing when the data is being saved along with any needed metadata (ie content type). That way when extracting it from storage, you can be confident that it is the correct type and size. That saves you having to worry about that later on.