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public class DownloadStreamedZippedFilesActionResult : FileResult { private const int Zip64CentralDirectoryRecordSize = 56; private const int Zip64CentralDirectoryLocatorSize = 20; /// /// This was obtained by debugging DotNetZip. It might change in newer versions of the library. The size is hardcoded with no constant available, not even a private one. /// private const int DotNetZipLocalFileHeaderExtraFieldLength = 20; private const int DotNetZipCentralDirectoryHeaderExtraFieldLength = 32; private const string ZipExtension = ".zip"; private static readonly ILogger Logger = ApplicationLogging.CreateLogger(); private readonly IReadOnlyCollection files; private readonly CancellationToken cancellationToken; public DownloadStreamedZippedFilesActionResult(string fileName, IEnumerable files, CancellationToken cancellationToken) : base(MediaTypeConverter.GetMimeType(ZipExtension)) { this.files = files.ToArray(); FileDownloadName = fileName + ZipExtension; this.cancellationToken = cancellationToken; } public DownloadStreamedZippedFilesActionResult(string fileName, IEnumerable files) : this(fileName, files, new CancellationToken(false)) { } public override Task ExecuteResultAsync(ActionContext context) { var response = context.HttpContext.Response; response.OnStarting(() => { var contentDisposition = new ContentDispositionHeaderValue("attachment"); contentDisposition.SetHttpFileName(FileDownloadName); response.Headers.Add("access-control-expose-headers", new[] { "content-disposition", "content-length" }); response.Headers.Add("content-disposition", contentDisposition.ToString()); response.Headers.ContentLength = CalculateContentLength(); return Task.FromResult(0); }); var callback = GetCallback(); return callback(response.Body); } private Func GetCallback() { return async (outputStream) => { byte[] buffer = new byte[4096]; try { var countingStream = new CountingStream(outputStream); using (outputStream) using (var zipStream = new Ionic.Zip.ZipOutputStream(countingStream, true)) { zipStream.CompressionLevel = Ionic.Zlib.CompressionLevel.Level0; zipStream.CompressionMethod = Ionic.Zip.CompressionMethod.None; zipStream.EnableZip64 = Zip64Option.Always; foreach (var file in files) { using (var fileContent = await file.GetContentAsync()) { zipStream.PutNextEntry(file.Name); StreamUtils.Copy(fileContent, zipStream, buffer); zipStream.Flush(); outputStream.Flush(); } cancellationToken.ThrowIfCancellationRequested(); } zipStream.Flush(); zipStream.Close(); outputStream.Flush(); } } catch (Exception e) { Logger.LogError(e, "Unexpected error streaming {FileName}", FileDownloadName); throw; } }; } private long CalculateContentLength() { /* ZIP file size constants according to the specification https://users.cs.jmu.edu/buchhofp/forensics/formats/pkzip.html and https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT * Please note this only works with compression type 0 (store, no compression). * This formula also takes into account the additional content DotNetZip adds in the extra fields. It might be different for other zip providers. * ZIP constants come from SharpZipLib https://github.com/icsharpcode/SharpZipLib/blob/master/src/ICSharpCode.SharpZipLib/Zip/ZipConstants.cs */ return files.Count * (ZipConstants.LocalHeaderBaseSize + ZipConstants.Zip64DataDescriptorSize + ZipConstants.CentralHeaderBaseSize + DotNetZipLocalFileHeaderExtraFieldLength + DotNetZipCentralDirectoryHeaderExtraFieldLength) + 2 * files.Sum(f => f.Name.Length) + files.Sum(f => f.Size) + ZipConstants.EndOfCentralRecordBaseSize + Zip64CentralDirectoryRecordSize + Zip64CentralDirectoryLocatorSize; } } public record class StreamedFile { private readonly Func> fileFunc; public StreamedFile(string name, long size, Func> fileFunc) { this.fileFunc = fileFunc; Name = name; Size = size; } public string Name { get; } public long Size { get; } /// /// Callback to get the file from S3, Azure Storage or wherever /// public Task GetContentAsync() => fileFunc(); }