SharpZipLib-561.cs

using (var outStream = new MemoryStream())
{
    var zipOutputStream = new ZipOutputStream(outStream);
    await outStream.FlushAsync(cancellationToken);
    outStream.Seek(0, SeekOrigin.Begin);
    zipOutputStream.SetLevel(CompressionMax);
    foreach (var file in files)
    {
        using (var inStream = new MemoryStream())
        {
            await _fileStorageApi.ReadToStreamAsync(inStream, file.Location.ContainerName,
                file.Location.FileName, cancellationToken);
            var inBuffer = new byte[inStream.Length];
            inStream.Seek(0, SeekOrigin.Begin);
            var length = await inStream.ReadAsync(inBuffer, 0, inBuffer.Length, cancellationToken);
            var entry = new ZipEntry(file.OriginalName)
            {
                DateTime = DateTime.Now, Size = inBuffer.Length
            };
            zipOutputStream.PutNextEntry(entry);
            await zipOutputStream.WriteAsync(inBuffer, 0, length, cancellationToken);
        }
    }

    outStream.Seek(0, SeekOrigin.Begin);
    var outBuffer = new byte[outStream.Length];
    await outStream.ReadAsync(outBuffer, 0, outBuffer.Length, cancellationToken);
    zipOutputStream.Finish();
    zipOutputStream.Close();
    return outBuffer;
}

using (var outStream = new MemoryStream())
{
    var zipOutputStream = new ZipOutputStream(outStream);
    await outStream.FlushAsync(cancellationToken);
    outStream.Seek(0, SeekOrigin.Begin);
    zipOutputStream.SetLevel(CompressionMax);
    foreach (var file in files)
    {
        using (var inStream = new MemoryStream())
        {
            await _fileStorageApi.ReadToStreamAsync(inStream, file.Location.ContainerName,
                file.Location.FileName, cancellationToken);
            var inBuffer = new byte[inStream.Length];
            inStream.Seek(0, SeekOrigin.Begin);
            var length = await inStream.ReadAsync(inBuffer, 0, inBuffer.Length, cancellationToken);
            var entry = new ZipEntry(file.OriginalName)
            {
                DateTime = DateTime.Now, Size = inBuffer.Length
            };
            zipOutputStream.PutNextEntry(entry);
            await zipOutputStream.WriteAsync(inBuffer, 0, length, cancellationToken);
        }
    }

    outStream.Seek(0, SeekOrigin.Begin);
    var outBuffer = new byte[outStream.Length];
    await outStream.ReadAsync(outBuffer, 0, outBuffer.Length, cancellationToken);
    zipOutputStream.Finish();
    zipOutputStream.Close();
    return outBuffer;
}