Parallel downloading multiple files from azure storage and create zip file.

Question

I have a Background Worker Process, where I need to download multiple files from Azure Storage and then create a ZIP file with those files. All the process will be done in memory.

So, I created the below code.

SemaphoreSlim semaphore = new(1, 1);
ParallelOptions options = new() { MaxDegreeOfParallelism = 10 };

using (MemoryStream zipStream = new MemoryStream())
{
	using (ZipArchive zipArchive = new ZipArchive(zipStream, ZipArchiveMode.Update, false))
	{
		Parallel.ForEach(supportingDocLists, options, supportingDoc =>
		{
			if (!string.IsNullOrWhiteSpace(supportingDoc.FileURL) && !string.IsNullOrWhiteSpace(supportingDoc.FileName))
			{
				BlobClient blobClient = containerClient.GetBlobClient(supportingDoc.FileURL);
				
				if (blobClient.Exists())
				{
					BlobDownloadInfo downloadInfo = blobClient.Download();

					// Store it in the zip file sequentially.
					//semaphore.Wait();
					
					ZipArchiveEntry zipEntry = zipArchive.CreateEntry(supportingDoc.FileName);
					lock(zipEntry)
					{
						using (Stream entryStream = zipEntry.Open())
						{
							blobClient.DownloadTo(entryStream);
						}
					}                                    
				}                                
			}
		});
	}

	zipStream.Position = 0;
	return zipStream.ToArray();

But the code is not working properly. I am having the exception (Cannot access a closed stream). Can anyone help me to resolve the problem.

Answer 1

Hi @Nasir Uddin,

When using using, it disposes the object and closes the stream being used. Do not use using statement for MemoryStream:

MemoryStream zipStream = new MemoryStream();
using (ZipArchive zipArchive = new ZipArchive(zipStream, ZipArchiveMode.Update, false))
{
}

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