Download blob using Azure Blob storage client library v12 for .NET

Question

I am using Azure.Storage.Blobs version=12.4.1. I have a REST endpoint that I want to use to download blobs from a storage account.

I need to stream the result to a HttpResponseMessage and I do not want to use a MemoryStream. I want to stream the result directly to the calling client. Is there a way to achieve this. How to get the downloaded blob in the HttpResponseMessage content? I do not want to use MemoryStream, since there will be a lot of download requests.

The BlobClient class has a method DownloadToAsync but it requires a Stream as a parameter.

        var result = new HttpResponseMessage(HttpStatusCode.OK);

        var blobClient = container.GetBlobClient(blobPath);
        if (await blobClient.ExistsAsync())
        {
            var blobProperties = await blobClient.GetPropertiesAsync();

            var fileFromStorage = new BlobResponse()
            {                    
                ContentType = blobProperties.Value.ContentType,
                ContentMd5 = blobProperties.Value.ContentHash.ToString(),
                Status = Status.Ok,
                StatusText = "File retrieved from blob"
            };

            await blobClient.DownloadToAsync(/*what to put here*/);
            return fileFromStorage;
        }

Answer 1

You could simply create a new memory stream and download the blob's content to that stream.

Something like:

        var connectionString = "UseDevelopmentStorage=true";
        var blobClient = new BlockBlobClient(connectionString, "test", "test.txt");
        var ms = new MemoryStream();
        await blobClient.DownloadToAsync(ms);

ms will have the blob's contents. Don't forget to reset memory stream's position to 0 before using it.

Answer 2

You need to use

 BlobDownloadInfo download = await blobClient.DownloadAsync();

download.Content is blob Stream. You can use it to copy directly to other stream.

using (var fileStream = File.OpenWrite(@"C:\data\blob.bin"))
{
    await download.CopyToAsync(fileStream);
}

Answer 3

Try to use the code as below to download blob into HttpResponseMessage.

try
{
    var storageAccount = CloudStorageAccount.Parse("{connection string}");
    var blobClient = storageAccount.CreateCloudBlobClient();
    var Blob = await blobClient.GetBlobReferenceFromServerAsync(new Uri("https://{storageaccount}.blob.core.windows.net/{mycontainer}/{blobname.txt}"));
    var isExist = await Blob.ExistsAsync();
    if (!isExist) {
        return Request.CreateErrorResponse(HttpStatusCode.NotFound, "file not found");
    }
    HttpResponseMessage message = new HttpResponseMessage(HttpStatusCode.OK);
    Stream blobStream = await Blob.OpenReadAsync();
    message.Content = new StreamContent(blobStream);
    message.Content.Headers.ContentLength = Blob.Properties.Length;
    message.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue(Blob.Properties.ContentType);
    message.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment")
    {
        FileName = "{blobname.txt}",
        Size = Blob.Properties.Length
    };
    return message;
}
catch (Exception ex)
{
    return new HttpResponseMessage
    {
        StatusCode = HttpStatusCode.InternalServerError,
        Content = new StringContent(ex.Message)
    };
}