使用 AspNet 从 Azure Blob 存储下载和重命名文件

Question

I am developing a web platform to manage the upload/download of files. The front-end is developed in React, the back-end in ASP.NET and Azure Blob Containers is used to store the uploaded files.

As for the upload, I'm using the Microsoft "Azure Storage Client Library" to send files directly from the client to Azure through SAS authentication. This Javascript library allow me to update a progress bar during the whole process.

As for the download, the process is more complicated: the file is first downloaded from the server (phase 1 or Azure->Server) and then it is downloaded from the client (phase 2 or Server->Client). Phase 1 creates two problems for me:

• I cannot display a progress bar to check the progress;
• It can take long time and, at this stage, the client cannot begin the download;

To solve these problems I would like one of the following solutions:

• download the file directly from the client using the Javascript library but in this case it is necessary to rename the file ;
• create a server-client communication to implement a progress bar relating to phase 1;

This is my current C # function to allow the download of a file

using Microsoft.WindowsAzure.Storage.Blob;

private IActionResult DownloadFile(...) {
    ...
    using (var blobStream = new MemoryStream()) {
        string blobName = ...
        CloudBlobContainer cloudBlobContainer = ...
        CloudBlockBlob blob = cloudBlobContainer.GetBlockBlobReference(blobName);
        blob.DownloadToStream(blobStream);
        return new FileContentResult(blobStream.ToArray(), "application/pdf");
    }
}

EDIT:

Below the code I use to generate the SAS token:

private string GetSasReadToken(string connectionString, string containerName) {

    var storageAccount = CloudStorageAccount.Parse(connectionString);
    CloudBlobClient cloudBlobClient = storageAccount.CreateCloudBlobClient();
    CloudBlobContainer cloudBlobContainer = cloudBlobClient.GetContainerReference(containerName);
    var sasConstraints = new SharedAccessBlobPolicy {
        SharedAccessExpiryTime = DateTimeOffset.UtcNow.AddMinutes(60),
        Permissions = SharedAccessBlobPermissions.Read
    };
    var sasContainerToken = cloudBlobContainer.GetSharedAccessSignature(sharedAccessBlobPolicy);
}

Answer 1

In order to make use of Content-Disposition , you will need to generate SAS token on a blob (currently you're creating a SAS token on a blob container). Then you will need to make use of SharedAccessBlobHeaders and define the content-disposition value there.

Here's the sample code (untested though):

private string GetSasReadToken(string connectionString, string containerName, string blobName) {

    var storageAccount = CloudStorageAccount.Parse(connectionString);
    CloudBlobClient cloudBlobClient = storageAccount.CreateCloudBlobClient();
    CloudBlobContainer cloudBlobContainer = cloudBlobClient.GetContainerReference(containerName);
    CloudBlockBlob cloudBlockBlob = cloudBlobContainer.GetBlockBlobReference(blobName);
    var sasConstraints = new SharedAccessBlobPolicy {
        SharedAccessExpiryTime = DateTimeOffset.UtcNow.AddMinutes(60),
        Permissions = SharedAccessBlobPermissions.Read,
    };
    var sasHeaders = new SharedAccessBlobHeaders();
    sasHeaders.ContentDisposition = "attachment;filename=<your-download-file-name>";
    var sasBlobToken = cloudBlockBlob.GetSharedAccessSignature(sharedAccessBlobPolicy, sasHeaders);
}