c# - How to download a file to browser from Azure Blob Storage

C# - How to download a file to browser from Azure Blob Storage

In C#, you can download a file from Azure Blob Storage and send it to a web browser in an ASP.NET application (such as ASP.NET Core or ASP.NET MVC). This involves retrieving the blob from Azure Blob Storage and then returning it as a file to the browser.

Prerequisites

To interact with Azure Blob Storage, you need the Azure.Storage.Blobs package. Ensure it's installed in your project via NuGet:

dotnet add package Azure.Storage.Blobs

Setting Up Azure Blob Storage

To connect to Azure Blob Storage, you need the connection string or a Shared Access Signature (SAS) URL, and details about the container and blob.

Controller to Download a File

In an ASP.NET Core controller, you can create an endpoint to download a file from Azure Blob Storage and send it to the browser. The key is to fetch the blob from Azure, then return it as a FileStreamResult or FileContentResult with appropriate headers.

Here's an example of a simple controller that downloads a file from Azure Blob Storage and sends it to the browser:

using Azure.Storage.Blobs;
using Microsoft.AspNetCore.Mvc;
using System.IO;
using System.Threading.Tasks;

public class FileDownloadController : Controller
{
    private readonly string connectionString = "Your_Azure_Storage_Connection_String";
    private readonly string containerName = "YourContainerName";

    // Download a specific blob from the Azure container
    public async Task<IActionResult> DownloadFile(string blobName)
    {
        // Create a BlobServiceClient to interact with Azure Blob Storage
        BlobServiceClient blobServiceClient = new BlobServiceClient(connectionString);
        
        // Get a reference to the specified container
        BlobContainerClient containerClient = blobServiceClient.GetBlobContainerClient(containerName);
        
        // Get a reference to the specified blob
        BlobClient blobClient = containerClient.GetBlobClient(blobName);

        // Download the blob content
        BlobDownloadInfo downloadInfo = await blobClient.DownloadAsync();

        // Return the content as a file to the browser
        return File(
            downloadInfo.Content,          // Content stream from Azure Blob Storage
            downloadInfo.ContentType,      // MIME type of the blob
            blobName                       // The suggested file name for the browser
        );
    }
}

Explanation

  • The BlobServiceClient connects to Azure Blob Storage using a connection string.
  • The BlobContainerClient is used to get a reference to the specific container.
  • The BlobClient gets a reference to the specific blob by its name.
  • BlobDownloadInfo is obtained using DownloadAsync, providing the content stream.
  • The File method creates a FileStreamResult or FileContentResult with the appropriate content type and suggested file name.
  • ContentType should be determined from the blob metadata or set manually based on known file types. Default to application/octet-stream if unknown.

Additional Considerations

  • Authorization and Security: Ensure you have proper authorization to access Azure Blob Storage. Consider using Azure Active Directory authentication for enhanced security.
  • Exception Handling: Handle exceptions like BlobNotFoundException for cases when the blob doesn't exist or network issues occur.
  • File Size: Consider the impact of large file downloads on server resources. Use streaming to manage memory efficiently.
  • CORS Policy: If you're accessing this endpoint from a different domain (e.g., an AJAX call), ensure you have a CORS policy that allows cross-origin requests.

With these steps, you can create an endpoint in ASP.NET Core that retrieves a file from Azure Blob Storage and sends it to a browser for download.

Examples

  1. Download File from Azure Blob Storage to Browser in ASP.NET MVC

    • Description: Use CloudBlockBlob to get a file from Azure Blob Storage and download it to the browser in ASP.NET MVC.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using System.Web.Mvc;

public class FileDownloadController : Controller
{
    public ActionResult DownloadFile(string blobName)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);
        var blobStream = blob.OpenRead();

        // Return a FileResult to download the file to the browser
        return File(blobStream, "application/octet-stream", blobName);
    }
}

  2. Download File from Azure Blob Storage with Presigned URL in ASP.NET

    • Description: Generate a presigned URL for a blob, allowing direct download from Azure Blob Storage.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using System;

public class PresignedUrlGenerator
{
    public string GetPresignedUrl(string blobName, int expirationMinutes = 10)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);

        var sasToken = blob.GetSharedAccessSignature(new SharedAccessBlobPolicy
        {
            Permissions = SharedAccessBlobPermissions.Read,
            SharedAccessExpiryTime = DateTime.UtcNow.AddMinutes(expirationMinutes)
        });

        return blob.Uri + sasToken;
    }
}

  3. Download File from Azure Blob Storage in ASP.NET Core

    • Description: Download a file from Azure Blob Storage to the browser in an ASP.NET Core application.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using Microsoft.AspNetCore.Mvc;
using System.IO;

public class FileDownloadController : Controller
{
    [HttpGet("download/{blobName}")]
    public IActionResult DownloadFile(string blobName)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);
        MemoryStream memoryStream = new MemoryStream();
        blob.DownloadToStream(memoryStream);

        memoryStream.Position = 0; // Reset stream position for download
        return File(memoryStream, "application/octet-stream", blobName);
    }
}

  4. Download File from Azure Blob Storage to Browser with Specific MIME Type in C#

    • Description: Download a file from Azure Blob Storage to the browser with a specific MIME type.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using System.Web.Mvc;

public class FileDownloadController : Controller
{
    public ActionResult DownloadFileWithMimeType(string blobName)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);
        var blobStream = blob.OpenRead();

        string mimeType = blob.Properties.ContentType; // Get the MIME type from the blob properties
        return File(blobStream, mimeType, blobName);
    }
}

  5. Download Large File from Azure Blob Storage in ASP.NET MVC

    • Description: Handle downloading large files from Azure Blob Storage in ASP.NET MVC to avoid memory issues.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using System.Web.Mvc;

public class FileDownloadController : Controller
{
    public ActionResult DownloadLargeFile(string blobName)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);
        var blobStream = blob.OpenRead();

        // Ensure that the response is not cached, preventing large files from causing memory issues
        Response.Buffer = false;
        Response.BufferOutput = false;

        return File(blobStream, "application/octet-stream", blobName);
    }
}

  6. Download File from Azure Blob Storage with Download Progress in C#

    • Description: Implement download progress indication when downloading a file from Azure Blob Storage.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using System;
using System.Web.Mvc;

public class FileDownloadController : Controller
{
    public ActionResult DownloadFileWithProgress(string blobName)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);
        var blobStream = blob.OpenRead();

        blob.DownloadProgress += (sender, e) =>
        {
            Console.WriteLine($"Download progress: {e.BytesTransferred} bytes transferred.");
        };

        return File(blobStream, "application/octet-stream", blobName);
    }
}

  7. Download File from Azure Blob Storage in Web API with C#

    • Description: Use a Web API to download a file from Azure Blob Storage to the browser.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using Microsoft.AspNetCore.Mvc;
using System.IO;

[ApiController]
[Route("[controller]")]
public class FileDownloadController : ControllerBase
{
    [HttpGet("download/{blobName}")]
    public IActionResult DownloadFile(string blobName)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);
        MemoryStream memoryStream = new MemoryStream();
        blob.DownloadToStream(memoryStream);

        memoryStream.Position = 0;
        return File(memoryStream, "application/octet-stream", blobName);
    }
}

  8. Download File from Azure Blob Storage in Blazor with C#

    • Description: Handle downloading files from Azure Blob Storage in a Blazor application.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using Microsoft.AspNetCore.Components;
using System.IO;
using System.Threading.Tasks;

public partial class FileDownloader : ComponentBase
{
    private string storageConnectionString = "your-storage-connection-string";

    public async Task DownloadFile(string blobName)
    {
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);

        MemoryStream memoryStream = new MemoryStream();
        await blob.DownloadToStreamAsync(memoryStream);
        memoryStream.Position = 0;

        using (var fileStream = new FileStream(blobName, FileMode.Create, FileAccess.Write))
        {
            memoryStream.CopyTo(fileStream);
        }
    }
}

  9. Download File from Azure Blob Storage with Content-Disposition in C#

    • Description: Ensure the file is downloaded with a specified name using the Content-Disposition header.
    • Code:
      using Microsoft.Azure.Storage;
using Microsoft.Azure.Storage.Blob;
using System.Web.Mvc;

public class FileDownloadController : Controller
{
    public ActionResult DownloadFileWithContentDisposition(string blobName)
    {
        string storageConnectionString = "your-storage-connection-string";
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString);
        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();
        CloudBlobContainer container = blobClient.GetContainerReference("your-container-name");

        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);
        var blobStream = blob.OpenRead();

        Response.AddHeader("Content-Disposition", $"attachment; filename=\"{blobName}\"");

        return File(blobStream, "application/octet-stream", blobName);
    }
}

  10. Download File from Azure Blob Storage with Client-Server Handshake in C#

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