Resolving 404 Error When Downloading Files From Azure Blob Storage Using Blazor WASM

Unlock the secrets to managing file downloads in your Blazor WebAssembly application backed by Azure Blob Storage and avoid those pesky `404 errors`!
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Troubleshooting File Downloads in Blazor WASM with Azure Blob Storage

If you’ve ever experienced the frustration of encountering a 404 error while attempting to download files from Azure Blob Storage in your Blazor WebAssembly (WASM) application, you’re not alone. This article dives into a common issue developers face when working with Blazor and Azure Blob Storage, and I will guide you through the steps to resolve it effectively.

Understanding the Issue

When using Azure Blob Storage with a Blazor application, the primary goal is to easily stream files to users. However, developers sometimes face obstacles, such as the infamous 404 error. This error indicates that the file cannot be found or served correctly. This issue typically arises from incorrect setups in the code responsible for handling file downloads.

Context of the Problem

In this example, a developer attempts to utilize the ABP.IO Blazor WASM framework alongside the Azurite emulator to download files from Azure Blob storage. While they can successfully list the blobs and use Swagger to confirm API functionality, the download action fails with a 404 error.

Steps to Resolve the Download Issue

Let’s break down the resolution into clearly defined sections.

1. Ensuring Correct API Endpoint

First, let’s ensure that the API endpoint for file downloads is correctly implemented. Here's a simplified version of the API code provided:

[[See Video to Reveal this Text or Code Snippet]]

Make sure this endpoint is accessible and correctly set up to fetch blobs by their name.

2. Update File Service Method

The next step is to ensure the method fetching the blob is using the correct approach. The deprecated DownloadAsync() should be replaced with DownloadContentAsync() as shown below:

[[See Video to Reveal this Text or Code Snippet]]

Ensure that ContentDto is correctly set up to handle a byte array instead of a stream, as efficiency during file transfers is key.

3. Validating the Blazor Download Logic

Once the server-side setup is ready, review the Blazor page that triggers the download. Take a look at the Download method:

[[See Video to Reveal this Text or Code Snippet]]

In this snippet, make sure that you have the JavaScript function download defined and included, as it handles the file download within the browser.

4. JavaScript Integration

Here’s the necessary JavaScript function for triggering the download:

[[See Video to Reveal this Text or Code Snippet]]

Make sure this script is correctly bundled into your application. If you're unable to execute the JavaScript file during the download, you must ensure it’s imported correctly in the application’s global script file via BundleContributor and then run the appropriate terminal commands to bundle it.

Conclusion

By following these steps, you can effectively handle file downloads from Azure Blob Storage in your Blazor WASM application, circumventing the dreaded 404 error. As always, careful attention to detail and ensuring that your setup is correct will save you from many debugging headaches.

Final Thoughts

Integrating files using Azure Blob with Blazor applications can enhance functionality and user experience. Be sure to test thoroughly and adapt your methods based on the unique needs of your application. Happy coding!