How can I get references to BlockBlob objects from CloudBlobDirectory.ListBlobs?

Question

I am using the Microsoft Azure .NET client libraries to interact with Azure cloud storage. I need to be able to access additional information about each blob in its metadata col

Answer 1

This approach has been developed for Java, but I hope it can somehow be modified to fit any other supported language. Despite the functionality you ask has not been explicitly developed yet, I think I found a different (hopefully less clunky) way to access CloudBlockBlob data from a ListBlobItem element.

The following code can be used to delete, for example, every blob inside a specific directory.

String blobUri;
CloudBlobClient blobClient = /* Obtain your blob client */

try{
     CloudBlobContainer container = /* Obtain your blob container */

     for (ListBlobItem blobItem : container.listBlobs(blobPrefix)) {

          if (blobItem instanceof CloudBlob) {
                blob = (CloudBlob) blobItem;
                if (blob.exists()){
                    System.out.println("Deleting blob " + blob.getName());
                    blob.delete();
                }
          }
     }
}catch (URISyntaxException | StorageException ex){
        Logger.getLogger(BlobOperations.class.getName()).log(Level.SEVERE, null, ex);
}

Answer 2

The previous answers are good. I just wanted to point out 2 things:

1) Nowadays ASYNC programming is recommended to do and supported by Azure SDK as well. So try to use it:

CloudBlobDirectory dir = container.GetDirectoryReference(dirPath);
var blobs = dir.ListBlobs(true);

foreach (IListBlobItem item in blobs)
{
    CloudBlockBlob blob = (CloudBlockBlob)item;
    await blob.FetchAttributesAsync(); //Use async calls...
}

2) Fetching Metadata in a separate call is not efficient. The code makes 2 HTTP request per blob object. ListBlobs() method supports getting Metadata with as well in one call by setting BlobListingDetails parameter:

CloudBlobDirectory dir = container.GetDirectoryReference(dirPath);
var blobs = dir.ListBlobs(useFlatBlobListing: true, blobListingDetails: BlobListingDetails.Metadata);

I recommend to use second code it it is possible. Since it is the most efficient way to fetch Metadata.

Answer 3

OK... I found a way to do this, and while it seems a little clunky and indirect, it does achieve the main thing I thought should be doable, which is to cast the IListBlobItem directly to a CloudBlockBlob object.

What I am doing is getting the list from the Directory object's ListBlobs() method and then looping over each item in the list and casting the item to a CloudBlockBlob object and then calling the FetchAttributes() method to retrieve the properties (including the metadata). Then add a new "info" object to a new list of info objects. Here's the code I'm using:

CloudBlobDirectory dir = container.GetDirectoryReference(dirPath);

var blobs = dir.ListBlobs(true);

foreach (IListBlobItem item in blobs)
{
    CloudBlockBlob blob = (CloudBlockBlob)item;
    blob.FetchAttributes();
    files.Add(new ImageInfo
    {
        FileUrl = item.Uri.ToString(),
        FileName = item.Uri.PathAndQuery.Replace(restaurantId.ToString().PadLeft(3, '0') + "/", ""),
        ImageName = blob.Metadata["Name"]
    });
}

The whole "Blob" concept seems needlessly complex and doesn't seem to achieve what I'd have thought would have been one of the main features of the Blob wrapper. That is, a way to expand search capabilities by allowing a query over name, directory, container and metadata. I'd have thought you could construct a linq query that would read somewhat like: "return a list of all blobs in the 'images' container, that are in the 'natural/landscapes/' directory path that have a metadata key of 'category' with the value of 'sunset'". There doesn't seem to be a way to do that and that seems to be a missed opportunity to me. Oh, well.

If I'm wrong and way off base here, please let me know.