Checking if a blob exists in Azure Storage

Question

The new API has the .Exists() function call. Just make sure that you use the GetBlockBlobReference, which doesn't perform the call to the server. It makes the function as easy as:

public static bool BlobExistsOnCloud(CloudBlobClient client, 
    string containerName, string key)
{
     return client.GetContainerReference(containerName)
                  .GetBlockBlobReference(key)
                  .Exists();  
}

Note: This answer is out of date now. Please see Richard's answer for an easy way to check for existence

No, you're not missing something simple... we did a good job of hiding this method in the new StorageClient library. :)

I just wrote a blog post to answer your question: http://blog.smarx.com/posts/testing-existence-of-a-windows-azure-blob.

The short answer is: use CloudBlob.FetchAttributes(), which does a HEAD request against the blob.

Seem lame that you need to catch an exception to test it the blob exists.

public static bool Exists(this CloudBlob blob)
{
    try
    {
        blob.FetchAttributes();
        return true;
    }
    catch (StorageClientException e)
    {
        if (e.ErrorCode == StorageErrorCode.ResourceNotFound)
        {
            return false;
        }
        else
        {
            throw;
        }
    }
}

If the blob is public you can, of course, just send an HTTP HEAD request -- from any of the zillions of languages/environments/platforms that know how do that -- and check the response.

The core Azure APIs are RESTful XML-based HTTP interfaces. The StorageClient library is one of many possible wrappers around them. Here's another that Sriram Krishnan did in Python:

http://www.sriramkrishnan.com/blog/2008/11/python-wrapper-for-windows-azure.html

It also shows how to authenticate at the HTTP level.

I've done a similar thing for myself in C#, because I prefer to see Azure through the lens of HTTP/REST rather than through the lens of the StorageClient library. For quite a while I hadn't even bothered to implement an ExistsBlob method. All my blobs were public, and it was trivial to do HTTP HEAD.

The new Windows Azure Storage Library already contains the Exist() method. It´s in the Microsoft.WindowsAzure.Storage.dll.

Available as NuGet Package
Created by: Microsoft
Id: WindowsAzure.Storage
Version: 2.0.5.1

See also msdn

This is the way I do it. Showing full code for those who need it.

        // Parse the connection string and return a reference to the storage account.
        CloudStorageAccount storageAccount = CloudStorageAccount.Parse(CloudConfigurationManager.GetSetting("AzureBlobConnectionString"));

        CloudBlobClient blobClient = storageAccount.CreateCloudBlobClient();

        // Retrieve reference to a previously created container.
        CloudBlobContainer container = blobClient.GetContainerReference("ContainerName");

        // Retrieve reference to a blob named "test.csv"
        CloudBlockBlob blockBlob = container.GetBlockBlobReference("test.csv");

        if (blockBlob.Exists())
        {
          //Do your logic here.
        }

Here's a different solution if you don't like the other solutions:

I am using version 12.4.1 of the Azure.Storage.Blobs NuGet Package.

I get an Azure.Pageable object which is a list of all of the blobs in a container. I then check if the name of the BlobItem equals to the Name property of each blob inside the container utilizing LINQ. (If everything is valid, of course)

using Azure.Storage.Blobs;
using Azure.Storage.Blobs.Models;
using System.Linq;
using System.Text.RegularExpressions;

public class AzureBlobStorage
{
    private BlobServiceClient _blobServiceClient;

    public AzureBlobStorage(string connectionString)
    {
        this.ConnectionString = connectionString;
        _blobServiceClient = new BlobServiceClient(this.ConnectionString);
    }

    public bool IsContainerNameValid(string name)
    {
        return Regex.IsMatch(name, "^[a-z0-9](?!.*--)[a-z0-9-]{1,61}[a-z0-9]$", RegexOptions.Singleline | RegexOptions.CultureInvariant);
    }

    public bool ContainerExists(string name)
    {
        return (IsContainerNameValid(name) ? _blobServiceClient.GetBlobContainerClient(name).Exists() : false);
    }

    public Azure.Pageable<BlobItem> GetBlobs(string containerName, string prefix = null)
    {
        try
        {
            return (ContainerExists(containerName) ? 
                _blobServiceClient.GetBlobContainerClient(containerName).GetBlobs(BlobTraits.All, BlobStates.All, prefix, default(System.Threading.CancellationToken)) 
                : null);
        }
        catch
        {
            throw;
        }
    }

    public bool BlobExists(string containerName, string blobName)
    {
        try
        {
            return (from b in GetBlobs(containerName)
                     where b.Name == blobName
                     select b).FirstOrDefault() != null;
        }
        catch
        {
            throw;
        }
    }
}

Hopefully this helps someone in the future.