CloudBlob.DownloadToStream returns null

Your problem is that your input stream pointer is set to end of the steam (See the screen shot, Length and Position both shows same value) that's why when you read it you always get null. You would need to set to input stream pointer to 0 using Stream.Position = 0 as below:

public static byte[] ReadFully(Stream input){    byte[] buffer = new byte[16 * 1024];    input.Position = 0; // Add this line to set the input stream position to 0    using (MemoryStream ms = new MemoryStream())    {        int read;        while ((read = input.Read(buffer, 0, buffer.Length)) > 0)        {            ms.Write(buffer, 0, read);        }        return ms.ToArray();    }} 

How about using the OpenRead() method on the CloudBlob object?

public static string ReadFully(string blobUri, string itemUri){    // e.g. itemUri == "foo.txt"    //   if there is a folder "bar" with foo.txt, provide instead: "bar/foo.txt"    CloudBlobContainer cloudBlobContainer = new CloudBlobContainer(new Uri(blobUri));    CloudBlob blobReference = cloudBlobContainer.GetBlobReference(itemUri);    using (var stream = blobReference.OpenRead())    {        using (StreamReader reader = new StreamReader(stream))        {            return reader.ReadToEnd();        }    }}

I have tried implementing the code above, but to my suprise, the function GetBlockBlobReference was not present in CloudBlobClient but in CloudBlockBlob.

Maybe the DLLs changed through time.

Therefore I present you with my adaptation:

public class BlobStorageHelper{    private readonly CloudBlobClient _blobClient;    protected readonly CloudStorageAccount StorageAccount;    public string _containerName { get; set; }    public BlobStorageHelper()    {        _blobClient = base.StorageAccount.CreateCloudBlobClient();        _containerName = ConfigurationManager.AppSettings["StorageContainerName"];        StorageAccount = CloudStorageAccount.Parse(ConfigurationManager.AppSettings["StorageConnectionString"]);    }    protected Stream DownloadBlobAsStream(string blobUri)    {        CloudStorageAccount account = this.StorageAccount;        CloudBlockBlob blob = GetBlockBlobReference(account, blobUri);        Stream mem = new MemoryStream();        if (blob != null)        {            blob.DownloadToStream(mem);                        }        return mem;    }    private CloudBlockBlob GetBlockBlobReference(CloudStorageAccount account, string blobUri)    {        string blobName = blobUri.Substring(blobUri.IndexOf("/" + _containerName + "/")).Replace("/" + _containerName + "/", "");        CloudBlobClient blobclient = account.CreateCloudBlobClient();        CloudBlobContainer container = _blobClient.GetContainerReference(_containerName);        container.CreateIfNotExists();        CloudBlockBlob blob = container.GetBlockBlobReference(blobName);        return blob;    }    public byte[] DownloadBlobAsByeArray(string blobUri)    {        Stream inputStream = DownloadBlobAsStream(blobUri);        byte[] buffer = new byte[16 * 1024];        inputStream.Position = 0; // Add this line to set the input stream position to 0        using (MemoryStream ms = new MemoryStream())        {            int read;            while ((read = inputStream.Read(buffer, 0, buffer.Length)) > 0)            {                ms.Write(buffer, 0, read);            }            return ms.ToArray();        }    }}