Azure ServiceBus from Xamarin Forms PCL

Question

How is brokered messaging via Azure Service Bus done from a Xamarin Forms PCL...is there an SDK, library or plugin? If there is a way to hand roll a brokered message, I suppose it could be accomplished with an HttpClient & the REST API ...

Answer 1

I finally have a working method for posting a message to an Azure Service Bus Queue from a Xamarin PCL! The way to do this is via an HttpClient.

Gotchas that made the solution elusive:

• The nuget package for Microsoft.ServiceBus.Messaging will happily install, but has no exposure to the portable class.

• WebClient examples abound, but there is no webclient available in the PCL!

• PCL has no native crypto for HMACSHA256 for Shared Access Signature tokens. Get the PCLCrypto package from nuget.
• Some documentation says request headers of Content-Type : application/atom+xml;type=entry;charset=utf-8, and BrokerProerties are required. Ain't so!

• The path to post to is: BaseServiceBusAddress + queue + "/messages"

  public const string ServiceBusNamespace = [YOUR SERVICEBUS NAMESPACE];
  
  public const string BaseServiceBusAddress = "https://" + ServiceBusNamespace + ".servicebus.windows.net/";
  
  /// <summary>
  /// The get shared access signature token.
  /// </summary>
  /// <param name="sasKeyName">
  /// The shared access signature key name.
  /// </param>
  /// <param name="sasKeyValue">
  /// The shared access signature key value.
  /// </param>
  /// <returns>
  /// The <see cref="string"/>.
  /// </returns>
  public static string GetSasToken(string sasKeyName, string sasKeyValue)
  {
      var expiry = GetExpiry();
      var stringToSign = WebUtility.UrlEncode(BaseServiceBusAddress ) + "\n" + expiry;
  
      var algorithm = WinRTCrypto.MacAlgorithmProvider.OpenAlgorithm(MacAlgorithm.HmacSha256);
      var hasher = algorithm.CreateHash(Encoding.UTF8.GetBytes(sasKeyValue));
      hasher.Append(Encoding.UTF8.GetBytes(stringToSign));
      var mac = hasher.GetValueAndReset();
      var signature = Convert.ToBase64String(mac);
  
      var sasToken = string.Format(CultureInfo.InvariantCulture, "SharedAccessSignature sr={0}&sig={1}&se={2}&skn={3}", WebUtility.UrlEncode(baseAddress), WebUtility.UrlEncode(signature), expiry, sasKeyName);
      return sasToken;
  }
  
  /// <summary>
  /// Posts an order data transfer object to queue.
  /// </summary>
  /// <param name="orderDto">
  /// The order data transfer object.
  /// </param>
  /// <param name="serviceBusNamespace">
  /// The service bus namespace.
  /// </param>
  /// <param name="sasKeyName">
  /// The shared access signature key name.
  /// </param>
  /// <param name="sasKey">
  /// The shared access signature key.
  /// </param>
  /// <param name="queue">
  /// The queue.
  /// </param>
  /// <returns>
  /// The <see cref="Task"/>.
  /// </returns>
  public static async Task<HttpResponseMessage> PostOrderDtoToQueue(OrderDto orderDto, string serviceBusNamespace, string sasKeyName, string sasKey, string queue)
  {
      using (var client = new HttpClient())
      {
          client.BaseAddress = new Uri(BaseServiceBusAddress);
          client.DefaultRequestHeaders.Accept.Clear();
  
          var token = GetSasToken(sasKeyName, sasKey);
          client.DefaultRequestHeaders.Add("Authorization", token);
  
          HttpContent content = new StringContent(JsonConvert.SerializeObject(orderDto), Encoding.UTF8);
          content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
  
          var path = BaseServiceBusAddress + queue + "/messages";
  
          return await client.PostAsync(path, content);
      }
  }
  
  /// <summary>
  ///     Gets the expiry for a shared access signature token
  /// </summary>
  /// <returns>
  ///     The <see cref="string" /> expiry.
  /// </returns>
  private static string GetExpiry()
  {
      var sinceEpoch = DateTime.UtcNow - new DateTime(1970, 1, 1);
      return Convert.ToString((int)sinceEpoch.TotalSeconds + 3600);
  }
  

  }