Azure functions with service bus trigger not working with REST API

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Hi everyone,

i am banging my head on the following task:

I have an AzureFunction that uses a ServiceBus-Trigger like so:

{
  "bindings": [
    {
      "name": "myQueueItem",
      "type": "serviceBusTrigger",
      "direction": "in",
      "connection": "MyConnection",
      "accessRights": "Listen",
      "topicName": "MyTopic",
      "subscriptionName": "MySubscription"
    }
  ],
  "disabled": false
}

public static void Run(byte[] myQueueItem, TraceWriter log)
{
    log.Info($"Incoming queue item was: {myQueueItem}");
}

On the sender side I use the following code:

private static void SendMessagesToServiceBus()
		{
			string senderConnectionString = CloudConfigurationManager.GetSetting("Microsoft.ServiceBus.ConnectionStringSender");
			TopicClient senderClient = TopicClient.CreateFromConnectionString(senderConnectionString);

			var content = Encoding.UTF8.GetBytes("Some string");
			BrokeredMessage message = new BrokeredMessage(content);
			message.ContentType = "text/plain";
			senderClient.Send(message);


			System.Console.WriteLine("Test message");
		}

That works like a charm. Now th trouble starts. I have a new client that has no reference to the Aszure.ServiceBus assembly. So I can't use BrokeredMessage to send the message to service bus. Instead I tried to use the REST API of the service bus and came up with the following:

private static void SendCheckpointServiceBusApi()
{
   var sas = createToken("sb://myendpoint.servicebus.windows.net/", "TopicSender", "MySecretKey");
   client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", sas);
   String message = "Some string";
   ByteArrayContent postContent = new ByteArrayContent(Encoding.UTF8.GetBytes(message));
   postContent.Headers.Add("ContentType", "text/plain");
   var response = client.PostAsync("https://myendpoint.servicebus.windows.net/topic" + "/messages" + "?timeout=60", postContent);
   response.Wait();
   response.Result.EnsureSuccessStatusCode();

}


Whenever sending this to the service bus I get the following exception

Exception while executing function: Functions.CheckpointToDatabase. Microsoft.Azure.WebJobs.Host: Exception binding parameter 'myQueueItem'. System.Runtime.Serialization: There was an error deserializing the object of type System.Byte[]. The input source is not correctly formatted. System.Runtime.Serialization: The input source is not correctly formatted.

I already learned that the BrokeredMessage intenally uses Serialization. That should be switched off by adding contenttype text/plain.

Any help is most appreciated.

What I have tried:

I already tried different combinations of ContentType (text/plain, application/xml, application/json) and parameter types (string, byte[] and stream). Everyhting works when the sender site has access to BrokeredMessage. Using rest and plain UTF8 content the azure function can not deserialize.