Saturday, 6 August 2011

Passing Objects between Silverlight Applications

This week I have been making some prototype applications using Silverlight, which has been awesome, and I decided that I wanted to pass a few objects between these applications. If this wasn't a quick prototype and didn't have to run off of a Sales person's laptop I would have whipped together a quick bit of RIA Services and got them talking that way. However these are just two out of browser applications.


Enter Local Messaging.

Silverlight has a great little feature, that I'm not sure is too often used, Local Messaging. Local Messaging allows you to send simple string messages between two Silverlight applications running on a local computer. This can be within the same Web Page or two Out of Browser Applications

Local messaging works by having a LocalMessageReciever listening for messages in one application and then a LocalMessageSender sending messages from the other. If you want two way communication you simply have a listener and sender in each application.

// In the receiving application:
LocalMessageReceiver messageReceiver = new LocalMessageReceiver("receiver");

// In the sending application:
LocalMessageSender messageSender = new LocalMessageSender("receiver");

I'm not going to explain how you send messages between the applications as MSDN does a far better job of doing so. However MSDN only talks about simple string messages, I wanted to send objects.


Serialising the objects

The obvious solution I thought was to just serialise the objects to a string and then back at the other end. Silverlight doesn't support binary serialisation and as we have to send the message as a string the Serialisation could be XML, which is common practice and I'm sure most people know instinctively how to do so, however XML is bloated. Alot of our message string would be information we aren't interested in. Now in most applications we don't worry about this and XML is fine however this local app messaging has a maximum string size of 40 kilobytes. We don't want to fill this up with XML notation, instead we want something alot leaner.

Although Silverlight doesn't support Binary Serialisation it does however support JSON. JSON is a far less verbose markup which will mean our messages will be more about the object data and structure rather than syntax which is a good thing

When it comes to JSON serialisation there are a few options, the built in DataContractJsonSerializer , JSON.Net etc. For the ease of setup I have gone for using the build in DataContractJsonSerializer. Here i take a generic object and serialise it to a stream which we can then use in our message.

DataContractJsonSerializer serialiser = new DataContractJsonSerializer(message.GetType());
var stream = new System.IO.MemoryStream();
serialiser.WriteObject(stream, message);

With this string we can simply use a streamreader to get the string that our sendasync message sender method requires.

 new System.IO.StreamReader(stream).ReadToEnd())

But there's a problem, we cant just send the json object, as at the other end we won't know the type. What we have to do then is send the type and the serialised form. This leaves us with the following:

stream.Position = 0;
sender.SendAsync(String.Format("{0}|!|{1}", message.GetType(), new System.IO.StreamReader(stream).ReadToEnd()));

Note: I have used |!| as a way to know where the type name ends and the stream begins, I could have said {type}={jsonserilaisedobject} but I chose not too, for neatness though this should be possible I just chose to use a delimiter I knew wouldn't appear in my serialised object.

Receiving the message and getting the object back

To get the object back in the other application, after receiving the message from the sender we have to split the string based on the delimeter and then deserialise the object.

var type = e.Message.Substring(0, e.Message.IndexOf("|!|"));
var data = e.Message.Substring(e.Message.IndexOf("|!|") + 3);

The DataContractJsonSerializer requires a stram not a string to deserialise an object so next take the data string and turn it into a stream. Then just deserialise.

var stream = new System.IO.MemoryStream(System.Text.Encoding.UTF8.GetBytes(data));
var serialiser = new DataContractJsonSerializer(Type.GetType(type));
if(type == "OurCustomType"){
    var result = (OurCustomType)serialiser.ReadObject(stream);
}

And that's it, obviously handling lots of types would mean not using an if statement, maybe a switch or whatever but the principle is the same.
I hope you find this of use, it certainly made my prototype far more functional and proved a neat way of doing it