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In the majority of cases, your Silverlight applications will access data through web services. However, Silverlight provides another mechanism that, though rarely used, can be quite powerful. This mechanism is socket communications. In this section, you will look at a greatly simplified example of communicating with a server via sockets and TCP. The main purpose here is to give you a taste of using sockets in Silverlight so you have a basic understanding of the process and can consider whether you would like to take this approach. If so, you can refer to a more advanced resource, such as Pro Silverlight 3 in C# 2008 by Matthew MacDonald (Apress, 2009). For this example, let s assume that you have a socket server running at the IP address 192.168.1.100 on port 4500. The socket server simply accepts text inputs and does something with them. In Silverlight, you want to connect to that socket server and send it text from a TextBox control. First, you make a connection to the socket server. To do this, you create an instance of a System.Net.Sockets.Socket object for IP version 4 (AddressFamily.InterNetwork). The type will be Stream, meaning it will accept a stream of bytes, and the protocol will be TCP. Socket socket; socket = new Socket( AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp); You need to execute the socket s ConnectAsync() method, but first you must create an instance of SocketAsyncEventArgs to pass to the method, using a statement similar to the following: SocketAsyncEventArgs socketArgs = new SocketAsyncEventArgs() { RemoteEndPoint = new IPEndPoint( IPAddress.Parse("192.168.1.100"), 4500) }; This statement sets the target for the socket connection as 192.168.1.100 on port 4500. In addition, since this is an asynchronous connection, you need to have notification when the connection has been established. To get this notification, you wire up an event handler to be triggered
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CHAPTER 6 DATA ACCESS AND NETWORKING
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on the SocketAsyncEventArgs.Completed event. Once you have that wired up, you simply call the ConnectAsync() method, passing it your SocketAsyncEventArgs instance. socketArgs.Completed += new EventHandler<SocketAsyncEventArgs>(socketArgs_Completed); socket.ConnectAsync(socketArgs); The method for this event handler will first remove the event handler, and then it will examine the response from the socket server. If it is successful, it will send a stream of bytes from your TextBox control to the socket server through your established connection. void socketArgs_Completed(object sender, SocketAsyncEventArgs e) { e.Completed -= socketArgs_Completed; if (e.SocketError == SocketError.Success) { SocketAsyncEventArgs args = new SocketAsyncEventArgs(); args.SetBuffer(bytes, 0, bytes.Length); args.Completed += new EventHandler<SocketAsyncEventArgs>(OnSendCompleted); socket.SendAsync(args); } } Once again, since the calls to the socket are asynchronous, you wire up another event handler called OnSendCompleted, which will fire when your SendAsync() method is completed. This event handler will do nothing more than close the socket. void OnSendCompleted(object sender, SocketAsyncEventArgs e) { socket.Close(); } Although this seems pretty simple, it is complicated by client-access policy permissions. In the same way that a Silverlight application can call a web service on a separate domain only if it has the proper client-access policy permissions, a Silverlight application can call a socket server only if that server contains the proper client-access policy permissions. The following is an example of a clientaccess policy for a socket server: < xml version="1.0" encoding ="utf-8" > <access-policy> <cross-domain-access> <policy> <allow-from> <domain uri="*" /> </allow-from> <grant-to> <socket-resource port="4500-4550" protocol="tcp" /> </grant-to> </policy> </cross-domain-access> </access-policy>
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CHAPTER 6 DATA ACCESS AND NETWORKING
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Recall that when you re using a web service, the client-access policy is contained in a file named clientaccesspolicy.xml, which is placed in the domain s root. In a socket access situation, things are a bit more complex. Before Silverlight will make a socket request to a server on whatever port is requested by the application, it will first make a socket request of its own to the server on port 943, requesting a policy file. Therefore, your server must have a socket service set up to listen to requests on port 943 and serve up the contents of the client-access policy in order for Silverlight applications to be able to make a socket connection.
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In this chapter, you focused on accessing data from your Silverlight applications through WCF services. I also discussed accessing data from different domains and cross-domain policy files. In addition, you looked at using sockets in Silverlight from a high level. In the next chapter, you will look at data validation within Silverlight.
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