.NET/Java PDF, Tiff, Barcode SDK Library

Notice that while methods in a callback interface require the usual OperationCon tract attribute, the interface itself does not need to be marked with ServiceContract. That s because this callback interface is not a contract in its own right it s one half of a duplex contract. So we need to modify the existing IChatService to associate it with this new callback interface (see Example 13-9).

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Bidirectional communication is problematic on the Internet today The vast majority of computers are behind firewalls Firewalls are usually configured to reject most incoming connections There will be exceptions for machines such as web servers and mail servers administrators set up firewalls to allow certain kinds of traffic through to such machines but the default presumption is that any incoming attempts to connect to a service should be blocked unless the firewall has been explicitly told to leave it open This is a good default from a security perspective, because the vast majority of unexpected incoming connections are from hackers Any machine connected directly to the Internet without a firewall will be subject to a continuous stream of traffic from hackers looking for machines that they might try to break into.

To get started, you need an application to translate. You ll use the SDI application from 4, with the additional features it was extended with in 8 (when file handling support was added). You can see a screenshot from the application in Figure 10-1. Because I m a native Swedish speaker, the task will be to translate the application into Swedish.

Typical firewall configuration insulates machines from this stream of attacks, providing an extra line of defense, just in case you get behind on installing OS updates or some hacker uses a so-called zeroday attack that exploits a bug which hasn t yet been fixed One problem with this is that it makes bidirectional communication difficult if you re using HTTP HTTP operations can be initiated only by the computer that opened the connection in the first place there s no way to open a connection to a web server and then wait for it to send a message to you HTTP is asymmetric, in that nothing happens until the client sends a request (The lower-level protocol that HTTP runs on top of [TCP] is more flexible than this, by the way that s one reason for using sockets.

The first is a string with the name of the data range In this case, it is Closing Price If you were superimposing data ranges on the chart (as in Figure 11-15), you would give them their distinct names here The second parameter is the PointPairList The third is the color for this range, which in this case is ColorSlateBlue, and the final parameter is the SymbolType used to indicate a point on the line If you refer to Figure 11-15, you will see that some points are indicated with triangles or diamonds You would specify these here Because the graph has a lot of points, these would cause it to look cluttered, so you won t use a symbol type for this example LineItem priceCurve = paneAddCurve("Closing Price", pt, ColorSlateBlue, SymbolType.

Either party in a TCP connection is free to send data at any time regardless of which end originally initiated the connection) To enable full bidirectional communication over HTTP, you need both ends to be running an HTTP server When using duplex communication with WCF in conjunction with an HTTP-based binding, WCF runs what is effectively a miniature web server in the client process Of course, this is only any use if the server is able to connect back to that client-side mini server If both the client and the server are behind the same firewall, that won t be a problem But if the server is on the Internet, publicly accessible to anyone, it almost certainly won t be able to connect back to most clients So the technique that is shown in Example 13-8 works only for private networks.

To make a chat program that works over the Internet requires the use of either TCP and sockets, or some slightly hacky HTTP tricks that are beyond the scope of this book The upshot of this is that you ll want to avoid duplex contracts for Internet-facing applications..

Figure 10-1. The SDI application The translations are kept in two different file formats: ts and qm. The ts files are used during development and contain all words found in the application in an easily maintainable XML file format. The qm files are used at run-time and contain the phrases in a portable compressed format. The idea is to use the ts files as source files during development. The ts files are then compiled into the distributable qm format used by the actual applications. The compilation is referred to as releasing the translation. Before you can start translating the application, you need to notify Qt of your intent. Since the target language is Swedish as spoken in Sweden, and the commonly used code for that locale is sv_SE, you can add it to the end of the application name: SDI_sv_SE.

[ServiceContract( CallbackContract=typeof(IChatClient), SessionMode=SessionMode.Required)] public interface IChatService {

[OperationContract] bool Connect(string name); [OperationContract] void PostNote(string note); [OperationContract] void Disconnect();

}