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Printing Data Matrix in visual basic.net s HANDLING EXCEPTIONS

Figure A-7. Solution Explorer As you can see, the top section of the Solution Explorer window has six icons. Consider them as six different commands that you can perform from within this window; they all have important roles to play. As you can see in Figure A-7, the first command helps you to probe the property of any selected file. The second command is to show all the files; this means to show Bin and Obj folders in the tree, which are hidden by default (see Figure A-8). Next in line is the Refresh command; you can click this button to refresh the Solution Explorer window. As you add or remove files from Solution Explorer, the VS 2005 IDE automatically refreshes the window content to its most current status. The next three buttons on the Toolbar help you to quickly access various aspects of selected files. From the left to the right, they display the code window, the designer associated with the file, and the class diagram.

Note Each signature file must appear before its corresponding implementation file in the compilation order for

You can see the results of this in Figure 16-4.

The initialization process is error-prone. You are likely to be making connections to resources that may be unavailable and over which you have no control. For example, your database server might be unavailable. Without a mechanism for handling such errors, your portlet could end up in an invalid state. As you would expect, the usual exception-handling mechanism comes into play here. The init() method is permitted to throw a PortletException. If it does so, the container is allowed to reattempt to load the portlet at any later time. When constructing an UnavailableException, the portlet can provide a message describing the problem. In this case, the portlet must not be restarted. Use this exception if a configuration setting of the portlet will have to be changed to get the portlet work to properly. For instance, the portlet may require version 9 of a database to connect to, but the database it tried connecting to was version 7.

No two people or organizations will settle on the exact same process for building and releasing their applications. You may encounter several other issues and ideas when preparing your builds.

As with the HttpChannel class, the TcpChannel class implements both the client- and the server-channel part for transmitting messages across the wire using the TCP protocol. Therefore, it is a combination of the TcpClientChannel as well as the TcpServerChannel. By default, it accepts and transmits messages in binary format. Usage example: TcpChannel channel = new TcpChannel(4711); ChannelServices.RegisterChannel(channel); Configuration example: <configuration> <system.runtime.remoting> <application> <channels>

In the course of issuing a request, you may realize that no handlers are available to service the request, but, so long as you know where at least one handler is located on the Internet, you can bring it down to the device. Simply create a fresh Invocation with the URL of the JAD file to install, and then execute the invocation, as in the following example.

Although server-side HTML technologies are popular and widespread, rich JavaScript-based client-centric manipulations are now commonly becoming part of web applications. This client-centric asynchronous interaction approach is popularly known as Ajax. With Ajax, JavaScript and DHTML are becoming more than mere animation decorations. In some cases, Ajax clients are able to provide some of the rich functionality that VM-based options provide. From a Flex perspective, interacting with JavaScript is easy. (You can read 9 to understand how to connect JavaScript and Flex). When replacing JavaScript user interfaces with Flex alternatives, the same remoting infrastructure can be reused, but often the user interface components themselves overlap with what Flex provides and need to be completely discarded. I will discuss a few bits of this in the context of your solutions later in this chapter. Next, let s take a quick look at the VM-based web applications.

A portlet uses a portlet response object to return content to the portal container, change the state of the portlet or the portal, and to pass any other information needed by the portal to build a portal page. Similar to the portlet request object, the PortletResponse interface class is the base class for both the ActionResponse interface and the RenderResponse interface.

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Both page and control events are executed on the server. When they re triggered, the parent page and its view state are posted to the server, where the event is handled and a response (the same or another page) is sent back to the client. A postback occurs if a page posts information to the server requesting the same page back. This round-trip scenario is common: the Reload button in Listing 14-2 and the Generate button in Listing 14-4 are examples of triggering a postback. In both of these cases, you have a postback because you remain on the same page; for example, the originating page handles the data submission by updating the content of a label (an HTML <span> element) in the button click handler. More often, though, data is posted to another page, in which case it s referred to as cross-page posting. You can easily enable cross-page posting by setting the PostBackUrl property of the submitting control. For instance, a SayHello button can be declared as follows: <asp:Button ID="btnSayHello" runat="server" Text="Greet" PostBackUrl="SayIt.aspx" /> There is a significant difference between how data submissions are handled via postback and cross-page posting: whereas the former is done in the submitting control s click event handler (for example, Reload_Click in the timer example), with cross-page posting, the submission event is handled in the Load event handler of the page to which the postback URL points. What slightly complicates matters is that the form data submitted isn t available directly on the receiving page: instead, you have to reference it through the page s PreviousPage property. Because you often place initialization or cross-page posting code in a page event (typically for the Load event), you need to make sure this code is executed only for initialization or when data is being posted to the page. You can do so by checking the IsPostBack property of the Page object, which returns false if the page is requested the first time.