.NET/Java PDF, Tiff, Barcode SDK Library

will be an asynchronous transaction. Next, it specifies the dotheupdate function to handle the readystate changes on the XMLHttpRequest object. The dotheupdate function checks to see whether the readyState is 4, indicating that the process is complete, and if it is, it updates the TOT field with the returned value from the XMLHttpRequest object. To get this application to work, add a new C# web form to the project, and keep the default name of WebForm2.aspx. In the page designer, delete all the default HTML so that the page contains just the ASPX declaration: <%@ Page language="c#" Codebehind="WebForm2.aspx.cs" AutoEventWireup="false" Inherits="Atlas1_1.WebForm2" %> Then, add the following code to the C# code file: private void Page_Load(object sender, System.EventArgs e) { int a = 0; int b = 0; if (Request.QueryString["A"] != null) { a = Convert.ToInt16(Request.QueryString["A"].ToString()); } if (Request.QueryString["B"] != null) { b = Convert.ToInt16(Request.QueryString["B"].ToString()); } Response.Write(a+b); } This handles the request from WebForm1.aspx, getting the values of A and B, and writes them back to the response buffer. The XMLHttpRequest object within WebForm1.aspx then handles the communication and asynchronously updates the Returned Total box.

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This chapter began with an image file format plugin for Qt; it continues by creating an image filtering application in which the filters are provided as plugins. A quick glance of what to expect can be seen in Figure 11-5: filters are on the left and right; the original image appears above the filtered image.

Summary

In this chapter, we reviewed the various types of errors that might occur in our software and looked at several strategies for handling them. These include ignoring the problem, aborting the application, returning errors, and throwing exceptions. We also saw some of the benefits and pitfalls of returning errors, and how exceptions can often provide a more robust and flexible means of alerting your clients to problems. We saw how we can handle exceptions in layers, sometimes catching, using, and then rethrowing an exception, sometimes wrapping an implementation exception in a public exception type, and sometimes allowing exceptions to bubble up to the next layer of handlers. We saw what happens when an unhandled exception pops out at the top of the stack, and how we can use finally blocks at each layer to ensure that application state remains consistent, and resources can be released, whether exceptions occur or not. We then took a quick review of some of the most common exceptions provided by the framework, and how we might use them. Finally, we looked at creating our own exception types and why we might (and might not) wish to do so. We ve come a long way in the past few chapters, covering all of the everyday C# programming concepts you ll need. In the next few chapters, we ll look at features of the .NET Framework in more detail, and how we can best use them in C#; starting with the collection classes.

An appdomain is a kind of process within a process. We ll talk about them a little more in 11 and 15, but they re mainly used by systems that need to host code, such as ASP.NET.

Most programs have to deal with multiple pieces of information. Payroll systems need to calculate the salary of every employee in a company; a space battle game has to track the position of all the ships and missiles; a social networking website needs to be able to show all of the user s acquaintances. Dealing with large numbers of items is a task at which computers excel, so it s no surprise that C# has a range of features dedicated to working with collections of information. Sets of information crop up so often that we ve already seen some of what C# has to offer here. So we ll start with a more detailed look at the collection-based features we ve already seen, and in the next chapter we ll look at the powerful LINQ (Language Integrated Query) feature that C# offers for finding and processing information in potentially large sets of information.

A filter is used to take one image and return a new image that is a transformed version of the given image, which means that it needs a method to take an image and return an image Because you re planning to load it as a plugin, the application can t know the name of each filter from the start thus it also needs a method returning its name How do you transform these lines into an actual plugin interface A Qt plugin interface is defined as a class consisting of pure virtual methods This means that all the methods that are a part of the plugin are made virtual and are left unimplemented Instead they are marked as =0 in the class declaration.

The ability to work with collections is so important that the .NET Framework s type system has a feature just for this purpose: the array. This is a special kind of object that can hold multiple items, without needing to declare a field for each individual item. Example 7-1 creates an array of strings, with one entry for each event coming up in one of the authors calendars over the next few days. You may notice a theme here (although one misfit appears to be a refugee from an earlier chapter s theme, but that s just how the author s weekend panned out; real data is never tidy).

string[] eventNames = { "Swing Dancing at the South Bank", "Saturday Night Swing", "Formula 1 German Grand Prix", "Swing Dance Picnic", "Stompin' at the 100 Club" };