ASP.NET PDF Viewer using C#, VB/NET

The lines that call ConsoleWriteLine all use the dynamic variable a with normal C# property syntax The first two do exactly what you d expect if the variable had been declared as MyType or var instead of dynamic: they just print out the values of the Text and Number properties The third one is more interesting it tries to use a property that does not exist If the variable had been declared as either MyType or var, this would not have compiled the compiler would have complained at our attempt to read a property that it knows is not there But because we ve used dynamic, the compiler does not even attempt to check this sort of thing at compile time.

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The close event can have several sources: the user might have clicked Close or Exit from the File menu, or the user might have closed the window by using features of the current platform. If the close event s source is the application exiting, an ignored close event means that no more windows will be closed. The user cancels the entire process of exiting, not just the closing of the current window, which makes it possible to cancel the entire closing-down process of the entire application using the Cancel button of the QMessageBox shown from a single document In 8, you will learn that it is really easy to integrate saving changes at close if you extend the isSafeToClose method. The structure looks unnecessarily complex now because you need to be able to handle the save before closing option as well.

So it compiles, and instead it fails at runtime that third line throws a RuntimeBinderException, with a message complaining that the target type does not define the Problem member we re looking for This is one of the prices we pay for the flexibility of dynamic behavior: the compiler is less vigilant Certain programming errors that would be caught at compile time when using the static style do not get detected until runtime And there s a related price: IntelliSense relies on the same compile-time type information that would have noticed this error If we were to change the variable in Example 18-11 s type to either MyType or var, we would see IntelliSense pop ups such as those shown in Figure 18-1 while writing the code.

Visual Studio is able to show the list of available methods because the variable is statically typed it will always refer to a MyType object But with dynamic, we get much less help As Figure 18-2 shows, Visual Studio simply tells us that it has no idea what s available In this simple example, you could argue that it should be able to work it out although we ve declared the variable to be dynamic, it can only ever be a MyType at this point in the program But Visual Studio does not attempt to perform this sort of analysis for a couple of reasons First, it would work for only relatively trivial scenarios such as these, and would fail to work anywhere you were truly exploiting the dynamic.

nature of dynamic and if you don t really need the dynamism, why not just stick with statically typed variables Second, as we ll see later, it s possible for a type to customize its dynamic behavior, so even if Visual Studio knows that a dynamic variable always refers to a MyType object, that doesn t necessarily mean that it knows what members will be available at runtime. Another upshot is that with dynamic variables, IntelliSense provides the rather less helpful pop up shown in Figure 18-2.

To create from the SdiWindow class, you need to provide a trivial main function that initializes a QApplication object before creating and showing an SdiWindow. The application then runs by itself, creating new windows for new documents and finishing when all documents have been closed. To build it, you also have to create a project file using the file created from running qmake -project is enough. Then simply run qmake followed by make to compile and link the application.

Example 18-11 just reads the properties, but as you d expect, we can set them, too. And we can also invoke methods with the usual syntax. Example 18-12 illustrates both features, and contains no surprises.

dynamic a = new MyType(); a.Number = 42; a.Text = "Foo"; Console.WriteLine(a); dynamic b = new MyType(); b.SetBoth("Bar", 99); Console.WriteLine(b);

Thus, the pendulum has swung back toward the thin client/fat server approach. Ironically, the thin client is probably fatter than the original servers, because it s an operating system that can support a browser that is extended to support XML (through parsers), scripting (through interpreters), and other plug-ins, as well as Java or .NET virtual machines! With all these runtime elements available to developers and a consistent server-side API (through the .NET Framework or Java server side), rich, high-performing applications built on a client/server model are now fully possible.

Our MyType example also overloads the + operator it defines what should occur when we attempt to add two of these objects together. This means we can take the two objects from Example 18-12 and pass them to the AddAnything method from Example 18-4, as Example 18-13 shows.