.NET/Java PDF, Tiff, Barcode SDK Library

The quick tests at the beginning of the method are sometimes called guard clauses or guards. Unless performance is more important to your application than correct operation (and it usually isn t), it is a good idea to check these preconditions before you attempt to execute the method. Sometimes you will also want a similar set of post-condition tests on exit from the method, to verify that your state is still valid at the end of the operation. The design-by-contract development philosophy requires you to specify these pre- and post conditions as a part of your method contract, and some languages such as Eiffel support declarative specification of these conditions. Microsoft Research is working on an extension of C# called Spec# which includes some of these design-by-contract features. You can read about it at http://research.mi crosoft.com/en-us/projects/specsharp/.

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Now you re ready to have a look at the ASCII art image plugin that can handle text images; the format is called ti. You ll also tell Qt to use ti as the preferred file extension for these text images. The TextImagePlugin class inherits from the QImageIOPlugin class, while the TextImageHandler inherits the QImageIOHandler class (nothing else is in the plugin). Let s start looking at the code, beginning with the class declaration of the TextImagePlugin in Listing 11-1. The interface consists of three methods: keys, capabilities, and create. The keys method returns a QStringList of the image formats that the plugin supports. The capabilities method takes a QIODevice and an image format as arguments and then returns a value indicating whether the plugin CanRead or CanWrite the specified format to or from the given device. The last method, create, creates a QImageIOHandler for a given device and format.

Rather than return an instance of an enum, we throw an instance of the InvalidOpera tionException class.

to be used when an operation fails because the current state of the object itself doesn t allow the method to succeed (rather than, say, because a parameter passed in to the method was incorrect). That seems to fit this case quite nicely, so we can make use of it.

Back before C# 3.0, you could throw an instance of any type you liked (e.g., a string). In C# 3.0, a constraint was added that only types derived from Exception can be thrown.

is supported. This means that it reads the image in several passes, making it possible to show the image gradually. The ASCII art image plugin never attempts to implement it.

If we take a look at the Exception class (see http://msdn.microsoft.com/library/system .exception) we ll see that it has a Message property. That s what we re setting with the string we pass to the constructor, and it can be any text we like preferably something that will help us (or one of our clients) debug the problem in the future. There s also a property called Data. This is a dictionary of key/value pairs that lets us associate more information with the exception, and it can be extremely useful for debugging or logging purposes. Replacing the return value with an exception, we will need to perform a bit of surgery on our application to get it to compile. First, let s change the Turtle.RunFor method so that it no longer returns a value, and delete the TurtleError enumeration (see Example 6-12).

// Run the turtle for the specified duration public void RunFor(double duration) { if (LeftMotorState == MotorState.Stopped && RightMotorState == MotorState.Stopped) { // If we are at a full stop, nothing will happen return; } // The motors are both // then we just drive if ((LeftMotorState == RightMotorState == (LeftMotorState == RightMotorState == { Drive(duration); } // // // if running in the same direction MotorState.Running && MotorState.Running) || MotorState.Reversed && MotorState.Reversed))

these privileges, you use the ASP .NET 2.0 role management service to configure these users into roles that give them the required functionality. To do this, select ASP.NET Configuration from the Website menu in Visual Studio .NET. This will take you to the ASP .NET Web Site Administration tool (see Figure 7-16).

{ }

The motors are running in opposite directions, so we don't move, we just rotate about the center of the rig ((LeftMotorState == MotorState.Running && RightMotorState == MotorState.Reversed) || (LeftMotorState == MotorState.Reversed && RightMotorState == MotorState.Running)) Rotate(duration);