ASP.NET Web PDF Document Viewer/Editor Control Library

Sadly, the dictionary has no way of knowing that the hash for that key has changed; so, when we come to look up the value using our original reference to our key, it will no longer find a match This can (and does!) cause all sorts of subtle bugs in applications built on runtimes that allow mutable strings But since NET strings are immutable, this problem cannot occur if you use strings as keys Another, related, benefit is that you avoid the buffer-overrun issues so prevalent on other runtimes Because you can t modify an existing string, you can t accidentally run over the end of your allocation and start stamping on other memory, causing crashes at best and security holes at worst Of course, immutable strings are not the only way the .

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Summary

NET designers could have addressed this problem, but they do offer a very simple solution that helps the developer fall naturally into doing the right thing, without having to think about it We think that this is a very neat piece of design So, we can obtain (ie, read) a character at a particular index in the string, using the square-bracket indexer syntax What about slicing and dicing the string in other ways.

For many years, programmers have written methods that detect errors, and which report those errors by returning an error code. Typically, this is a Boolean value of some kind, with True representing success and False failure. Or you might use either an int or an enum if you need to distinguish lots of different types of errors.

Before we add an error return value, we should remove the code we just added that silently enforces the constraints. We can delete EnsurePlat formSize and any references to it. (Or if you re following along in Visual Studio and don t want to delete the code, just comment out all the relevant lines.)

The first case where you can see some Ajax functionality in action is when you attempt to log in. If you haven t yet used the application or registered any users, try logging in with the username Arnie and any password (see Figure 7-11). When you attempt to do this and log in, you ll see that the Login pane updates without a complete page refresh (see Figure 7-12).

Using files on different platforms usually means trouble. The incompatibilities are found on all levels: file names, directory paths, line breaks, endianess, and so on. You can avoid problems with paths, drives, and file names by using the QDir and QFileInfo classes. After you locate a file, you can open it by using QFile. Qt has streams to read and write data. If you use the QTextStream class, you can handle text files with ease; if you use the QDataStream class, it is easy to serialize and read back your data from binary files. Just think about the potential stream-versioning problem. Even if you use the same Qt versions for all your application deployments, you will get more versions in the future. A simple setVersion call can save days of frustration. One alternative to storing your data as text or in a custom binary format is to use XML. Qt enables you to use DOM, which allows you to read an entire XML document into memory, modify it, and then write it back to a file. If you want to read an XML file without having to load it all at once, you can use Qt s SAX classes. When you use XML, you need to add the line QT += xml to your project file because the XML support is implemented in a separate module. This module is not included in all editions of Qt, so verify that you have access to it before trying to use it. Finally, you saw the missing piece of the SDI application. Adding the methods covered in the final section of this chapter makes it easy to build applications that support file loading and saving.

So where are we going to return the error from Our first instinct might be to put it in the RunFor method, where we suggested earlier; but look at the code there s nothing substantive there. The problem actually occurs in Rotate. What happens if we change the Rotate method later so that it depends on different properties Do we also update RunFor to check the new constraints Will we remember It is Rotate that actually uses the properties, so as a rule of thumb we should do the checking there. It will also make the debugging easier later we can put breakpoints near the origin of the error and see what is going wrong. Let s change the Rotate method and see what happens (see Example 6-6).

private bool Rotate(double duration) { if (PlatformWidth <= 0.0) { return false; }

}

// This is the total circumference of turning circle double circum = Math.PI * PlatformWidth; // This is the total distance traveled double d = duration * MotorSpeed; if (LeftMotorState == MotorState.Reversed) { // And we're going backwards if the motors are reversed d *= -1.0; } // So we've driven it this proportion of the way round double proportionOfWholeCircle = d / circum; // Once round is 360 degrees (or 2pi radians), so we have traveled CurrentOrientation = CurrentOrientation + (Math.PI * 2.0 * proportionOfWholeCircle); return true;