ASP.NET Web PDF Document Viewer/Editor Control Library

Notice how we re able to initialize the field to a default value right where we declare it, by using the = operator. (This sort of code is called, predictably enough, a field initializer.) Alternatively, we could have initialized it inside a constructor, but if the default is a constant value, it is conventional to set it at the point of declaration. What about the first example of a field that we saw the one we used as the backing data for a property in Example 3-12 We didn t explicitly initialize it. In some other languages that would be a ghastly mistake. (Failure to initialize fields correctly is a major source of bugs in C++, for example.) Fortunately, the designers of .NET decided that the trade-off between performance and robustness wasn t worth the pain, and kindly initialize all fields to a default value for us numeric fields are set to zero and fields of other types get whatever the nearest equivalent of zero is. (Boolean fields are initialized to false, for example.)

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There s also a security reason for this initialization. Because a new object s memory is always zeroed out before we get to see it, we can t just allocate a whole load of objects and then peer at the uninitialized values to see if anything interesting was left behind by the last object that used the same memory.

Defining a field for our scale factor is an improvement, but we could do better. Our 1.609344 isn t ever going to change. There are always that many kilometers per mile, not just for this instance of a Plane, but for any Plane there ever will be. Why allocate the storage for the field in every single instance Wouldn t it be better if we could define this value just once, and not store it in every Plane instance

C# provides a mechanism for declaring that a field holds a constant value, and will never, ever change. You use the const modifier, as Example 3-14 shows.

If you thought initialized flag handling was simple, you will find the methods in Listing 6-28 even easier. The methods isContainer(),icon(),toolTip(), and whatsThis() return as little as possible. You can easily give your widget a custom icon, a tooltip, and What s this text. Listing 6-28. Simple methods returning the least possible bool CircleBarPlugin::isContainer() const { return false; } QIcon CircleBarPlugin::icon() const { return QIcon(); } QString CircleBarPlugin::toolTip() const { return ""; } QString CircleBarPlugin::whatsThis() const { return ""; }

Once you ve added these controls, you ll see them on the tab you created earlier (see Figure 6-6).

const double kilometersPerMile = 1.609344;

The platform now takes advantage of the fact that this can never change, and allocates storage for it only once, no matter how many instances of Plane you new up. Handy. This isn t just a storage optimization, though. By making the field const, there s no danger that someone might accidentally change it for some reason inside another function he s building in the class the C# compiler prevents you from assigning a value to a const field anywhere other than at the point of declaration.

In general, when we are developing software, we re trying to make it as easy as possible for other developers (including our future selves ) to do the right thing, almost by accident. You ll often hear this approach called designing for the pit of success. The idea is that people will fall into doing the right things because of the choices you ve made.

The includeFile(),name(), and domXml() methods return standardized strings built from the class name. It is important to return the same class name from both the name and domXml methods. Notice that the name is case sensitive. You can see the methods in Listing 6-29. Listing 6-29. Returning XML for the widget, header file name, and class name QString CircleBarPlugin::includeFile() const { return "circlebar.h"; } QString CircleBarPlugin::name() const { return "CircleBar"; } QString CircleBarPlugin::domXml() const { return "<widget class=\"CircleBar\" name=\"circleBar\">\n" "</widget>\n"; }

Some aspects of an object don t fit well as either a normal modifiable field or a constant value. Take the plane s identifier, for example that s fixed, in the sense that it never changes after construction, but it s not a constant value like kilometersPerMile. Different planes have different identifiers. .NET supports this sort of information through read-only properties and fields, which aren t quite the same as const.

In Example 3-5, we made our Plane class s Identifier property private. This prevented users of our class from setting the property, but our class is still free to shoot itself in the foot. Suppose a careless developer added some code like that in Example 3-15, which prints out messages in the SpeedInMilesPerHour property perhaps in order to debug some problem he was investigating.

public double SpeedInMilesPerHour { get { return SpeedInKilometersPerHour / kilometersPerMile; } set { Identifier += ": speed modified to " + value; Console.WriteLine(Identifier); SpeedInKilometersPerHour = value * kilometersPerMile; } }