Simple .NET/ASP.NET PDF document editor web control SDK

Default and named parameters are very useful features, but we need to warn you of a subtle potential problem. Although they are more-or-less equivalent to providing a bunch of different function overloads, as far as the syntax for the caller goes, under the covers, they are implemented very differently. The compiler marks a parameter to indicate that it is optional using the OptionalAttri bute and there s a DefaultParameterValueAttribute to specify a default value. These two attributes have been around for quite a while they were originally added for the benefit of Visual Basic, long before C# started using them. (Attributes are discussed in 17.) When you call a method (or constructor), the C# compiler always emits a complete call the compiled code passes a full set of arguments to the method, even if your source code left some arguments out. For example, in our Plane example, if you wrote:

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SendMessage("SomeMessage");

s Note The <ErrorTemplate> element is meaningful only when used in conjunction with an UpdatePanel control because it triggers the asynchronous communication used by the UpdatePanel control. So, although the <ErrorTemplate> is defined on the ScriptManager control, it will do nothing until an error is received during an asynchronous update.

but the Plane class only has the method shown in Example 3-34, the compiler actually generates code equivalent to this:

SendMessage("SomeMessage", default(TimeSpan));

Note If w or h is less than 0, the corner specified by x, y is not the top-left corner of the rectangle.

In other words, it plugs in the default value at compile time. This means if you re using some external library that uses default values, and a newer version of the library comes out that changes the default values for some method or constructor, your code won t pick up those new values unless you recompile your code. There s also a subtler problem you can run into. Some parts of the .NET Framework require you to provide a particular constructor overload. For example, it you write a custom control for WPF, and you want to use it from Xaml, it must have a default constructor. (WPF and Xaml are described in 20.) If all your constructors take parameters, then even if you provide default values for all the parameters, that s not good enough. You can write, say, new MyControl() in C#, but only because the C# compiler is implicitly passing the missing values for you. Not everything in the world of .NET understands the concept of default arguments. (C# itself didn t until version 4.0.) Sometimes only a genuine no-arguments constructor will do.

This is not just limited to normal methods you can use this same syntax to provide default values for parameters in your constructors, if you wish. Being forced to delete the extra constructor we tried to add back in Example 3-31 was a little disappointing we re constraining the number of ways users of our type can initialize it. Named arguments and default values have helped, but can we do more

Until C# 3.0, the only real solution to this was to write one or more factory methods. These are described in the sidebar below. But now we have another option.

The second pair of rectangles is drawn according to a given QRect class, which holds the coordinates for the rectangle. In the drawRoundRect call, the rect variable is used directly. In the drawRect call, the rectangle specified by rect is translated, or moved, 45 pixels down. This is achieved by using the translated(int x, int y) method that returns a rectangle of the same size, but moved by the amount of pixels specified. The results of the drawing operations are shown in Figure 7-9. Listing 7-5. Drawing rectangles to a pixmap QPixmap pixmap( 200, 100 ); pixmap.fill( Qt::white ); QPainter painter( &pixmap ); painter.setPen( Qt::black ); painter.drawRect( 10, 10, 85, 35 ); painter.drawRoundRect( 10, 55, 85, 35 ); QRect rect( 105, 10, 85, 35 ); painter.drawRoundRect( rect ); painter.drawRect( rect.translated( 0, 45 ) );

A factory method is a static method that builds a new object. There s no formal support for this in C#, it s just a common solution to a problem a pattern, as popular idioms are often called in programming. We can get around the overload ambiguity problems by providing factory methods with different names. And the names can make it clear how we re initializing the instance:

public static PolarPoint3D FromDistanceAndAngle( double distance, double angle) { return new PolarPoint3D(distance, angle, 0); } public static PolarPoint3D FromAngleAndAltitude( double angle, double altitude) { return new PolarPoint3D(0, angle, altitude); }

It s important to remember the ID values used in this example. You must always use these values (errorMessageLabel and okButton). The ScriptManager control generates XML script

We rather like this approach, although some people frown on it as insufficiently discoverable. (Most developers aren t expecting to find static methods that act rather like constructors, and if nobody finds these methods, we re wasting our time in providing them.) However, this pattern is used all over the .NET Framework libraries DateTime, TimeSpan, and Color are popular types that all use this technique.