Information Technology Reference
In-Depth Information
// Work with the circle.
// ...
// Convert to an ellipse.
Ellipse
e =
new
Ellipse
(c);
Flatten(e);
The variable e holds the flattened ellipse. By replacing the conversion oper-
ator with a constructor, you have not lost any functionality; you've merely
made it clearer when new objects are created. (Veteran C++ programmers
should note that C# does not call constructors for implicit or explicit con-
versions. You create new objects only when you explicitly use the new oper-
ator, and at no other time. There is no need for the
explicit
keyword on
constructors in C#.)
Conversion operators that return fields inside your objects will not exhibit
this behavior. They have other problems. You've poked a serious hole in the
encapsulation of your class. By casting your type to some other object,
clients of your class can access an internal variable. That's best avoided for
all the reasons discussed in Item 26.
Conversion operators introduce a form of substitutability that causes
problems in your code. You're indicating that, in all cases, users can rea-
sonably expect that another class can be used in place of the one you cre-
ated. When this substituted object is accessed, you cause clients to work
with temporary objects or internal fields in place of the class you created.
Yo u t h e n m o d i f y t e m p o r a r y o b j e c t s a n d d i s c a r d t h e r e s u l t s . T h e s e s u b t l e
bugs are hard to find because the compiler generates code to convert these
objects. Avoid conversion operators in your APIs.
Overloads
C# now has support for named parameters at the call site. That means the
names of formal parameters are now part of the public interface for your
type. Changing the name of a public parameter could break calling code.
That means you should avoid using named parameters in many situations,
and also you should avoid changing the names of the formal parameters
on public, or protected methods.
Of course, no language designer adds features just to make your life diffi-
cult. Named parameters were added for a reason, and they have positive
