It is a design goal of C# that user-defined classes can have all the functionality of built-in types. For example, suppose you have defined a type to represent fractions. Ensuring that this class has all the functionality of the built-in types means that you must be able to perform arithmetic on instances of your fractions (e.g., add two fractions, multiply, etc.) and convert fractions to and from built-in types such as integer (
int). You could, of course, implement methods for each operation and invoke them by writing statements such as:
Fraction theSum = firstFraction.Add(secondFraction);
Although this will work, it is ugly, and is not how the built-in types are used. It would be much better to write:
Fraction theSum = firstFraction + secondFraction;
Statements like this are intuitive and consistent with how built-in types, such as
int, are added.
In this chapter, you will learn techniques for adding standard operators to your user-defined types. You will also learn how to add conversion operators so that your user-defined types can be implicitly and explicitly converted to other types.
In C#, you implement operators by creating static methods whose return values represent the result of an operation and whose parameters are the operands. When you create an operator for a class you say that you have "overloaded" that operator, much as you might overload any member method. Thus, to overload the addition operator (
+), you would ...