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In some situations, you will want to use an object of a class in an expression involving other types of data Sometimes, overloading one or more operators can provide the means of doing this However, in other cases, what you want is a simple type conversion from the class type to the target type To handle these cases, C# allows you to create a special type of operator method called a conversion operator A conversion operator converts an object of your class into another type Conversion operators help fully integrate class types into the C# programming environment by allowing objects of a class to be freely mixed with other data types as long as a conversion to those other types is defined There are two forms of conversion operators, implicit and explicit The general form for each is shown here: public static operator implicit target-type(source-type v) { return value; } public static operator explicit target-type(source-type v) { return value; } Here, target-type is the target type that you are converting to; source-type is the type you are converting from; and value is the value of the class after conversion The conversion operators return data of type target-type, and no other return type specifier is allowed If the conversion operator specifies implicit, then the conversion is invoked automatically, such as when an object is used in an expression with the target type When the conversion operator specifies explicit, the conversion is invoked when a cast is used You cannot define both an implicit and explicit conversion operator for the same target and source types To illustrate a conversion operator, we will create one for the ThreeD class Suppose you want to convert an object of type ThreeD into an integer so it can be used in an integer expression Further, the conversion will take place by using the product of the three dimensions To accomplish this, you will use an implicit conversion operator that looks like this:.

public static implicit operator int(ThreeD op1) { return op1x * op1y * op1z; }

Part I:

2 (1/ 2 _ x)

Let s examine this program closely First, as explained, Area( ) is declared as virtual in the TwoDShape class and is overridden by Triangle and Rectangle Inside TwoDShape, Area( ) is given a placeholder implementation that simply informs the user that this method must be overridden by a derived class Each override of Area( ) supplies an implementation that is suitable for the type of object encapsulated by the derived class Thus, if you were to implement an ellipse class, for example, then Area( ) would need to compute the area of an ellipse There is one other important feature in the preceding program Notice in Main( ) that shapes is declared as an array of TwoDShape objects However, the elements of this array are assigned Triangle, Rectangle, and TwoDShape references This is valid because a base class reference can refer to a derived class object The program then cycles through the array, displaying information about each object Although quite simple, this illustrates the power of both inheritance and method overriding The type of object stored in a base class reference variable is determined at runtime and acted on accordingly If an object is derived from TwoDShape, then its area can be obtained by calling Area( ) The interface to this operation is the same no matter what type of shape is being used

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Sometimes you will want to create a base class that defines only a generalized form that will be shared by all of its derived classes, leaving it to each derived class to fill in the details Such a class determines the nature of the methods that the derived classes must implement, but does not, itself, provide an implementation of one or more of these methods One way this situation can occur is when a base class is unable to create a meaningful implementation for a method This is the case with the version of TwoDShape used in the preceding example The definition of Area( ) is simply a placeholder It will not compute and display the area of any type of object You will see as you create your own class libraries that it is not uncommon for a method to have no meaningful definition in the context of its base class You can handle this situation two ways One way, as shown in the previous example, is to simply have it report a warning message Although this approach can be useful in certain situations such as debugging it is not usually appropriate You may have methods that must be overridden by the derived class in order for the derived class to have any meaning Consider the class Triangle It is incomplete if Area( ) is not defined In such a case, you want some way to ensure that a derived class does, indeed, override all necessary methods C# s solution to this problem is the abstract method An abstract method is created by specifying the abstract type modifier An abstract method contains no body and is, therefore, not implemented by the base class Thus, a derived class must override it it cannot simply use the version defined in the base class As you can probably guess, an abstract method is automatically virtual, and there is no need to use the virtual modifier In fact, it is an error to use virtual and abstract together To declare an abstract method, use this general form: abstract type name(parameter-list); As you can see, no method body is present The abstract modifier can be used only on instance methods It cannot be applied to static methods Properties and indexers can also be abstract.