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// Error! Can't access protected data from object instance.

Employee emp = new Employee();

emp.empName = "Fred";

}

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Note

Although 

protected

field data can break encapsulation, it is quite safe (and useful) to define 

protected

methods. When building class hierarchies, it is very common to define a set of methods that are only for use by

derived types.

Adding a Sealed Class

Recall that a sealed class cannot be extended by other classes. As mentioned, this technique is most

often used when you are designing a utility class. However, when building class hierarchies, you

might find that a certain branch in the inheritance chain should be “capped off,” as it makes no

sense to further extend the linage. For example, assume you have added yet another class to your

program (PTSalesPerson) that extends the existing SalesPerson type. Figure 6-5 shows the current

update.

Figure 6-5.

The PTSalesPerson class

PTSalesPerson is a class representing (of course) a part-time salesperson. For the sake of

argument, let’s say that you wish to ensure that no other developer is able to subclass from

PTSalesPerson. (After all, how much more part-time can you get than “part-time”?) Again, to

prevent others from extending a class, make use of the sealed keyword:

sealed class PTSalesPerson : SalesPerson

{

public PTSalesPerson(string fullName, int age, int empID,

float currPay, string ssn, int numbOfSales)

:base (fullName, age, empID, currPay, ssn, numbOfSales)

{

}

// Assume other members here...

}

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Given that sealed classes cannot be extended, you may wonder if it is possible to reuse the

code within a class marked sealed. If you wish to build a new class that leverages the functionality

of a sealed class, your only option is to forego classical inheritance and make use of the contain-

ment/delegation model (aka the “has-a” relationship).