Chapter 8 Object-Based Programming 317 System) (Web domain) to make
Chapter 8 Object-Based Programming 317 System) to make this request. The garbage collector is not guaranteed to collect all objects that are currently available for collection. If the garbage collector decides to collect objects, the garbage collector first invokes the destructor of each object. It is important to understand that the garbage collector executes as an independent entity called a thread. (Threads are discussed in Chapter 14, Multithreading.) It is possible for multiple threads to execute in parallel on a multiprocessor system or to share a processor on a single-processor system. Thus, a program could run in parallel with garbage collection. For this reason, we call static method WaitForPendingFinalizers of class GC (line 37), which forces the program to wait until the garbage collector invokes the destructors for all objects that are ready for collection and reclaims those objects. When the program reaches lines 41, we are assured that both destructor calls completed and that the value of counthas been decremented accordingly. In this example, the output shows that the destructor was called for each Employee, which decrements the count value by two (once per Employee being collected). Lines 39 41 use property Count to obtain the value of count after invoking the garbage collector. If the objects had not been collected, the count would be greater than zero. Toward the end of the output, notice that the Employee object for BobJones was finalized before the Employee object for SusanBaker. However, the output of this program on your system could differ. The garbage collector is not guaranteed to collect objects in a specific order. 8.12 constand readonly Members C# allows programmers to create constants whose values cannot change during program execution. Testing and Debugging Tip 8.3 If a variable should never change, make it a constant. This helps eliminate errors that might occur if the value of the variable were to change. To create a constant data member of a class, declare that member using either the const or readonly keyword. Data members declared as const implicitly are static and must be initialized in their declaration. Data members declared as readonly can be initialized in their declaration or in their class s constructor. Neither const nor read- only values can be modified once they are initialized, except that readonly variables can be assigned values in several constructors (only one of which will be called when an object is initialized). Common Programming Error 8.7 Declaring a class data member as const but failing to initialize it in that class s declaration is a syntax error. Common Programming Error 8.8 Assigning a value to a const variable after that variable is initialized is a compilation error. Common Programming Error 8.9 The declaration of a const member as static is a syntax error, because a const member implicitly is static.
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