Conclusion

Defining new classes and structures that represent complex data structures is a key aspect of any sufficiently complex program. Choosing between a class and a structure depends on the different ways that the types are allocated and used. Although the Framework handles many memory and resource management tasks , properly tracking and releasing memory and resources is still a consideration for programmers. Now that we have introduced user -defined types, the next chapter begins the discussion of type members with methods .

Here are some style points to consider.

• The Framework's method of memory management is very different from COM's method of memory management. Instead of garbage collection, COM uses a scheme called reference counting to track when values on the heap should be released. Reference counting requires a program, rather than the system, to keep track of how many references remain to a particular value. This has the advantage of allowing a program to free a value the moment the last reference to it is released. Reference counting is vulnerable to bugs (because of programs incorrectly tracking reference counts) and cannot handle circular references (i.e., object A has a reference to object B, which has a reference to object A).

• In general, it is best to set reference variables to Nothing as soon as you are finished with them so that the memory can be reclaimed by the garbage collector. However, a variable that reaches the end of its lifetime (for example, a local at the end of a subroutine) is automatically set to Nothing by the Framework and does not need to be set to Nothing .

• For clarity, shared members should always be accessed through the type name , not an instance.

• Dispose should be written so it can be called multiple times, so that a programmer using the class does not have to keep track of whether Dispose has already been called.

• When using a disposable type, always put the call to Dispose in a Finally block. This ensures that even if an exception occurs while the class is being used, the class will still be disposed of.