Unit Initialization and Finalization

On the .NET platform, units that you depend on will be initialized prior to initializing your own unit. However, there is no way to guarantee the order in which units are initialized. Nor is there a way to guarantee when they will be initialized. Be aware of initialization code that depends on another unit's initialization side effects, such as the creation of a file. Such a dependency cannot be made to work reliably on the .NET platform.

Unit finalization is subject to the same constraints and difficulties as the Finalize method of objects. Specifically, unit finalization is asynchronous, and, there no way to determine when it will happen (or if it will happen, though under most circumstances, it will).

Typical tasks performed in a unit finalization include freeing global objects, unregistering objects that are used by other units, and freeing resources. Because .NET is a memory managed environment, the garbage collector will free global objects even if the unit finalization section is not called. The units in an application domain are loaded and unloaded together, so you do not need to worry about unregistering objects. All units that can possibly refer to each other (even in different assemblies) are released at the same time. Since object references do not cross application domains, there is no danger of something keeping a dangling reference to an object type or code that has been unloaded from memory.

Freeing resources (such as file handles or window handles) is the most important consideration in unit finalization. Because unit finalization sections are not guaranteed to be called, you may want to rewrite your code to handle this issue using finalizers rather than relying on the unit finalization.

The main points to keep in mind for unit initialization and finalization on the .NET platform are:

  1. The Finalize method is called asynchronously (both for objects, and for units).
  2. Finalization and destructors are used to free unmanaged resources such as file handles. You do not need to destroy object member variables; the garbage collector takes care of this for you.
  3. Classes should rely on the compiler's implementation of IDisposable, and provide a destructor called Destroy.
  4. If a class implements IDisposable itself, it cannot have a destructor called Destroy.
  5. Reference counting is deprecated. Try to use the destructor Destroy; override; pattern wherever possible.
  6. Unit initialization should not depend on side effects produced by initialization of dependent units.
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