Delphi about Resource Release Free_release_freeandnil

A little experience about resource release (Free/release/freeandnil) in Delphi
After this period of time to Delphi Free and release of careful study, summed up the following:
1. Form.free immediately after releasing the resource, the OnDestroy event is invoked, but the form pointer is not changed to nil
2. Form.release waits for the resource to be freed after it is no longer in use, and also without modifying the form pointer to nil, it is best to use release for resource deallocation in Destroy.
3. When a local variable frees a resource, consider releasing the resource with Freeandnil (), which can be more efficient and increase memory utilization.
4. When working on a form, it is not enough to judge whether a form has been created simply by assigned (Form1), because if the form is freed by Free/release, Form1 points to an unknown address instead of nil, so you need to use (not assigned (ShowForm)) and (ShowForm is TForm2) to determine if the form has been created in memory