Form.Release + NIL

delphi winapi vcl
Holgerwa picture Holgerwa · Nov 8, 2008 · Viewed 7.6k times · Source

if Form.Release is called after using the form, it will free all related memory but not set the form variable to nil.

if not assigned (Form1) then
  begin
    Application.CreateForm(Tform1, Form1);
    try
      // Do something
    finally
      Form1.Release
    end;
  end;

To be able to call the same code again, Form1 would have to be set to nil at some point. From the description of Release I cannot do

Form1 := nil;

right after Release, because the Release procedure will return directly after being called and before the form is actually freed. I cannot detect when Form.Release is finished to set the form var to nil.

What is the best way to do this?

Answer

a.delphi.developer picture a.delphi.developer · Nov 8, 2008

Put the line 

  Form1 := nil;

just after the call to Release.

Release is just posting a CM_RELEASE message to the Form which allows the Form to finish what's in its queue (event handlers) before handling the CM_RELEASE message which means normally just calling Free.
So, after calling Release, you should not assume that the Form variable still points to a valid Form, thus putting nil into the variable.

Related questions

How can I check if my application has focus?

What I want to do is check if my application has focus because if it is not then I will popup an Alert Window just over the Notification Area to display some message to the end user.

Read More
Why does CreateProcess give error 193 (%1 is not a valid Win32 app)

The code below fails to start documents. I get error 193 (%1 is not a valid Win32 app). Starting executables work fine. The files are properly associated, they start the corresponding app when double clicked. I have searched SO and elsewhere for …

Read More
System Error. Code: 8. Not enough storage is available to process this command

We have a few Win32 applications (coded in Delphi 2006) where sometimes the user gets an error message saying "System Error. Code: 8. Not enough storage is available to process this command.". From the stacktrace it looks like it is always during …

Read More