How to hide main form rather than closing it?

Question

If the user clicks X on my main form, i want the form to hide, rather than close. This sounds like a job for the OnClose form event:

Use OnClose to perform special processing when the form closes. The OnClose event specifies which event handler to call when a form is about to close. The handler specified by OnClose might, for example, test to make sure all fields in a data-entry form have valid contents before allowing the form to close.

A form is closed by the Close method or when the user chooses Close from the form's system menu.

The TCloseEvent type points to a method that handles the closing of a form. The value of the Action parameter determines if the form actually closes. These are the possible values of Action:

• caNone: The form is not allowed to close, so nothing happens.
• caHide: The form is not closed, but just hidden. Your application can still access a hidden form.
• caFree: The form is closed and all allocated memory for the form is freed.
• caMinimize: The form is minimized, rather than closed. This is the default action for MDI child forms.

Which i test in an empty application with one form:

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
    Action := caHide;
end;

So now when i click X, (rather than hiding) the form closes and the application terminates:

...which sounds like a job for the OnClose event...

Bonus Reading

Vcl.Forms.pas

procedure TCustomForm.Close;
var
   CloseAction: TCloseAction;
begin
   if fsModal in FFormState then
      ModalResult := mrCancel
   else if CloseQuery then
   begin
      if FormStyle = fsMDIChild then
         if biMinimize in BorderIcons then
            CloseAction := caMinimize 
         else
            CloseAction := caNone
      else
         CloseAction := caHide;

      DoClose(CloseAction);
      if CloseAction <> caNone then
      begin
         if Application.MainForm = Self then //Borland doesn't hate developers; it just hates me
            Application.Terminate
         else if CloseAction = caHide then   
            Hide
         else if CloseAction = caMinimize then 
            WindowState := wsMinimized
         else 
            Release;
      end;
   end;
end;

Bonus Reading

• How to make hovering over Minimize, Maximize, and Close buttons behave?
• Hide form instead of closing when close button clicked
• How to show a modal dialog from a modeless form? (Did Windows, WinForms, WPF, MessageBox, TaskDialog, ProgressDialog, SHFileOperation, IFileOperation all get it wrong? Nobody ever uses modeless windows?)

Answer 1

When the user closes a window, it receives a WM_CLOSE message, which triggers TForm to call its Close() method on itself. Calling Close() on the project's MainForm always terminates the app, as this is hard-coded behavior in TCustomForm.Close():

procedure TCustomForm.Close;
var
  CloseAction: TCloseAction;
begin
  if fsModal in FFormState then
    ModalResult := mrCancel
  else
    if CloseQuery then
    begin
      if FormStyle = fsMDIChild then
        if biMinimize in BorderIcons then
          CloseAction := caMinimize else
          CloseAction := caNone
      else
        CloseAction := caHide;
      DoClose(CloseAction);
      if CloseAction <> caNone then
        if Application.MainForm = Self then Application.Terminate // <-- HERE
        else if CloseAction = caHide then Hide
        else if CloseAction = caMinimize then WindowState := wsMinimized
        else Release;
    end;
end;

Only secondary TForm objects respect the output of the OnClose handler.

To do what you are asking for, you can either:

• handle WM_CLOSE directly and skip Close().

  private
    procedure WMClose(var Message: TMessage); message WM_CLOSE;
  
  procedure TForm1.WMClose(var Message: TMessage);
  begin
    Hide;
    // DO NOT call inherited ...
  end;
  

• have your MainForm's OnClose handler call Hide() directly and return caNone:

  procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
  begin
    Hide;
    Action := caNone;
  end;
  

Answer 2

Try the OnCloseQuery event. Hide the form and set CanClose to False. You should be good.

procedure TForm1.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
begin
  Hide;
  CanClose := False;
end;