What is the difference between these Form Delphi creation options?

Question

We have the following options:

Application.CreateForm(TForm, Form);

and

Form := TForm.Create(Application);

There’s a difference between the two?

I know it exists because depending on where we call each of them the variable "Form" will not be available for other Formularies

What exactly is the difference if Application is proprietary?

Answer 1

Placing the Difference Between Forms Construction Methods in Delphi

1 - Application.CreateForm(TForm, Form);

Creates a global scope variable that can be instantiated in any other form, of course with the previous addition of the Unit in the form where you want to use it, example roughly:

Application.CreateForm(TFormCliente, FormCliente);
  Application.CreateForm(TFormDetalhes, FormDetalhes);
    Application.CreateForm(TFormSubDetalhes, FormSubDetalhes);
    FormDetalhes.SubShowModal;

       ... rotina ...
       Aqui estará disponpivel o formulário FormCliente e FormDetalhes
       ... Aguarda FormSubDetalhes ser fechado para continuar

  FormDetalhes.ShowModal;

    Idem

FormCliente.ShowModal;

In this case the Formclient form would be available in the other two created later ( provided that added to Unit Formclient in USES )

2 - Form := Tform.Create(Application);

Even saying that the owner is Application it will not be available in a call from a Showmodal form subsequent to it, example roughly:

FormCliente := TFormCliente.Create(Application);
  FormDetalhes := TFormDetalhes.Create(Application);
    FormSubDetalhes := TFormSubDetalhes.Create(Application);
    FormDetalhes.SubShowModal;

       ... rotina ...
       Aqui não estará disponpivel o formulário FormCliente nem FormDetalhes
       ... Aguarda FormSubDetalhes ser fechado para continuar

  FormDetalhes.ShowModal;

    Idem

FormCliente.ShowModal;

Form := Tform.Create(Application); Creates a local variable, so trying to use the form in other Units will give the Accessviolation error, because the form (variable) was not created globally

Another way of understanding:

Within a function we can create variables that will be of exclusive use of that function, example 1

function fTeste(): Integer;
var vlocal: Integer;
begin
  result := vlocal+1;
end;

If you try to use the vlocal variable outside the function you will have an error because the variable was not created globally.

Example 2:

public // Declarando a variável para ser publica (global)
  vpublica: Integer;

function fTeste(): Integer;
begin
  result := vpublica+1;
end;

In example 2 the vpublica variable will be available in any part of the current form and any form you wish to use this Unit

Descriptive summary:

I confused the creation of the Form with the creation of the Form Variable, I explain, when creating the form we can say who owns the form, if the application or others, this is necessary to know for the moment of destruction of the application, for the correct release of memory, already the creation of the variable whether it will be global or local, will give you power to use it locally or globally, I thought that when we put "Application" the created form would be available in any and all part of the application.

I hope I was clear.

Answer 2

The difference is that with Application.Createform the created form will be the "Mainform" (main form) if it is not yet defined. There are no other differences with respect to the visibility or use of the form.