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Display a LogIn / Password Dialog Before the Main Form is Created

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Login/Password Dialog BEFORE Delphi Main Form

Login/Password Dialog BEFORE Delphi Main Form

The MainForm of a Delphi application is a form (window) that is the first one created in the main body of the application.

If you need to implement some kind of authorization for your Delphi application you might want to display a login / password dialog *before* the main form is created and displayed to the user.

In short, the idea would be to create, display, and destroy the "login" dialog before creating the main form.

The Delphi MainForm

When a new Delphi project is created, "Form1" automatically becomes the value of the MainForm property (of the global Application object). To assign a different form to the MainForm property, use the Forms page of the Project|Options dialog box at design time.

When the main form closes, the application terminates.

Login/Password Dialog

Let's start by creating the main form of the application. Create a new Delphi project containing one form. This form is, by design, the main form.

If you change the name of the form to "TMainForm" and save the unit as "main.pas", the project's source code looks like (the project was saved as "PasswordApp"):

 program PasswordApp;
 
 uses
   Forms,
   main in 'main.pas' {MainForm};
 
 {$R *.res}
 
 begin
   Application.Initialize;
   Application.CreateForm(TMainForm, MainForm) ;
   Application.Run;
 end.
 
Now, add a second form to the project.

By design, the second form added, gets listed in the "Auto-Create Forms" list on the Project Options dialog.

Name the second form "TLoginForm" and *remove* ir from the "Auto-Create Forms" list. Save the unit as "login.pas".

Add a Label, an Edit and a Button on the form.

Add a class method to create, show and close the login/password dialog. The method "Execute" returns true if the user has entered the correct password in the password edit box.

Here's the full source code:

 unit login;
 
 interface
 
 uses
   Windows, Messages, SysUtils, Variants, Classes,
   Graphics, Controls, Forms, Dialogs, StdCtrls;
 
 type
   TLoginForm = class(TForm)
     LogInButton: TButton;
     pwdLabel: TLabel;
     passwordEdit: TEdit;
     procedure LogInButtonClick(Sender: TObject) ;
   public
     class function Execute : boolean;
   end;
 
 implementation
 {$R *.dfm}
 
 class function TLoginForm.Execute: boolean;
 begin
   with TLoginForm.Create(nil) do
   try
     Result := ShowModal = mrOk;
   finally
     Free;
   end;
 end;
 
 procedure TLoginForm.LogInButtonClick(Sender: TObject) ;
 begin
   if passwordEdit.Text = 'delphi' then
     ModalResult := mrOK
   else
     ModalResult := mrAbort;
 end;
 
 end. 
The Execute method dynamically creates an instance of the TLoginForm and displays it modaly using the ShowModal method.

ShowModal does not return until the form closes. When the form closes, it returns the value of the ModalResult property.

The "LogInButton" OnClick event handler assigns "mrOk" to the ModalResult property is the user has entered the correct password ('delphi' in the above example). If the user has provided a wrong password, ModalResult is set to "mrAbort" (can be anything *except* "mrNone").

Setting a value to the ModalResult property closes the form ... Execute returns true is ModalResult equals "mrOk" - if the user has entered the correct password.

Don't Create MainForm Before Login

You now only need to make sure the main form is not created if the user failed to provide the correct password.

Here's how the project's source code should look :

 program PasswordApp;
 
 uses
   Forms,
   main in 'main.pas' {MainForm},
   login in 'login.pas' {LoginForm};
 
 {$R *.res}
 
 begin
   if TLoginForm.Execute then
   begin
     Application.Initialize;
     Application.CreateForm(TMainForm, MainForm) ;
     Application.Run;
   end
   else
   begin
     Application.MessageBox('You are not authorized to use the application. The password is "delphi".', 'Password Protected Delphi application') ;
   end;
 end.
 
Note the usage of the if then else block to determine if the main form should be created. If "Execute" returns false, MainForm is not created and the application terminates without starting, so to say.

That's all!

Source Code

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