delphi thread error in subform

Question

I have problem when run a thread in subform.

main form

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

uses Unit2;

procedure TForm1.Button1Click(Sender: TObject);
begin
TForm2.create(form1).ShowModal;
end;

SUBform

type
TMthread=class(Tthread)
protected
procedure execute; override;
end;
type
TForm2 = class(TForm)
    Label1: TLabel;
    procedure FormShow(Sender: TObject);
  private
  public
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

uses Unit1;

procedure TMthread.Execute;
begin
  synchronize( procedure 
               begin 
                 sleep(200);
                 freeonterminate:=true;
                 sleep(200);
                 form2.label1.Caption:='beep';
                 form1.button1.Caption:='beep'; 
               end);
end;

procedure TForm2.FormShow(Sender: TObject);
var Loadcombo2: TMthread;
begin
  Loadcombo2:=TMthread.Create(False);
end;

Program

program Project1;

uses
  Vcl.Forms,
  Unit1 in 'Unit1.pas' {Form1},
  Unit2 in 'Unit2.pas' {Form2};

{$R *.res}

begin
  Application.Initialize;
  Application.MainFormOnTaskbar := True;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

I got error in Execute Procedure when trying to access Form2.Label1.caption.

my test:

When I add the subform(Form2) in the Initialize section(last code) the the application runs without error, but doesn't change Label1.caption on the Form2.(Button1.caption on the main form is changed)

When I put exactly the same thread in main form it works without issues.

Do not rely on global variables for forms like Form1, Form2. These are normally used by auto-created forms, that means if Application.CreateForm is called. But what do you do if two instances of the forms exits? Or none is auto-created like in your code? — nil
You say twice error, but not once do you tell us what error you got. It's on the screen right in front of you; unfortunately, we can't see your screen from where we're sitting. Yes, it's clear to us from looking at the code, but it won't be to someone who is searching for a solution to a problem in the future. Please include the error information in your question (and while you're making the edit to do so, you can also actually ask a question - your current post does not). — Ken White

David Heffernan David Heffernan · Accepted Answer · 2017-11-16T12:23:51

The variable Form2 is never assigned. Because it is a global variable, its value is nil. Thus you encounter an error when you attempt to reference members of Form2.

You create an instance of Form2 like this:

TForm2.Create(Form1).ShowModal;

I suspect that instead you mean to write something like this:

Form2 := TForm2.Create(Form1);
Try
  Form2.ShowModal;
Finally
  Form2.Free;
End;

delphi thread error in subform

1 Answers