Access variable from one form to another

[Namenloser]

If you want to access a property of Form1 (Unit1) from within Form2 (Unit2), you have to add Unit1 to the uses clause in Unit2.

Delphi-Quellcode:

unit Unit1;
interface

uses
  Windows, Classes, SysUtils, Forms, Controls, Graphics, StdCtrls;

type
  TForm1 = class(TForm)
    {...}
  end;

var
  Form1: TForm1;

implementation

uses Unit2; // <--

procedure TForm1.FormClick(Sender: TObject);
begin
  ShowMessage(IntToStr(Form2.OldId));
end;

Delphi-Quellcode:

unit Unit2;
interface

uses
  Windows, Classes, SysUtils, Forms, Controls, Graphics, StdCtrls;

type
  TForm2 = class(TForm)
    {...}
  private
    FOldId: integer;
  public
    property OldId: integer read FOldId write FOldId
  end;

var
  Form2: TForm2;

implementation

{...}

I generally wouldn't recommend doing this though because it leads to spaghetti code.

Instead I would outsource the data and business logic to a separate class and then create all forms that operate on a specific object on demand:

Delphi-Quellcode:

unit MyData;

type
  TMyData = class
  private
    FOldId: integer;
  public
    OldId: integer read FOldId write FOldId;
  end;

  {...}

Delphi-Quellcode:

unit Main;

uses
  Windows, Classes, SysUtils, Forms, Controls, Graphics, StdCtrls,
  MyData;

type
  TFrmMain = class(TForm)
    { ... }
    FMyData: TMyData;
  end;

var
  frmMain: TFrmMain;

implementation

procedure TForm1.FormClick(Sender: TObject);
var
  DialogForm: TFrmDialog;
begin
  DialogForm := TFrmDialog.Create(FMyData);
  DialogForm.Show; // Or perhaps DialogForm.ShowModal
end;

Delphi-Quellcode:

unit FrmDialog;

uses
  Windows, Classes, SysUtils, Forms, Controls, Graphics, StdCtrls,
  MyData;

type
  TFrmDialog = class(TForm)
    { ... }
    FMyData: TMyData;
    constructor Create(MyData: TMyData);
  end;

// You should dispose of this global variable. You'll also have to edit the
// project's source file for this
//var
//  FrmDialog: TFrmDialog ;

implementation

constructor TFrmDialog.Create(MyData: TMyData);
begin
  inherited Create;
  FMyData := MyData;
end;

procedure TFrmDialog.FormClick(Sender: TObject);
begin
  FMyData.OldId := 1; // change the data object
end;