unit Main;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.Menus, Vcl.ComCtrls;

type
  TfrmMain = class(TForm)
    StatusBar: TStatusBar;
    MainMenu: TMainMenu;
    mmiForm1: TMenuItem;
    mmiForm2: TMenuItem;
    mmiForm3: TMenuItem;
    procedure mmiForm1Click(Sender: TObject);
    procedure mmiForm2Click(Sender: TObject);
    procedure mmiForm3Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  frmMain: TfrmMain;

implementation

{$R *.dfm}

uses Form1, Form2, Form3;

procedure TfrmMain.mmiForm1Click(Sender: TObject);
begin
  Form1Show;
end;

procedure TfrmMain.mmiForm2Click(Sender: TObject);
begin
  Form2Show;
end;

procedure TfrmMain.mmiForm3Click(Sender: TObject);
begin
  Form3Show;
end;

end.

unit CustomMdiFrm;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes,
  Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs;

type
  TfrmCustomMDI = class(TForm)
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
  public
  end;

function CheckMDIChild(Sender: TForm): Boolean;

implementation

{$R *.dfm}

function CheckMDIChild(Sender: TForm): Boolean;
var
  i: Integer;
begin
  Result := False;

  for i := 0 to (Application.MainForm.MDIChildCount - 1) do
  begin
    if Application.MainForm.MDIChildren[i] = Sender then
    begin
      if Sender.WindowState = wsMinimized
        then Sender.WindowState := wsNormal;
      Sender.BringToFront;
      Result := True;
      exit;
    end;
  end;
end;

procedure TfrmCustomMDI.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caFree;
end;

end.

unit Form1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes,
  Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs, CustomMdiFrm, Vcl.StdCtrls;

type
  TfrmForm1 = class(TfrmCustomMDI)
    Label1: TLabel;
  private
  public
  end;

var
  frmForm1: TfrmForm1;

procedure Form1Show;

implementation

{$R *.dfm}

procedure Form1Show;
begin
  if CheckMDIChild(frmForm1) then
  begin
    if frmForm1.WindowState = wsMinimized
      then frmForm1.WindowState := wsNormal;
    frmForm1.BringToFront;
  end
  else
  begin
    frmForm1 := TfrmForm1.Create(Application);
  end;
end;

end.