I'm working with Delphi 12.
I want to create a visual component derived from TPanel with several components grouped together. One piece is to create a PageControl with 2 (at the moment) tabsheets, and one cxGrid on the tabsheets.

The same code that I use to create the pagecontrol and childs sheets is working on a normal "one form project", but raise an "Control xx has no parent window" when is used inside the component.

After I isolate the code I'm 100% sure that creating tabsheet is the problem. If I create the page control with no sheet has no problem.

After I read some stackoverflow sugested article I still not figure out what to do. Some articles are talking about overriding CreateWnd procedure but i dont understand it.

function  CreatePageControl(AOwner: TComponent; AParent : TWinControl; const aNAME : String = '') : TPageControl;
begin
  Result := TPageControl.Create(AOwner);
  if aNAME = '' then begin
    Randomize;
    Result.Name := 'pc_' + Random(999999).ToString;
  end;
  Result.Parent      := AParent;
  Result.TabPosition := tpTop;
  Result.TabWidth    := 100;
  Result.Top         := 100;
  Result.Left        := 100;
end;

function  CreateTabSheet(AOwner: TPageControl; const aNAME : String = '') : TTabSheet;
begin
  Result := TTabSheet.Create(AOwner);
  if aNAME = '' then begin
    Randomize;
    Result.Name := 'tc_' + Random(999999).ToString;
  end;
  Result.PageControl := AOwner;
end;

constructor TCustomGr.Create(AOwner: TComponent);
var workSheet : TTabSheet;
begin
  inherited Create(AOwner);

  fPanelTop               := TPanel.Create(Self);
  fPanelTop.Parent        := Self;

  fPageControlMain        := CreatePageControl(Self, Self);

  workSheet               := CreateTabSheet(fPageControlMain); <- error here
  workSheet               := CreateTabSheet(fPageControlMain);
  InitComponents;
end;

I'm working with Delphi 12.
I want to create a visual component derived from TPanel with several components grouped together. One piece is to create a PageControl with 2 (at the moment) tabsheets, and one cxGrid on the tabsheets.

The same code that I use to create the pagecontrol and childs sheets is working on a normal "one form project", but raise an "Control xx has no parent window" when is used inside the component.

After I isolate the code I'm 100% sure that creating tabsheet is the problem. If I create the page control with no sheet has no problem.

After I read some stackoverflow sugested article I still not figure out what to do. Some articles are talking about overriding CreateWnd procedure but i dont understand it.

function  CreatePageControl(AOwner: TComponent; AParent : TWinControl; const aNAME : String = '') : TPageControl;
begin
  Result := TPageControl.Create(AOwner);
  if aNAME = '' then begin
    Randomize;
    Result.Name := 'pc_' + Random(999999).ToString;
  end;
  Result.Parent      := AParent;
  Result.TabPosition := tpTop;
  Result.TabWidth    := 100;
  Result.Top         := 100;
  Result.Left        := 100;
end;

function  CreateTabSheet(AOwner: TPageControl; const aNAME : String = '') : TTabSheet;
begin
  Result := TTabSheet.Create(AOwner);
  if aNAME = '' then begin
    Randomize;
    Result.Name := 'tc_' + Random(999999).ToString;
  end;
  Result.PageControl := AOwner;
end;

constructor TCustomGr.Create(AOwner: TComponent);
var workSheet : TTabSheet;
begin
  inherited Create(AOwner);

  fPanelTop               := TPanel.Create(Self);
  fPanelTop.Parent        := Self;

  fPageControlMain        := CreatePageControl(Self, Self);

  workSheet               := CreateTabSheet(fPageControlMain); <- error here
  workSheet               := CreateTabSheet(fPageControlMain);
  InitComponents;
end;

• delphi
• vcl

• 1 Why are you creating a component for this? Have you considered creating a Frame instead? It's literally designed for this kind of task. – Remy Lebeau Commented Jan 20 at 18:02
• In my experience frames have a bad functionality (after scroll, same name of components etc), when are multiple instances on the same form. – Popa Ovidiu-Razvan Commented Jan 21 at 14:04
• 1 One comment, unrelated to your question: You should call Randomize only once during execution of your program, for example in the initialization section of your main unit. Calling Randomize; Random; multiple times with short time delay may result in very non-random numbers. – Matej Commented Jan 21 at 16:57

2 Answers 2

The error occurs because the TCustomGr does not have its Parent assigned yet in its constructor, so it cannot create its HWND yet when the TPageControl and TTabSheet try to create their HWNDs. The TTabSheet requires the TPageControl's HWND, so you need to delay the creation of the TTabSheet until after the TCustomGr has created its own HWND first.

Well i have found an answer.
I have created all the "objects" on the constructor, with absolutely no references to each other (only the create method), and in the CreateWnd overrided procedure I put all the logical (parents and so on) dependences between objects.