MDIChild form cannot be created

Question

Have a main form (Form1) where is created a MDIForm (Form2) and a MDIChild (Form3) form respectivelly in execution time. In my tests the MDIForm (Form2) is show like expected but when try show the MDIChild (Form3) i get the following error that say:

Cannot create form. No mdi forms are currently active

Some idea about how fix this?

program Project1;

uses
  Vcl.Forms,
  Unit1 in 'Unit1.pas' {Form1},
  Unit2 in 'Unit2.pas' {Form2},
  Unit3 in 'Unit3.pas' {Form3};

{$R *.res}

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

Form:

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

var
  Form1: TForm1;

implementation

uses
  Unit2, Unit3;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  Form2 := TForm2.Create(Self);
  Form2.Show;
  Form3 := TForm3.Create(Form2);
  Form3.Show;
end;

end.

Your edit is asks a new question. I've removed it by rolling back. If you have a new question, ask a new question. However, you should not simply ask it in the form of "please show a code example" because that would be off topic. — David Heffernan, Aug 30 '19 at 10:52

Remy Lebeau · Accepted Answer · 2019-08-30T03:37:25.153

The VCL (not the Win32 API) is hard-coded to allow only the Application.MainForm to be set to fsMDIForm for hosting fsMDIChild Forms. Your MainForm is not the fsMDIForm parent Form, which is why you are getting the error.

Using a secondary Form as the fsMDIForm parent is technically possible, but not out of the box. It requires a bit of manual work hacking up the VCL's internals to make it work, and even then there are holes and gotchas. See my Multiple MDI Parent Forms in a single Application submission on CodeCentral for an example (I haven't updated it in over a decade, so it may need some tweaking for modern VCL versions). The old Quality Central (not Quality Portal!) ticket it refers to can be found on archive.org: #12006: Hosting MDI child forms in non-MainForm forms.

That being said, MDI is a dead technology, Microsoft abandoned it a long time ago, and modern Windows versions have poor support for MDI, especially when Visual Styles are used. You are best off not even bothering with MDI in modern software, there are other/better UI design choices available.

@BrowJr you can either move your MDI logic to the MainForm, or else use Frames instead of child Forms, but to match everything that MDI offers will be a lot of manual work. There is really no good reason to use/simulate MDI in a modern UI. Rethink your UI design. — Remy Lebeau, Aug 30 '19 at 03:40

MDIChild form cannot be created

1 Answers1