Question

I am creating a form in a dll. No packages. The form in the dll is called by using the exported procedure:

procedure ShowAbout(const AppHandle: THandle); stdcall;
  var
    aHandle: THandle;
    form:  TfrmAbout;  / my form in some other unit in the dll
  begin

    aHandle:= Application.Handle;
    Application.Handle:= AppHandle;

    form :=TfrmAbout.Create(Application);
    form.ShowModal;
    form.Free;
    Application.Handle:= aHandle;
  end;

The form displays well and there are no problems. Now, the only thing I would like it to do is to behave positioning as poMainFormCenter (I want it to display always over the main form (the form that is calling the dll).

I have tried using form :=TfrmAbout.Create(Application.MainForm); etc but no luck.

Any tricks which would help here?

Answer 1

The VCL Position mechanism relies on the other forms in the application all running with the same version of the VCL. This is clearly not the case here and you will have to position the form manually.

Find out the position of the main form by calling GetWindowRect() passing the main form handle. Then you need to work out where your form needs to go to be in the center of that form.

procedure PositionForm(Form: TForm; MainWindow: HWND);
var
  MainBounds: TRect;
  MainWidth, MainHeight: Integer;
begin
  if GetWindowRect(MainWindow, MainBounds) then
  begin
    MainWidth := MainBounds.Right-MainBounds.Left;
    MainHeight := MainBounds.Bottom-MainBounds.Top;
    Form.Left := MainBounds.Left + (MainWidth - Form.Width) div 2;
    Form.Top := MainBounds.Top + (MainHeight - Form.Height) div 2
  end;

By the way, the handle you are passing is an HWND rather than a THandle. You should change you code accordingly. It won’t change behaviour, but it is logically correct to do so.