Author: Manuel Sarmiento

Displaying and using forms from a DLL can be difficult if you don't know the way. 
Fortunately enough, Delphi is quite flexible and the managing forms from a DLL is 
quite easy.

Answer:

The first thing you will need a Handle to the application’s main window. Assuming 
that the application is running on top, you can get the window’s handle using 
GetActiveWindow, like this: 

MainApplicationHandle := GetActiveWindow;

In order to be able to take advantage of Delphi’s window controlling features, 
however, you need a window control, not a window handle. You can get a window 
control uising FindControl (Controls Unit) 
1   
2   fWinControl := FindControl(MainApplicationHandle);


fWinControl is assumed to be of TwinControl type.. 

I have found it useful to create the Modal (or modeless window) since the 
begiinning: 
3   
4   MyForm := TMyForm.create(fWinControl);


The three items can be placed confortably in the initialization section of the DLL: 

5   MainApplicationHandle := GetActiveWindow;
6   fWinControl := FindControl(MainApplicationHandle);
7   MyForm := TMyForm.create(fWinControl);


Finally, when you need the Form, you just show it, but be carfeul to redraw it if 
something changed within it: 

8   MyForm.Repaint;
9   winresult := MyForm.showmodal


(winresult is defined to be of typo longint) You just have to be sure that your 
form includes wither buttons that produce a modal result (by setting ModalResult to 
something else than mrNone) 
If you’d rather prefer a modeless window (for example, to display a progress bar) , 
you can use the following approach: 

10  MyForm.Visible := true;
11  while {some condition set} do
12  begin
13    {change something in the window}
14    MyForm.repaint;
15  end;
16  MyForm.Hide;