Author: Tomas Rutkauskas

What is the best way to avoid a form being created more than once in a MDI 
application?

Answer:

1   unit WindowFunctions;
2   
3   interface
4   
5   uses
6     Classes, Forms;
7   
8   function IsChildWindow(AFormClass: TFormClass; AiTag: integer): Boolean;
9   procedure CreateChildWin(AOwner: TComponent; AFormClass: TFormClass; AiTag: 
10  integer);
11  
12  implementation
13  
14  uses
15    Dialogs, Controls;
16  
17  function IsChildWindow(AFormClass: TFormClass; AiTag: integer): boolean;
18  var
19    i: integer;
20  begin
21    Result := False; {The window does not exist}
22    for i := 0 to (Screen.FormCount - 1) do
23    begin
24      if (Screen.Forms[i] is AFormClass) and (AiTag = Screen.Forms[i].Tag) then
25      begin
26        {The window was found}
27        Screen.Forms[i].BringToFront;
28        Result := True;
29        break;
30      end;
31    end;
32  end;
33  
34  procedure CreateChildWin(AOwner: TComponent; AFormClass: TFormClass; AiTag: 
35  integer);
36  begin
37    if not IsChildWindow(AFormClass, AiTag) then
38    begin
39      with AFormClass.Create(AOwner) do
40      begin
41        Tag := AiTag;
42      end;
43    end;
44  end;
45  
46  end.