Author: Jonas Bilinkevicius

In Access and PB applications, I have implemented menus based on database tables. 
Ie., you select a particular entry and it load a particular form based on the text 
in the table. The question is, how do I do something similar in Delphi?

Answer:

Solve 1:

Let's assume that you want to show one of the forms of class TForm1, TForm2 or 
TForm3 and that the classname is held in a string called FormString.

Add the units holding the forms to a Uses statement - implementation section if 
possible (i.e Uses Form1, Form2, Form3)

Add an initialization section (before final end.):

1   initialization
2     RegisterClasses([TForm1, TForm2, TForm3]);
3   end.
4   
5   write a procedure to show a form. for example:
6   
7   procedure TForm1.ShowForm(const fname: string);
8   begin
9     with TFormClass(FindClass(fname)).Create(Application) do
10    try
11      ShowModal;
12    finally
13      Free;
14    end;
15  end;


Thus you would call this via:

ShowForm(FormString);


Solve 2:

Yes, you can do this. The requirement is that you register all your forms (at least 
those that need to be created by name) using the RegisterClass method. That adds 
the form class and its name to an internal list maintained by the VCL. You can now 
call FindClass or GetClass to get the class reference back using the class name.

16  var
17    fc: TFormClass; {class of TForm, a class reference type}
18    form: TForm;
19  begin
20    fc := TFormClass(getClass(formclassnamefromdatabase));
21    if Assigned(fc) then
22    begin
23      form := fc.Create(Application);
24      try
25        form.ShowModal
26      finally
27        form.free
28      end;
29    end;
30  end;


The main problem here is that you cannot use the default form variables the IDE 
puts into the form units Interface, so better delete them to prevent accidental 
references to them. There is no mechanism to find a variable by name build into the 
VCL, but if you really need to use the form variables you could build your own 
registration mechanism that would build a list associating form class name, the 
form class, and the address of the form variable.

Once the form is created you can find a reference to it from elsewhere in the app 
by iterating through the Screen.Forms array, looking for a form with a specific 
class name.


Solve 3:

Here are a couple of approaches. As long as you are only accessing Methods and 
Properties common to TForm, the following will work for you. Pay special attention 
to the RegisterClasses call in the initialization section:

31  unit GenericFormCreate;
32  
33  interface
34  
35  uses
36    Windows, Messages, SysUtils, Classes, Graphics, Controls,
37    Forms, Dialogs, StdCtrls, FileCtrl, Buttons, ExtCtrls;
38  
39  type
40    TFrmGenericFormCreate = class(TForm)
41      BtnCreateFromString: TButton;
42      BtnCreateFromClass: TButton;
43      RgrSelect: TRadioGroup;
44      procedure FormCreate(Sender: TObject);
45      procedure BtnCreateFromStringClick(Sender: TObject);
46      procedure BtnCreateFromClassClick(Sender: TObject);
47    private
48      procedure CreateFromClass(AForm: TClass);
49      procedure CreateFromClassName(AForm: string);
50    public
51    end;
52  
53  var
54    FrmGenericFormCreate: TFrmGenericFormCreate;
55  
56  implementation
57  
58  {$R *.DFM}
59  
60  uses
61    Unit2, Unit3;
62  
63  procedure TFrmGenericFormCreate.FormCreate(Sender: TObject);
64  begin
65    RgrSelect.ItemIndex := 0;
66  end;
67  
68  procedure TFrmGenericFormCreate.CreateFromClass(AForm: TClass);
69  begin
70    with TFormClass(AForm).Create(Application) do
71    try
72      ShowModal;
73    finally
74      Release;
75    end;
76  end;
77  
78  procedure TFrmGenericFormCreate.CreateFromClassName(AForm: string);
79  begin
80    try
81      with TFormClass(FindClass(AForm)).Create(Application) do
82      try
83        ShowModal;
84      finally
85        Release;
86      end;
87    except
88      ShowMessage(Format('Class %s not found', [AForm]));
89    end;
90  end;
91  
92  procedure TFrmGenericFormCreate.BtnCreateFromStringClick(Sender: TObject);
93  begin
94    CreateFromClassName('TForm' + IntToStr(RgrSelect.ItemIndex + 2));
95    RgrSelect.SetFocus;
96  end;
97  
98  procedure TFrmGenericFormCreate.BtnCreateFromClassClick(Sender: TObject);
99  begin
100   case RgrSelect.ItemIndex of
101     0: CreateFromClass(TForm2);
102     1: CreateFromClass(TForm3);
103   end;
104   RgrSelect.SetFocus;
105 end;
106 
107 initialization
108   RegisterClasses([TForm2, TForm3]);
109 end.