Creating an Associator

unit appassoc;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls, Buttons,registry;
type
  TForm1 = class(TForm)
    Panel1: TPanel;
    Panel2: TPanel;
    Panel3: TPanel;
    StaticText1: TStaticText;
    StaticText2: TStaticText;
    edappname: TEdit;
    edExt: TEdit;
    StaticText3: TStaticText;
    btnAssoc: TBitBtn;
    btnClose: TBitBtn;
    StaticText4: TStaticText;
    btnbrowse: TSpeedButton;
    StaticText5: TStaticText;
    op: TOpenDialog;
    StaticText7: TStaticText;
    procedure btnCloseClick(Sender: TObject);
    procedure btnbrowseClick(Sender: TObject);
    procedure btnAssocClick(Sender: TObject);

  private
    { Private declarations }
  public
    { Public declarations }
  end;
var
  Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.btnCloseClick(Sender: TObject);
begin
close;
end;
procedure TForm1.btnbrowseClick(Sender: TObject);
begin
if op.Execute then begin
edappname.Text:=op.FileName;
end
end;
procedure TForm1.btnAssocClick(Sender: TObject);
var
reg:tregistry;
error:boolean;
begin
if (edappname.text = '') OR (edext.text = '') then begin
MessageDlg('Please ensure that you''ve filled in ALL the fields.', mtError,      [mbOk], 0);
exit;
end;
reg := TRegistry.Create;
reg.RootKey := HKEY_CLASSES_ROOT;
reg.LazyWrite := false;
try
if
{Add Program Support}
 reg.OpenKey('.'+edExt.Text+'shellopencommand', true) then begin
{Invoke the program passing the file name as the first parameter}
reg.WriteString('', edappname.Text+' %1 ');
{Add Icon Display}
reg.CloseKey;
error:=false;
end
else begin
error:=true;
end;
if not error then begin
reg.OpenKey('.'+edExt.Text+'DefaultIcon', true);
 {Use the first icon in the executable to display}
 reg.WriteString('', edappname.Text+',0');
 reg.CloseKey;
 end;
 if error then begin
 MessageDlg('There''s been an error creating this association, please check that you''ve entered the information correctly.', mtInformation   ,

      [mbOk], 0);
exit;
      end
      else begin
 if MessageDlg(extractfilename(edappname.Text)+ ' have now been associated ' +#13#10+ 'with ' +'extention - '+ '.'+edExt.Text+'!'#13#10+'Would you like to make another Association?', mtInformation          , [mbYes, mbNo], 0) = mrYes then
  begin
    edext.Clear;
    edappname.Clear;
   end
   else begin
   close;
   end;
   end;
      //************
finally
reg.Free;
end;
close;
end;
end.

Conclusion

That's it! You can now associate as many extensions with your application as you like. Because this application involves changing the system registry, I would strongly advice you to back up your registry, in the unlikely case that something goes wrong.