program Project7;
uses Dialogs, Controls;

type
 TParent = class
           protected
            Field: string;
           public
            constructor Create; virtual;
            procedure VeryComplexAlgo;
           end;

 TChildA = class(TParent)
           public
            constructor Create; override;
           end;

 TChildB = class(TParent)
           public
            constructor Create; override;
           end;

 TParentClass = class of TParent;

 function GetClassFromUser:TParentClass;
 begin
 if MessageDlg('Use TChildA?', mtConfirmation, [mbYes, mbNo], 0)=mrYes
    then result := TChildA
    else result := TChildB
 end;

{ TParent }

constructor TParent.Create;
begin
Field := 'TParent'
end;

procedure TParent.VeryComplexAlgo;
begin
ShowMessage(Field)
end;

{ TChildA }

constructor TChildA.Create;
begin
inherited;
Field := Field + ' TChildA';
end;

{ TChildB }

constructor TChildB.Create;
begin
inherited;
Field := Field + ' TChildB';
end;

begin
GetClassFromUser.Create.VeryComplexAlgo
end.