uses stdctrls;

var
 Selecting: boolean = false;
 FirstPoint: TPoint;
 sel: TRect;

procedure DrawRect;
begin
 with Form1.Canvas do begin
 Pen.Style := psDot;
 Pen.Color := clGray;
 Pen.Mode := pmXor;
 Brush.Style := bsClear;
 Rectangle(sel.Left, sel.Top, sel.Right, sel.Bottom);
 end;
end;

procedure NormalRect(p1, p2: TPoint);
begin
 if p1.x < p2.x then begin
 sel.Left := p1.x;
 sel.Right := p2.x;
 end else begin
 sel.Left := p2.x;
 sel.Right := p1.x;
 end;
 if p1.y < p2.y then begin
 sel.Top := p1.y;
 sel.Bottom := p2.y;
 end else begin
 sel.Top := p2.y;
 sel.Bottom := p1.y;
 end;
end;

procedure TForm1.FormCreate(Sender: TObject);
var i: integer;
begin
 randomize;
 for i := 1 to random(5) + 5 do begin
 with TLabel.Create(Form1) do begin
 Caption := 'Label' + IntToStr(i);
 Left := random(Form1.ClientWidth - Width);
 Top := random(Form1.ClientHeight - Height);
 Visible := true;
 Parent := Form1;
 end;
 end;
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
 Shift: TShiftState; X, Y: Integer);
begin
 if selecting or (Button < > mbLeft) then Exit;
 SetCapture(Form1.Handle);
 Selecting := true;
 FirstPoint := Point(X, Y);
 sel := Bounds(X, Y, 0, 0);
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
 Y: Integer);
 procedure SelectLebel(lb: TLabel; r: TRect);
 var
 select: boolean;
 res: TRect;
 begin
 select := IntersectRect(res, lb.BoundsRect, r);
 if select and (lb.Color = clNavy) then Exit;
 if select then begin
 lb.Color := clNavy;
 lb.Font.Color := clWhite;
 end else begin
 lb.Color := clBtnFace;
 lb.Font.Color := clBlack;
 end;
 end;
var
 i: integer;
begin
 if not Selecting then Exit;
 DrawRect;
 NormalRect(FirstPoint, Point(X, Y));
 for i := 0 to Form1.ComponentCount - 1 do
 if (Form1.Components[i] is TLabel) then
 SelectLebel(Form1.Components[i] as TLabel, sel);
 Application.ProcessMessages;
 DrawRect;
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
 Shift: TShiftState; X, Y: Integer);
begin
 if (not Selecting) or (Button < > mbLeft) then Exit;
 NormalRect(FirstPoint, Point(X, Y));
 DrawRect;
 ReleaseCapture;
 Selecting := false;
end;