void __fastcall TForm1::PaintBox1Paint(TObject *Sender)
{
  RECT a;         
  String Buf;

  if(GetUpdateRect(Sender,&a,true))
  {   
    Buf=a.left;
    Label5->Caption=Buf;
    Buf=a.right;
    Label6->Caption=Buf;
    Buf=a.top;
    Label7->Caption=Buf;
    Buf=a.bottom;
    Label8->Caption=Buf;
  }
  PaintBox1->Canvas->Pen->Width=12;
  PaintBox1->Canvas->Pen->Color=clRed;
  PaintBox1->Canvas->MoveTo(0,0);
  PaintBox1->Canvas->LineTo(100,0);
  PaintBox1->Canvas->LineTo(40,100);
}

Invalidate()就用InvalidateRect(...)吧，不过要自己确定重画范围

谢谢