private void Canvas_MouseDown(object sender, MouseEventArgs e)
{
drawing = true;
var a = random.Next (32, 256);
var r = random.Next (0, 256);
var g = random.Next (0, 256);
var b = random.Next (0, 256);
var polyline = new Polyline (Color.FromArgb (a, r, g, b), random.Next (1, 10));
polyline.Add (e.Location);
polylines [currentPolyline++] = polyline;
}