This seems to work
protected override void OnMouseMove(MouseEventArgs e)
{
if (e.Button == MouseButtons.Left)
{
rec.Width = e.X - rec.X;
rec.Height = e.Y - rec.Y;
this.Invalidate();
}
if (e.Button == MouseButtons.Right)
{
rec.Location = new Point((e.X-MouseDownLocation.X) + rec.Left, (e.Y-MouseDownLocation.Y) + rec.Top);
MouseDownLocation = e.Location;
this.Invalidate();
}
}