// 選択範囲を更新する
private void UpdateArea(bool init)
{
var mouse = PointToClient(Cursor.Position);
// 初回時
if (init)
{
startPoint = new Point(mouse.X, mouse.Y);
select = new Rectangle(mouse.X, mouse.Y, 0, 0);
}
// 更新
var expanded = new Rectangle(select.X - 10, select.Y - 10, select.Width + 20, select.Height + 20);
select = RenderUtil.GetFixedArea(startPoint.Value.X, startPoint.Value.Y, mouse.X, mouse.Y);
// レンダリング
Invalidate(expanded);
}