pictureBox里用滚轮实现 放大缩小功能

[DllImport("user32.dll")]
public static extern int WindowFromPoint(int xPoint, int yPoint);

void Form1_MouseWheel(object sender, MouseEventArgs e)
{
System.Drawing.Point p = PointToScreen(e.Location);
if (WindowFromPoint(p.X, p.Y) == pictureBox1.Handle.ToInt32())
{

//向前
if (e.Delta > 0)
{
float w = this.pictureBox1.Width * 0.9f; //每次縮小 20%
float h = this.pictureBox1.Height * 0.9f;
this.pictureBox1.Size = Size.Ceiling(new SizeF(w, h));

}

//向后
else if (e.Delta < 0)
{

float w = this.pictureBox1.Width * 1.1f; //每次放大 20%
float h = this.pictureBox1.Height * 1.1f;
this.pictureBox1.Size = Size.Ceiling(new SizeF(w, h));
pictureBox1.Invalidate();

}
}
}