c#windows 应用程序picturebox形状怎样变成圆形

using System.Drawing.Drawing2D;
private void button1_Click(object sender, EventArgs e)
{
GraphicsPath gp = new GraphicsPath();
gp.AddEllipse(pictureBox1.ClientRectangle);
Region region = new Region(gp);
pictureBox1.Region = region;

pictureBox2.Region=region;

pictureBox3.Region=region;
gp.Dispose();
region.Dispose();
}