C#求圆的周长、面积、体积

窗体应用程序

private void button1_Click(object sender, EventArgs e)
{
double r;
r = Convert.ToInt32(textBox1.Text);
textBox2.Text = Convert.ToString(2 * r * System.Math.PI);
textBox3.Text = Convert.ToString(r * r * System.Math.PI);
textBox4.Text = Convert.ToString(r * r * r * 4 / 3 * System.Math.PI);
}