private void button1_Click(object sender, EventArgs e)
        {
           double r,l,s,v;  //这里说是有错误,无效的表达式,意外的字符,应输入;
            r=Convert.ToDouble(textBox1.Text);
            l=2*Math.PI*r;
            s=Math.PI*r*r;
            v=4.0*Math.PI*r*r*r/3;
            textBox2.Text=Convert.ToSingle(l);
            textBox3.Text=Convert.ToSingle(s);
            textBox4.Text=Convert.ToSingle(v);
        }

private void button1_Click(object sender, EventArgs e)
        {
           double r,l,s,v;
            r=Convert.ToSingle(textBox1.Text);
            l=2*Math.PI*r;
            s=Math.PI*r*r;
            v=4.0*Math.PI*r*r*r/3.0;
            textBox2.Text=Convert.ToString(l);
            textBox3.Text=Convert.ToString(s);
            textBox4.Text=Convert.ToString(v);
        }

谢谢啊,这样的写法挺不错的,简洁啊~