C#学习笔记(1)

using System;using System.Collections.Generic;using System.ComponentModel;using System.Data;using System.Drawing;using System.Linq;using System.Text;using System.Threading.Tasks;using System.Windows.Forms;namespace demo1{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();        }        private void button1_Click(object sender, EventArgs e)        {            string r = textBox1.Text.Trim();// 将输入的圆半径赋给字符串r            double DouR = Convert.ToDouble(r);// 将字符类型转换为数值类型            const double PI = 3.1415926;// 定义圆周率            label1.Text = "圆的面积为：" + PI * DouR * DouR;// 计算圆面积并输出        }    }}