设计一个windows应用程序，实现不同运算。

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

namespace WindowsFormsApplication2

{

    public partial class Form1 : Form

    {

        string s;

        public Form1()

        {

            InitializeComponent();

        }

        private void button1_Click(object sender, EventArgs e)

        {

            if(textBox1.Text.Length==0||textBox2.Text.Length==0)

            {

                MessageBox.Show("请输入数据！");

                return;

           

            }

            try

            {

                double x = double.Parse(textBox1.Text);

                double y = double.Parse(textBox2.Text);

                //textBox3.Text=(x y).ToString();

                switch (s)

                {

                    case "+": textBox3.Text = (x + y).ToString(); break;

                    case "-": textBox3.Text = (x - y).ToString(); break;

                    case "*": textBox3.Text = (x * y).ToString(); break;

                    case "/": textBox3.Text = (x / y).ToString(); break;

                    default: textBox3.Text = "错误"; break;

                }

            }

            catch(Exception ex)

            {MessageBox.Show(ex.Message);}

        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)

        {

            s = comboBox1.Text;

            //MessageBox.Show(s);

        }

    }

}