C# 四则运算及省市选择及日月选择

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 WindowsFormsApp
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            button1.Text = "If语法计算";
            button2.Text = "switch语法计算";
            label2.Text = "省";
            label3.Text = "市";
        }

        private void Button1_Click(object sender, EventArgs e)
        {
            int text1 = Convert.ToInt32(textBox1.Text);
            int text2 = Convert.ToInt32(textBox2.Text);
            int text3 = ; //用if的时候需要对变量进行赋值，不然在if语句里会报错
            if (comboBox1.Text == "+" && comboBox1.SelectedItem == "+")
            //由上一行可以看出combox的text和selecteditem的值一样，
            //但是建议使用后者，所以好的写法如下面的
            {
                text3 = text1 + text2;
            }
            else if (comboBox1.Text == "-")
            {
                text3 = text1 - text2;
            }
            else if (comboBox1.Text == "*")
            {
                text3 = text1 * text2;
            }
            else if (comboBox1.Text == "/")
            {
                if (text2 == )
                {
                    MessageBox.Show("除数不能为0");
                    return;
                }
                else
                {
                    text3 = text1 / text2;
                }
            }
            else
            {
                MessageBox.Show("不知道该怎么计算");
            }
            textBox3.Text = Convert.ToString(text3);
        }

        private void Button2_Click(object sender, EventArgs e)
        {
            try
            {
                int text1 = Convert.ToInt32(textBox1.Text);
                int text2 = Convert.ToInt32(textBox2.Text);
                int text3;//switch时候就不需要先对变量赋值
                switch (comboBox1.SelectedItem)
                {
                    case "+":
                        text3 = text1 + text2;
                        break;
                    case "-":
                        text3 = text1 - text2;
                        break;
                    case "*":
                        text3 = text1 * text2;
                        break;
                    case "/":
                        //if (text2 == 0)
                        //{
                        //    MessageBox.Show("除数不能为0");
                        //    return;
                        //}
                        text3 = text1 / text2;
                        break;
                    default:
                        MessageBox.Show("不知道该怎么计算");
                        return;
                        //throw new Exception("不知道怎么计算");也可以用这个
                }
                textBox3.Text = Convert.ToString(text3);
            }
            catch (Exception ex)
            {

                MessageBox.Show("程序出现意外" + ex.Message);
            }

        }
        private void ComboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (comboBox2.SelectedItem == "江苏省")
            {
                comboBox3.Items.Clear();//每次选择省时，清空市的内容
                comboBox3.Items.Add("苏州");
                comboBox3.Items.Add("张家港");
                comboBox3.Items.Add("昆山");
                comboBox3.Items.Add("吴江");
            }
            if (comboBox2.SelectedItem == "山东省")
            {
                comboBox3.Items.Clear();
                comboBox3.Items.Add("青岛1");
                comboBox3.Items.Add("青岛2");
                comboBox3.Items.Add("青岛3");
                comboBox3.Items.Add("青岛4");
            }
            if (comboBox2.SelectedItem == "浙江省")
            {
                comboBox3.Items.Clear();
                comboBox3.Items.Add("杭州");
                comboBox3.Items.Add("义乌");
                comboBox3.Items.Add("温州");
                comboBox3.Items.Add("台州");
            }
        }
        private void ComboBox3_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        //初始化月份的下拉选择项
        {
            comboBox4.DropDownStyle = ComboBoxStyle.DropDownList;
            comboBox4.Items.Clear();
            for (int i = ; i < ; i++)
            {
                //comboBox4.Items.Add(i.ToString());//这个也是类型转换
                comboBox4.Items.Add(Convert.ToString(i));
            }
        }
        private void ComboBox4_SelectedIndexChanged(object sender, EventArgs e)
        {
            switch (Convert.ToInt32(comboBox4.SelectedItem))
            {
                case :
                case :
                case :
                case :
                case :
                case :
                case :
                    comboBox5.Items.Clear();
                    for (int i = ; i < ; i++)
                    {
                        comboBox5.Items.Add(Convert.ToString(i));
                    }
                    break;
                case :
                    comboBox5.Items.Clear();
                    for (int i = ; i < ; i++)
                    {
                        comboBox5.Items.Add(Convert.ToString(i));
                    }
                    break;
                default:
                    comboBox5.Items.Clear();
                    for (int i = ; i < ; i++)
                    {
                        comboBox5.Items.Add(Convert.ToString(i));
                    }
                    break;
            }
        }
        private void Button3_Click(object sender, EventArgs e)
        {
            string str = string.Format("你选择了{0}月{1}日", comboBox4.SelectedItem, comboBox5.SelectedItem);
            MessageBox.Show(str);
            string str1 = comboBox4.SelectedItem + "月" + comboBox5.SelectedItem + "日";
            MessageBox.Show(str1);
        }
    }
}