组合框使用方法，按照下面图片上提示，设计一个windows应用程序，实现不同运算。

界面设计：

代码：

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 combox{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();            label1.Text = "=";            button1.Text = "选择不同运算符进行计算";            comboBox1.Items.Add("+");            comboBox1.Items.Add("-");            comboBox1.Items.Add("*");            comboBox1.Items.Add("/");        }        private void label1_Click(object sender, EventArgs e)        {        }        private void button1_Click(object sender, EventArgs e)        {            float a, b, c;            a = Convert.ToSingle(textBox1.Text);            b = Convert.ToSingle(textBox2.Text);            if (comboBox1.Text== "+")            {                c = a + b;                textBox3.Text = c.ToString();             }            else if (comboBox1.Text == "-")            {                c = a - b;                textBox3.Text = c.ToString() ;            }            else if (comboBox1.Text == "*")            {                c = a * b;                textBox3.Text = c.ToString();            }            else            {                c = a / b;                textBox3.Text = c.ToString();            }        }        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)        {        }    }}

运行：