程序代码：

using System;
using System.Collections.Generic;
using using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormTest
{
    public partial class Form1 : Form
    {
        private double n1;
        private double n2;

        public Form1()
        {
            InitializeComponent();

            this.n1 = 0;
            this.n2 = 0;
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.MaximizeBox = false;
            this.Text = "Calculator";

            this.comboBox1.Items.Add("+");
            this.comboBox1.Items.Add("-");
            this.comboBox1.Items.Add("x");
            this.comboBox1.Items.Add("/");
            this.comboBox1.SelectedIndex = 0;

            this.label1.Text = "=";

            this.button1.Text = "OK";
            this.button2.Text = "Exit";
            this.button3.Text = "Clear";
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (!txtValidate()) return;

            double result = 0d;

            if(this.comboBox1.SelectedIndex==0)
            {
                result = this.n1 + this.n2;
            }
            else if(this.comboBox1.SelectedIndex==1)
            {
                result = this.n1 - this.n2;
            }
            else if(this.comboBox1.SelectedIndex==2)
            {
                result = this.n1 * this.n2;
            }
            else
            {
                result = this.n1 / this.n2;
            }

            this.textBox3.Text = result.ToString();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            this.textBox1.Clear();
            this.textBox2.Clear();
            this.textBox3.Clear();
            this.comboBox1.SelectedIndex = 0;
        }

        private bool txtValidate()
        {
            try
            {
                this.n1 = double.Parse(this.textBox1.Text);
            }
            catch(Exception e)
            {
                MessageBox.Show(e.Message);
                return false;
            }

            try
            {
                this.n2 = double.Parse(this.textBox2.Text);
            }
            catch (Exception e)
            {
                MessageBox.Show(e.Message);
                return false;
            }

            if(this.comboBox1.SelectedIndex==3&&double.Parse(this.textBox2.Text)==0d)
            {
                MessageBox.Show("Can't divide zero!");
                return false;
            }

            return true;
        }
    }
}