c#计算器设计

Question

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;namespace 计算器
{
    public partial class Form1 : Form
    {
        int flag;
        string left, right;
        string op;
        public Form1()
        {
            InitializeComponent();
        }
        private void button5_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(0);
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            this.textBox1.Text = Convert.ToString(0);
            flag = 0;
            left = "";
            right = "";
            op = "";
        }        private void button4_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(1);
            else
                this.textBox1.Text = Convert.ToString(1);
        }        private void button18_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(2);
            else
                this.textBox1.Text = Convert.ToString(2);
        }        private void button19_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(3);
            else
                this.textBox1.Text = Convert.ToString(3);
        }        private void button3_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(4);
            else
                this.textBox1.Text = Convert.ToString(4);
        }        private void button14_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(5);
            else
                this.textBox1.Text = Convert.ToString(5);
        }        private void button15_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(6);
            else
                this.textBox1.Text = Convert.ToString(6);
        }        private void button2_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(7);
            else
                this.textBox1.Text = Convert.ToString(7);
        }        private void button10_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(8);
            else
                this.textBox1.Text = Convert.ToString(8);
        }        private void button11_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "0")
                this.textBox1.Text = this.textBox1.Text + Convert.ToString(9);
            else
                this.textBox1.Text = Convert.ToString(9);
        }        private void button22_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text.IndexOf('.') > 0)
                ;
            else
                this.textBox1.Text = this.textBox1.Text + ".";
        }        private void button8_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text.StartsWith("-"))
                this.textBox1.Text = this.textBox1.Text.Substring(this.textBox1.Text.IndexOf('-') + 1);
            else
                this.textBox1.Text = "-" + this.textBox1.Text;
        }        private void button23_Click(object sender, EventArgs e)
        {
            if (flag == 1)
            {
                right = this.textBox1.Text;
                double ld = Convert.ToDouble(left);
                double rd = Convert.ToDouble(right);
                if (op == "+")
                {
                    this.textBox1.Text = null;
                    this.textBox1.Text = Convert.ToString(ld + rd);
                }
                if (op == "-")
                    this.textBox1.Text = Convert.ToString(ld - rd);
                if (op == "*")
                    this.textBox1.Text = Convert.ToString(ld * rd);
                if (op == "/")
                    this.textBox1.Text = Convert.ToString(ld / rd);
                left = this.textBox1.Text;
                op = "+";
                flag = 1;
            }
            if (flag == 0)
            {
               left = this.textBox1.Text;
                op = "+";
                flag = 1;
            }
        }        private void button20_Click(object sender, EventArgs e)
        {
            if (flag == 1)
            {
                right = this.textBox1.Text;
                double ld = Convert.ToDouble(left);
                double rd = Convert.ToDouble(right);
                if (op == "+")
                    this.textBox1.Text = Convert.ToString(ld + rd);
                if (op == "-")
                    this.textBox1.Text = Convert.ToString(ld - rd);
                if (op == "*")
                    this.textBox1.Text = Convert.ToString(ld * rd);
                if (op == "/")
                    this.textBox1.Text = Convert.ToString(ld / rd);
                left = this.textBox1.Text;
                op = "-";
                flag = 1;
            }
            if (flag == 0)
            {
                left = this.textBox1.Text;
                op = "-";
                flag = 1;
            }
        }        private void button16_Click(object sender, EventArgs e)
        {
            if (flag == 1)
            {
                right = this.textBox1.Text;
                double ld = Convert.ToDouble(left);
                double rd = Convert.ToDouble(right);
                if (op == "+")
                    this.textBox1.Text = Convert.ToString(ld + rd);
                if (op == "-")
                    this.textBox1.Text = Convert.ToString(ld - rd);
                if (op == "*")
                    this.textBox1.Text = Convert.ToString(ld * rd);
                if (op == "/")
                    this.textBox1.Text = Convert.ToString(ld / rd);
                left = this.textBox1.Text;
                op = "*";
                flag = 1;
            }
            if (flag == 0)
            {
                left = this.textBox1.Text;
                op = "*";
                flag = 1;
            }
        }        private void button12_Click(object sender, EventArgs e)
        {
            if (flag == 1)
            {
                right = this.textBox1.Text;
                double ld = Convert.ToDouble(left);
                double rd = Convert.ToDouble(right);
                if (op == "+")
                    this.textBox1.Text = Convert.ToString(ld + rd);
                if (op == "-")
                    this.textBox1.Text = Convert.ToString(ld - rd);
                if (op == "*")
                    this.textBox1.Text = Convert.ToString(ld * rd);
                if (op == "/") 
                {
                    if (rd == 0) 
                    {
                        ;
                    }
                    else
                        this.textBox1.Text = Convert.ToString(ld / rd);
                } 
                left = this.textBox1.Text;
                op = "/";
                flag = 1;
            }
            if (flag == 0)
            {
                left = this.textBox1.Text;
                op = "/";
                flag = 1;
            }
        }        private void button21_Click(object sender, EventArgs e)
        {
            right = this.textBox1.Text;
            double ld = Convert.ToDouble(left);
            double rd = Convert.ToDouble(right);
            if (op == "+")
                this.textBox1.Text = Convert.ToString(ld + rd);
            if (op == "-")
                this.textBox1.Text = Convert.ToString(ld - rd);
            if (op == "*")
                this.textBox1.Text = Convert.ToString(ld * rd);
            if (op == "/")
                this.textBox1.Text = Convert.ToString(ld / rd);
            left = "";
            right = "";
            op = "";
            flag = 0;
        }
        private void button7_Click(object sender, EventArgs e)
        {
            this.textBox1.Text = Convert.ToString(0);
            flag = 0;
            left = "";
            right = "";
            op = "";
        }
        private void button6_Click(object sender, EventArgs e)
        {
            this.textBox1.Text = this.textBox1.Text.Substring(0,this.textBox1.Text.Length-1);
            if (this.textBox1.Text == "") 
            { this.textBox1.Text = "0"; }
            return;
        }
    }
}
例如：输入一个1，再输入一个加号，再输入一个1，文本框里显示的是11。怎么解决？

Answer 1

你那button21就是等号按钮吧？转换类型的时候判断一下值为不为空，为空就会出现异常