winform计算器

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 FirstForm
{
public partial class JiSuanQi : Form
{
public JiSuanQi()
{
InitializeComponent();
}

private double sum=0;
private string biaodashi;
private string Preyunsuanfu;//存放之前的运算符
private bool isok=true;//用来判断是否刚刚点过运算符

private void btnling_Click(object sender, EventArgs e)
{
Button btn = (Button)sender;
if (isok)//是否点击过运算符
{
txtshuru.Text = btn.Text;
//isok = false;
}
else
{
txtshuru.Text += btn.Text;
}

}

private void btnjia_Click(object sender, EventArgs e)
{
Button btn = (Button)sender;
string yunsuanfu = btn.Text;
if (Preyunsuanfu == null)
{
biaodashi = txtshuru.Text;//“+”可以去掉
sum = double.Parse(txtshuru.Text);
}
else
{
if (!isok)
{
if (Preyunsuanfu == "+")
{
sum += double.Parse(txtshuru.Text);
}
if (Preyunsuanfu == "-")
{
sum -= double.Parse(txtshuru.Text);
}
if (Preyunsuanfu == "*")
{
sum *= double.Parse(txtshuru.Text);
}
if (Preyunsuanfu == "/")
{
sum /= double.Parse(txtshuru.Text);
}
if (Preyunsuanfu == "%")
{
sum %= double.Parse(txtshuru.Text);
}

biaodashi = biaodashi+ Preyunsuanfu + txtshuru.Text;
txtshuru.Text = sum.ToString();
}
}
txtjisuanshi.Text = biaodashi + yunsuanfu;
Preyunsuanfu = yunsuanfu;
isok = true;

}

private void btnClear_Click(object sender, EventArgs e)
{

}

private void btndeng_Click(object sender, EventArgs e)
{

}

}
}