求c#高手帮助，c#设计的计算器怎样实现先乘除后加减
比如：2+3*5 怎么让程序自动判别先乘除后加减啊？
程序代码：using System;
using System.Collections.Generic;
using using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace Calculator
{
public partial class Form1 : Form
{
//定义一个控制标志
int flag=-1;
//存储第一个操作数
int num1,num2;
//存储最后的结果
int results;
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
}
//当用户按下0-9数字键时触发
private void num_Click(object sender, EventArgs e)
{
Button a = (Button)sender;
txtOutput.Text = txtOutput.Text + a.Text;
}
//当用户按下运算符时触发
private void operator_Click(object sender, EventArgs e)
{
Button b = (Button)sender;
if (b.Text == "+")
{
if (flag == -1)
{
num1 = int.Parse(txtOutput.Text);
}
else
{
switch(flag)
{
case 0:
num1 = num1 + int.Parse(txtOutput.Text);
break;
case 1:
num1 = num1 - int.Parse(txtOutput.Text);
break;
case 2:
num1 = num1 *int.Parse(txtOutput.Text);
break;
case 3:
num1 = num1 / int.Parse(txtOutput.Text);
break;
}
};
if (num2==-1)
{
num1 = 0;
}
txtOutput.Text = "";
flag = 0;
}
if (b.Text == "-")
{
if (flag == -1)
{
num1 = int.Parse(txtOutput.Text);
}
else
{
switch (flag)
{
case 0:
num1 = num1 + int.Parse(txtOutput.Text);
break;
case 1:
num1 = num1 - int.Parse(txtOutput.Text);
break;
case 2:
num1 = num1 * int.Parse(txtOutput.Text);
break;
case 3:
num1 = num1 / int.Parse(txtOutput.Text);
break;
}
};
txtOutput.Text = "";
flag = 1;
}
if (b.Text == "*")
{
if (flag == -1)
{
num1 = int.Parse(txtOutput.Text);
}
else
{
switch (flag)
{
case 0:
num1 = num1 + int.Parse(txtOutput.Text);
break;
case 1:
num1 = num1 - int.Parse(txtOutput.Text);
break;
case 2:
num1 = num1 * int.Parse(txtOutput.Text);
break;
case 3:
num1 = num1 / int.Parse(txtOutput.Text);
break;
}
}
txtOutput.Text = "";
flag = 2;
}
if (b.Text == "/")
{
if (flag == -1)
{
num1 = int.Parse(txtOutput.Text);
}
else
{
switch (flag)
{
case 0:
num1 = num1 + int.Parse(txtOutput.Text);
break;
case 1:
num1 = num1 - int.Parse(txtOutput.Text);
break;
case 2:
num1 = num1 * int.Parse(txtOutput.Text);
break;
case 3:
num1 = num1 / int.Parse(txtOutput.Text);
break;
}
}
txtOutput.Text = "";
flag = 3;
}
if (b.Text == "C")
{
txtOutput.Text = "";
num1 = 0;
txtOutput.Focus();
}
if (b.Text == "=")
{
if (flag == 0)
{
results = num1 + int.Parse(txtOutput.Text);
flag = -1;
}
if (flag == 1)
{
results = num1 - int.Parse(txtOutput.Text);
flag = -1;
}
if (flag == 2)
{
results = num1 * int.Parse(txtOutput.Text);
flag = -1;
}
if (flag == 3)
{
results = num1 / int.Parse(txtOutput.Text);
flag = -1;
}
txtOutput.Text = results.ToString();
}
}
private void btnDivide_Click(object sender, EventArgs e)
{
}
}
}
