我写了一个计算器 但是怎么让他实现累加呢？

代码如下
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 jisuanqi
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button2_Click(object sender, EventArgs e)
{
textBox1.Text += "4";
}
private void button1_Click(object sender, EventArgs e)
{
textBox1.Text += "1";
}
private void button3_Click(object sender, EventArgs e)
{
textBox1.Text += "2";
}
string a = "";
double b = 0;
double c = 0;
double d = 0;
private void button8_Click(object sender, EventArgs e)
{
b = double.Parse(textBox1.Text);
textBox1.Text = "";
a = "+";
//int datetime double long
}
private void button16_Click(object sender, EventArgs e)
{
if(a=="+")
{
c=double.Parse(textBox1.Text);
textBox1.Text=(b + c).ToString();
}
else if(a=="-")
{
c = double.Parse(textBox1.Text);
textBox1.Text = (b -c).ToString();
}
else if (a == "*")
{
c = double.Parse(textBox1.Text);
textBox1.Text = (b * c).ToString();
}
else if (a == "/")
{
c = double.Parse(textBox1.Text);
textBox1.Text = (b * c).ToString();
}
else if(a=="ca")
{
c = double.Parse(textBox1.Text);
textBox1.Text = (b - b).ToString();
}
}
private void button4_Click(object sender, EventArgs e)
{
textBox1.Text += "3";
}
private void button7_Click(object sender, EventArgs e)
{
textBox1.Text += "5";
}
private void button6_Click(object sender, EventArgs e)
{
textBox1.Text += "6";
}
private void button5_Click(object sender, EventArgs e)
{
textBox1.Text += "7";
}
private void button9_Click(object sender, EventArgs e)
{
textBox1.Text += "8";
}
private void button10_Click(object sender, EventArgs e)
{
textBox1.Text += "9";
}
private void button15_Click(object sender, EventArgs e)
{
textBox1.Text += "0";
}
private void button14_Click(object sender, EventArgs e)
{
textBox1.Text = "0";
}
private void button12_Click(object sender, EventArgs e)
{
b = double.Parse(textBox1.Text);
textBox1.Text = "";
a = "*";
}
private void button13_Click(object sender, EventArgs e)
{
b = double.Parse(textBox1.Text);
textBox1.Text = "";
a = "/";
}
private void button11_Click(object sender, EventArgs e)
{
b = double.Parse(textBox1.Text);
textBox1.Text = "";
a = "-";
}
}
}