Calc program in c#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace CALC
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
String s = "",str="";
char c = '+';
double res = 0, opr = 0;
private void button2_Click(object sender, EventArgs e)
{
str += "2";
textBox1.Text = str;
}
public String compute()
{
res = Convert.ToInt32(textBox1.Text);
s = "";
str = "";
if(c=='+')
res+=opr;
if(c=='-')
res=opr-res;
if(c=='*')
res*=opr;
if(c=='/')
res=opr/res;
return ""+res;
}
private void button1_Click(object sender, EventArgs e)
{
str += "1";
opr = res;
textBox1.Text = str;
}
private void button3_Click(object sender, EventArgs e)
{
str += "3";
opr = res;
textBox1.Text = str;
}
private void button4_Click(object sender, EventArgs e)
{
str += "4";
opr = res;
textBox1.Text = str;
}

private void button5_Click(object sender, EventArgs e)
{
str += "5";
opr = res;
textBox1.Text = str;
}
private void button6_Click(object sender, EventArgs e)
{
str += "6";
opr = res;
textBox1.Text = str;
}
private void button7_Click(object sender, EventArgs e)
{
str += "7";
opr = res;
textBox1.Text = str;
}
private void button8_Click(object sender, EventArgs e)
{
str += "8";
opr = res;
textBox1.Text = str;
}
private void button9_Click(object sender, EventArgs e)
{
str += "9";
opr = res;
textBox1.Text = str;
}
private void button10_Click(object sender, EventArgs e)
{
str += "0";
opr = res;
textBox1.Text = str;
}
private void button16_Click(object sender, EventArgs e)
{
if (str.Length != 0)
str = str.Substring(0, str.Length - 1);
s = str;
textBox1.Text = s;
}
private void button11_Click(object sender, EventArgs e)
{
String temp = compute();
c = '+';
textBox1.Text = temp;
}
private void button13_Click(object sender, EventArgs e)
{
String temp = compute();
c = '*';
textBox1.Text = temp;
}
private void button14_Click(object sender, EventArgs e)
{
String temp = compute();
c = '/';
textBox1.Text = temp;
}
private void button15_Click(object sender, EventArgs e)
{
String temp = compute();
c = '+';
opr = 0;
textBox1.Text = temp;
}
private void button12_Click(object sender, EventArgs e)
{
String temp = compute();
c = '-';
textBox1.Text = temp;
}
}
}