yazılım derslerini vereyim mi laen ibneler

using System;
using System. Collections.Generic;
using System. ComponentModel;
using System. Data;
using System. Drawing;
using System. Text;
using System. Windows.Forms;

namespace WindowsApplication1
{
public partial class Form1 : Form
{
int a, b;
string d;

public Form1()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "1";
}
else
{
textBox1.Text = textBox1.Text + "1";
}
}

private void button2_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "2";
}
else
{
textBox1.Text = textBox1.Text + "2";
}
}

private void button3_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "3";
}
else
{
textBox1.Text = textBox1.Text + "3";
}
}

private void button4_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "4";
}
else
{
textBox1.Text = textBox1.Text + "4";
}
}

private void button5_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "5";
}
else
{
textBox1.Text = textBox1.Text + "5";
}
}

private void button6_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "6";
}
else
{
textBox1.Text = textBox1.Text + "6";
}
}

private void button7_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "7";
}
else
{
textBox1.Text = textBox1.Text + "7";
}
}

private void button8_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "8";
}
else
{
textBox1.Text = textBox1.Text + "8";
}
}

private void button9_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "9";
}
else
{
textBox1.Text = textBox1.Text + "9";
}
}

private void button11_Click(object sender, EventArgs e)
{
if (textBox1.Text == "0")
{
textBox1.Text = "0";
}
else
{
textBox1.Text = textBox1.Text + "0";
}
}

private void button10_Click(object sender, EventArgs e)
{
a = Convert. ToInt16(textBox1.Text);
d = "+";
textBox1.Text = "";
}

private void button12_Click(object sender, EventArgs e)
{
a = Convert. ToInt16(textBox1.Text);
d = "-";
textBox1.Text = "";
}

private void button14_Click(object sender, EventArgs e)
{
a = Convert. ToInt16(textBox1.Text);
d = "*";
textBox1.Text = "";
}

private void button15_Click(object sender, EventArgs e)
{
a = Convert. ToInt16(textBox1.Text);
d = "/";
textBox1.Text = " ";
}

private void button17_Click(object sender, EventArgs e)
{
b=Convert. ToInt16(textBox1.Text);
if (d

" ")

if (d

"+")
{
textBox1.Text = Convert. ToString(a + b);
}
if (d == "-")
{
textBox1.Text = Convert. ToString(a-b);
}
if (d == "*")
{
textBox1.Text = Convert. ToString(a * b);
}
if (d == "/")
{
textBox1.Text = Convert. ToString(a / b);
}
}

private void Form1_Load(object sender, EventArgs e)
{
d = " ";
}

private void button16_Click(object sender, EventArgs e)
{
a = 0;
b = 0;
d = "";
textBox1.Text = "0";
}

private void button18_Click(object sender, EventArgs e)
{

}

private void button19_Click(object sender, EventArgs e)
{
textBox1.Text. Substring(1, 5);
}

private void button13_Click(object sender, EventArgs e)
{
MessageBox. Show("inci giber:)", "kutsall");
}
}
}