Notes

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

namespace _20170928_Form_HesapMakinesi
{
public partial class frmMain : Form
{
double Sonuc = 0;
double Islenen1 = 0;
double Islenen2 = 0;
string Islem = "";

public frmMain()
{
InitializeComponent();
}

#region Sayılar

private void button1_Click(object sender, EventArgs e)
{
lblSayi.Text += btn1.Text;
}

private void btn2_Click(object sender, EventArgs e)
{
lblSayi.Text += btn2.Text;
}

private void btn3_Click(object sender, EventArgs e)
{
lblSayi.Text += btn3.Text;
}

private void btn4_Click(object sender, EventArgs e)
{
lblSayi.Text += btn4.Text;
}

private void btn5_Click(object sender, EventArgs e)
{
lblSayi.Text += btn5.Text;
}

private void btn6_Click(object sender, EventArgs e)
{
lblSayi.Text += btn6.Text;
}

private void btn7_Click(object sender, EventArgs e)
{
lblSayi.Text += btn7.Text;
}

private void btn8_Click(object sender, EventArgs e)
{
lblSayi.Text += btn8.Text;
}

private void btn9_Click(object sender, EventArgs e)
{
lblSayi.Text += btn9.Text;
}

private void btn0_Click(object sender, EventArgs e)
{
lblSayi.Text += btn0.Text;
}


#endregion

private void btnCE_Click(object sender, EventArgs e)
{
lblSayi.Text = string.Empty;
}

private void btnEsittir_Click(object sender, EventArgs e)
{
Hesapla();
lblSonuc.Text = Sonuc.ToString();
}

private void btnTopla_Click(object sender, EventArgs e)
{
lblIslem.Text += lblSayi.Text + " + ";

Hesapla();

lblSonuc.Text = Sonuc.ToString();
lblSayi.Text = "";

Islem = "+";
}

private void btnCikar_Click(object sender, EventArgs e)
{
lblIslem.Text += lblSayi.Text + " - ";

Hesapla();

lblSonuc.Text = Sonuc.ToString();
lblSayi.Text = "";

Islem = "-";
}

private void btnCarp_Click(object sender, EventArgs e)
{
lblIslem.Text += lblSayi.Text + " * ";

Hesapla();

lblSonuc.Text = Sonuc.ToString();
lblSayi.Text = "";

Islem = "*";
}

private void btnBol_Click(object sender, EventArgs e)
{
lblIslem.Text += lblSayi.Text + " / ";

Hesapla();

lblSonuc.Text = Sonuc.ToString();
lblSayi.Text = "";

Islem = "/";
}

void Hesapla()
{
if (Islem == "+")
{
Islenen1 = Sonuc;
Islenen2 = double.Parse(lblSayi.Text);
Sonuc = Islenen1 + Islenen2;
}
else if (Islem == "-")
{
if (Sonuc == 0)
Sonuc = double.Parse(lblSayi.Text);
else
{
Islenen1 = Sonuc;
Islenen2 = double.Parse(lblSayi.Text);
Sonuc = Islenen1 - Islenen2;
}
}
else if (Islem == "*")
{
if (Sonuc == 0)
Sonuc = 1;
else
{
Islenen1 = Sonuc;
Islenen2 = double.Parse(lblSayi.Text);
Sonuc = Islenen1 * Islenen2;
}
}
else if (Islem == "/")
{
if (Sonuc == 0)
Sonuc = double.Parse(lblSayi.Text);
else
{
Islenen1 = Sonuc;
Islenen2 = double.Parse(lblSayi.Text);
Sonuc = Islenen1 / Islenen2;
}
}
else
Sonuc = double.Parse(lblSayi.Text);
}
}
}