HESAP MAKİNASI BİR TEXTBOX

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 WindowsFormsApplication2

{

    public partial class Form1 : Form

    {double sayi1,sayi2,sonuc;

    string islem;

       

        public Form1()

        {

            InitializeComponent();

        }

        private void sayi()

        {

            sayi1 = Convert.ToDouble(textBox1.Text);

            textBox1.Clear();

        }

        private void button1_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "1";

        }

        private void button2_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "2";

        }

        private void button3_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "3";

        }

        private void button4_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "4";

        }

        private void button5_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "5";

        }

        private void button6_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "6";

        }

        private void button7_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "7";

        }

        private void button8_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "8";

        }

        private void button9_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "9";

        }

        private void button11_Click(object sender, EventArgs e)

        {

            textBox1.Text = textBox1.Text + "0";

        }

        private void button14_Click(object sender, EventArgs e)

        {

            sayi();

            islem = "/";

        }

        private void button13_Click(object sender, EventArgs e)

        {

            sayi();

            islem = "*";

        }

        private void button10_Click(object sender, EventArgs e)

        {

            sayi();

            islem = "-";

        }

        private void button12_Click(object sender, EventArgs e)

        {

            sayi();

            islem = "+";

        }

        private void button16_Click(object sender, EventArgs e)

        {

            textBox1.Clear();

        }

        private void button15_Click(object sender, EventArgs e)

        {

            if (textBox1.Text != " ")

            {

                sayi2 = Convert.ToDouble(textBox1.Text);

                switch (islem)

                {

                    case "+":

                        sonuc = sayi1 + sayi2;

                        textBox1.Text = Convert.ToString(sonuc);

                        break;

                    case "-":

                        sonuc = sayi1 - sayi2;

                        textBox1.Text = Convert.ToString(sonuc);

                        break;

                    case "*":

                        sonuc = sayi1 * sayi2;

                        textBox1.Text = Convert.ToString(sonuc);

                        break;

                    case "/":

                        sonuc = sayi1 / sayi2;

                        textBox1.Text = Convert.ToString(sonuc);

                        break;

                }

            }

            else

            {

                textBox1.Text = "İkinci Değeri Giriniz";

            }

        }

    }

}