Emon

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 WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        double FirstNumber;
        string Operation;
        public Form1()
        {
            InitializeComponent();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void button10_Click(object sender, EventArgs e)
        {
            string num0 = "0";
            textBox1.Text = textBox1.Text.ToString() + num0;

        }

        private void button7_Click(object sender, EventArgs e)
        {
            string num1 = "1";
            textBox1.Text = textBox1.Text.ToString() + num1;
        }

        private void button8_Click(object sender, EventArgs e)
        {
            string num2 = "2";
            textBox1.Text = textBox1.Text.ToString() + num2;
        }

        private void button9_Click(object sender, EventArgs e)
        {
            string num3 = "3";
            textBox1.Text = textBox1.Text.ToString() + num3;
        }

        private void button4_Click(object sender, EventArgs e)
        {
            string num4 = "4";
            textBox1.Text = textBox1.Text.ToString() + num4;
        }

        private void button5_Click(object sender, EventArgs e)
        {
            string num5 = "5";
            textBox1.Text = textBox1.Text.ToString() + num5;
        }

        private void button6_Click(object sender, EventArgs e)
        {
            string num6 = "6";
            textBox1.Text = textBox1.Text.ToString() + num6;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string num7 = "7";
            textBox1.Text = textBox1.Text.ToString() + num7;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            string num8 = "8";
            textBox1.Text = textBox1.Text.ToString() + num8;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            string num9 = "9";
            textBox1.Text = textBox1.Text.ToString() + num9;
        }

        private void button11_Click(object sender, EventArgs e)
        {
            FirstNumber = Convert.ToDouble(textBox1.Text);
            textBox1.Text = "";
            Operation = "+";
        }

        private void button12_Click(object sender, EventArgs e)
        {
            FirstNumber = Convert.ToDouble(textBox1.Text);
            textBox1.Text = "";
            Operation = "-";
        }

        private void button13_Click(object sender, EventArgs e)
        {
            FirstNumber = Convert.ToDouble(textBox1.Text);
            textBox1.Text = "";
            Operation = "*";
        }

        private void button14_Click(object sender, EventArgs e)
        {
            FirstNumber = Convert.ToDouble(textBox1.Text);
            textBox1.Text = "";
            Operation = "/";
        }

        private void button15_Click(object sender, EventArgs e)
        {
            double SecondNumber;
            double Result;

            SecondNumber = Convert.ToDouble(textBox1.Text);

            if (Operation == "+")
            {
                Result = (FirstNumber + SecondNumber);
                textBox1.Text = Convert.ToString(Result);
                FirstNumber = Result;
            }
            if (Operation == "-")
            {
                Result = (FirstNumber - SecondNumber);
                textBox1.Text = Convert.ToString(Result);
                FirstNumber = Result;
            }
            if (Operation == "*")
            {
                Result = (FirstNumber * SecondNumber);
                textBox1.Text = Convert.ToString(Result);
                FirstNumber = Result;
            }
            if (Operation == "/")
            {
                    Result = (FirstNumber / SecondNumber);
                    textBox1.Text = Convert.ToString(Result);
                    FirstNumber = Result;
            }
        }

    }
}