C# Calculator program reading and writing calculations to a file

Question

I need to be able to save and load answers that are entered by the User in a simple calculator program. Where should I put the code to read and write in the following program?. I will also need to be able to load previously entered calculations. I would also need to create a load function to the GUI. Thanks for your time GUI

using System;
using System.Linq;
using System.IO;

namespace Calculator_Project
{
    public partial class Form1: Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        //sets user input and answer return
        double num1, ans;
        double count;
        private void calculate_button_Click(object sender, EventArgs e)
        {
            compute(count);
        }
        //
        public void compute(double count) 
        {
            switch (count)
            {
                case 1:
                    ans = num1 - float.Parse(output_display.Text);
                    output_display.Text = ans.ToString();
                    break;
                case 2:
                    ans = num1 + float.Parse(output_display.Text);
                    output_display.Text = ans.ToString();
                    break;
                case 3:
                    ans = num1 * float.Parse(output_display.Text);
                    output_display.Text = ans.ToString();
                    break;
                case 4:
                    ans = num1 / float.Parse(output_display.Text);
                    output_display.Text = ans.ToString();
                    break;
                default:
                    break;
            }
        }
        // Math Operantion Click events
        private void number1_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 1;
        }

        private void number2_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 2;
        }
        private void number3_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 3;
        }

        private void number4_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 4;
        }

        private void number5_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 5;
        }

        private void number6_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 6;
        }

        private void number7_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 7;
        }

        private void number8_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 8;
        }

        private void number9_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 9;
        }

        private void numberZero_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 0;
        }

        private void numberDoubleZero_button_Click(object sender, EventArgs e)
        {
            output_display.Text = output_display.Text + 0 + 0;
        }

        private void Decimal_button_Click(object sender, EventArgs e)
        {
            int c = output_display.TextLength;
            int flag = 0;
            string text = output_display.Text;
            for (int i = 0; i < c; i++)
            {
                if (text[i].ToString() == ".")
                {
                    flag = 1; break;
                }
                else
                {
                    flag = 0;
                }
            }
            if (flag == 0)
            {
                output_display.Text = output_display.Text + ".";
            }
        }
        // Math Formulas for click event: *, +, -, sgrt
        private void clear_button_Click(object sender, EventArgs e)
        {
            output_display.Clear();
            count = 0;
        }

        private void divide_button_Click(object sender, EventArgs e)
        {
            num1 = float.Parse(output_display.Text);
            output_display.Clear();
            output_display.Focus();
            count = 4;
        }
        
        private void multiply_button_Click(object sender, EventArgs e)
        {
            num1 = float.Parse(output_display.Text);
            output_display.Clear();
            output_display.Focus();
            count = 3;
        }

        private void subtract_button_Click(object sender, EventArgs e)
        {
            num1 = float.Parse(output_display.Text);
            output_display.Clear();
            output_display.Focus ();
        }

        private void Sqrt_button_Click(object sender, EventArgs e)
        {
            output_display.Text = Convert.ToString(Math.Sqrt(Convert.ToDouble(output_display.Text)));
        }

        private void add_button_Click(object sender, EventArgs e)
        {
            num1 = float.Parse(output_display.Text);
            output_display.Clear();
            output_display.Focus();
            count = 2;
        }

    }

}