how to use .cs in .sln file

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;
using System.Diagnostics;


namespace ROD_PISTON1
{
    public partial class Form1 : Form
    {
        private static Microsoft.Office.Interop.Excel.Workbook mWorkBook;
        private static Microsoft.Office.Interop.Excel.Sheets mWorkSheets;
        private static Microsoft.Office.Interop.Excel.Worksheet mWSheet1;
        private static Microsoft.Office.Interop.Excel.Application oXL;
        private static Microsoft.Office.Interop.Excel.Worksheet mWSheet2;
        public Form1()
        {
            InitializeComponent();
        }

        private void label2_Click(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            string path = @"C:\Users\Dhaval\Desktop\BOOK1.xlsx";

            oXL = new Microsoft.Office.Interop.Excel.Application();

            oXL.Visible = false;

            oXL.DisplayAlerts = false;

            mWorkBook = oXL.Workbooks.Open(path, 0, false, 5, "", "", false, Microsoft.Office.Interop.Excel.XlPlatform.xlWindows, "", true, false, 0, true, false, false);

            //Get all the sheets in the workbook

            mWorkSheets = mWorkBook.Worksheets;

            //Get the allready exists sheet

            mWSheet1 = (Microsoft.Office.Interop.Excel.Worksheet)mWorkSheets.get_Item("INPUT");

            // Microsoft.Office.Interop.Excel.Range range = mWSheet1.UsedRange;
            //mWSheet1.Cells[3, 3] = comboBox1.Text;
            //mWSheet1.Cells[4, 3] = comboBox2.Text;
            //mWSheet1.Cells[5, 3] = textBox3.Text;
            //mWSheet1.Cells[6, 3] = textBox4.Text;
            //mWSheet1.Cells[7, 3] = textBox5.Text;
            //mWSheet1.Cells[8, 3] = textBox6.Text;
            //mWSheet1.Cells[9, 3] = textBox7.Text;
            //mWSheet1.Cells[10, 3] = textBox8.Text;
            //mWSheet1.Cells[11, 3] = textBox9.Text;
            //mWSheet1.Cells[12, 3] = textBox10.Text;
            //mWSheet1.Cells[13, 3] = comboBox3.Text;
            //("textBox"+i.ToString()).Text;

            mWSheet2 = (Microsoft.Office.Interop.Excel.Worksheet)mWorkSheets.get_Item("OUTPUT");

            Microsoft.Office.Interop.Excel.Range range = mWSheet2.UsedRange;

            double i = (mWSheet2.Cells[3, 3].Value2);
            textBox12.Text = i.ToString();

            double j = (mWSheet2.Cells[4, 3].Value2);
            textBox13.Text = j.ToString();

            double k = (mWSheet2.Cells[5, 3].Value2);
            textBox14.Text = k.ToString();
            mWorkBook.Save();
            mWorkBook.Close();
            Process p = new Process();
            p.StartInfo.FileName = "C:\\Users\\Dhaval\\Desktop\\ug\\rod.prt";
            p.Start();

        }
    }
}