Program.cs

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace Activity3Salarda
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new FrmName());
        }
    }
}

Form1.cs

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

namespace Activity3Salarda
{
    public partial class FrmName : Form
    {
        private clsArithmetic objArithmetic = new clsArithmetic();
        public FrmName()
        {
            InitializeComponent();
            objArithmetic = new clsArithmetic();
        }

        private void txtNum1_TextChanged(object sender, EventArgs e)
        {
            try
            {
                objArithmetic.Num1 = int.Parse(txtNum1.Text);
            }
            catch (Exception Ex)
            {
                MessageBox.Show("Incorrect Format.\n" + Ex.Message, "Error", MessageBoxButtons.OK,MessageBoxIcon.Error);
            }

        }

        private void txtNum2_TextChanged(object sender, EventArgs e)
        {
            try
            {
                objArithmetic.Num2 = int.Parse(txtNum2.Text);
            }
            catch (Exception Ex)
            {
                MessageBox.Show("Incorrect Format. \n" + Ex.Message, "Error", MessageBoxButtons.OK,MessageBoxIcon.Error);
            }
           

        }

        private void btnAdd_Click(object sender, EventArgs e)
        {
            MessageBox.Show(objArithmetic.getsum().ToString());
          
        }

        private void btnSubtract_Click(object sender, EventArgs e)
        {
            MessageBox.Show(objArithmetic.getdif().ToString());
        }

        private void btnMultiply_Click(object sender, EventArgs e)
        {
            MessageBox.Show(objArithmetic.getpro().ToString());
        }

        private void btnDivide_Click(object sender, EventArgs e)
        {
            MessageBox.Show(objArithmetic.getquo().ToString());
        }
    }
}

Class.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace Activity3Salarda
{
    class clsArithmetic
    {

        private int _Num1 = 0;
        public int Num1
        {
            get { return _Num1; }
            set { _Num1 = value; }
        }
        private int _Num2 = 0;
        public int Num2
        {
            get { return _Num2; }
            set { _Num2 = value; }
        }
        public int getsum()
        {
            return _Num1 + _Num2;
        }
        public int getdif()
        {
            return _Num1 - _Num2;
        }
        public int getquo()
        {
            return _Num1 / _Num2;
        }
        public int getpro()
        {
            return _Num1 * _Num2;
        }
    }
}