Bobo

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 WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void textBox3_TextChanged(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            int a = Convert.ToInt32(textBox1.Text);
            int b = Convert.ToInt32(textBox2.Text);
            textBox3.Text = Convert.ToString(a + b);
            label2.Text = "+";
        }

        private void button2_Click(object sender, EventArgs e)
        {
            int a = Convert.ToInt32(textBox1.Text);
            int b = Convert.ToInt32(textBox2.Text);
            textBox3.Text = Convert.ToString(a - b);
            label2.Text = "-";
        }

        private void button3_Click(object sender, EventArgs e)
        {
            decimal a = Convert.ToDecimal(textBox1.Text);
            decimal b = Convert.ToDecimal(textBox2.Text);
            textBox3.Text = Convert.ToString(a * b);
            label2.Text = "*";
        }

        private void button4_Click(object sender, EventArgs e)
        {
            decimal a = Convert.ToDecimal(textBox1.Text);
            decimal b = Convert.ToDecimal(textBox2.Text);
            textBox3.Text = Convert.ToString(a / b);
            label2.Text = "/";
        }

        private void button5_Click(object sender, EventArgs e)
        {
            textBox1.Clear();
            textBox2.Clear();
            textBox3.Clear();
        }

        private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            char a = e.KeyChar;

            if (!Char.IsDigit(a) && a != 8 && a != 42)
            {
                e.Handled = true;
            }
        }

        private void textBox2_KeyPress(object sender, KeyPressEventArgs e)
        {
            char a = e.KeyChar;

            if (!Char.IsDigit(a) && a != 8 && a != 42)
            {
                e.Handled = true;
            }
        }

        private void textBox3_KeyPress(object sender, KeyPressEventArgs e)
        {
            char a = e.KeyChar;

            if (!Char.IsDigit(a) && a != 8 && a != 42)
            {
                e.Handled = true;
            }
        }
    }
}