Código: Seleccionar todo

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

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

        private void button2_Click(object sender, EventArgs e)
        {
            txtbox1.Text="0";
            txtbox2.Text="0";
            Lbl4.Text = "0";
          
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Lbl4.Text = Convert.ToString(
                Int64.Parse(txtbox1.Text) * Int64.Parse(txtbox2.Text)
                );
        }
    }
}