Form1.cs

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 kalkulatorsederhana
{
    public partial class Form1 : Form
    {
        double nilai_1, nilai_2;
        public Form1()
        {
            InitializeComponent();
        }

        private void tambah_button_Click(object sender, EventArgs e)
        {
            this.hasil_textbox.Text = (nilai_1 + nilai_2).ToString();
        }

        private void kurang_button_Click(object sender, EventArgs e)
        {
            this.hasil_textbox.Text = (nilai_1 - nilai_2).ToString();
        }

        private void kali_button_Click(object sender, EventArgs e)
        {
            this.hasil_textbox.Text = (nilai_1 * nilai_2).ToString();
        }

        private void bagi_button_Click(object sender, EventArgs e)
        {   
            if (nilai_2 == 0)
            {
                this.hasil_textbox.Text = "NULL, dibagi dengan 0";
            }
            else {
                this.hasil_textbox.Text = (nilai_1 / nilai_2).ToString();
            }
        }

        private void hapus_button_Click(object sender, EventArgs e)
        {
            this.hasil_textbox.Text = "";
            this.nilai1_textbox.Text = "";
            this.nilai2_textbox.Text = "";
        }
        private void nilai1_textbox_TextChanged(object sender, EventArgs e)
        {
            if(this.nilai1_textbox.Text!="")
                this.nilai_1 = double.Parse(this.nilai1_textbox.Text);
        }
        private void nilai2_textbox_TextChanged(object sender, EventArgs e)
        {
            if (this.nilai1_textbox.Text != "")
                this.nilai_2 = double.Parse(this.nilai2_textbox.Text);
        }
    }
}