Элемент "textbox1" не существует в данном контексте - C#

using System.Drawing;
using System.Windows.Forms;
using System;
class SimpleWindowsForm : Form
{
ListBox listbox1;
public SimpleWindowsForm()
    {
        this.Size = new Size(400, 400);
        PictureBox picturebox1 = new PictureBox();
        picturebox1.SizeMode = PictureBoxSizeMode.StretchImage;
        Bitmap image1 = new Bitmap("C://Users//Компьютер//Desktop//Автар//Deadpool-Marvel-фэндомы-Deathstroke-1197435.png");
        picturebox1.ClientSize = new Size(this.Width, 150);
        picturebox1.Image = (Image)image1;
        this.Controls.Add(picturebox1);
        this.Text = "Калькулятор";
        // Окно
        Button button1 = new System.Windows.Forms.Button();
        button1.Text = "Unknown";
        button1.Location = new Point(150, 160);
        button1.Size = new Size(100, 30);
        button1.Click += new System.EventHandler(button1_Click);
        this.Controls.Add(button1);
        // 1 кнопка
        listbox1 = new System.Windows.Forms.ListBox();
        listbox1.Location = new System.Drawing.Point(20, 200);
        listbox1.Size = new Size(100, 100);
        listbox1.Items.Add("Умножение");
        listbox1.Items.Add("Деление");
        listbox1.Items.Add("Сложение");
        listbox1.Items.Add("Вычитание");
        listbox1.SelectedIndex = 2;
        this.Controls.Add(listbox1);
        // Список
        TextBox textbox1 = new TextBox();
        textbox1.Multiline = true;
        textbox1.Text = "";
        textbox1.Location = new Point(250, 200);
        textbox1.Width = 50; textbox1.Height = 20;
        this.Controls.Add(textbox1);
        // 1 число
        TextBox textbox2 = new TextBox();
        textbox2.Multiline = true;
        textbox2.Text = "";
        textbox2.Location = new Point(250, 220);
        textbox2.Width = 50; textbox1.Height = 20;
        this.Controls.Add(textbox2);
    }
 
static void Main()
    {
        Application.Run(new SimpleWindowsForm());
        // Запуск
    }
void button1_Click(object a, EventArgs b)
        {
 
            string x;
            string y;
            int v;
            int n;
            x = textbox1.Text;
            y = textbox2.Text;
            int TheResultOfActionPlus = v + n;
            switch(listbox1.SelectedItem.ToString())
                {
                    case ("Сложение"): MessageBox.Show("Ваш ответ: " + TheResultOfActionPlus); break;
                }
        }
private void InitializeComponent()
        {
            this.SuspendLayout();
            this.BackColor = System.Drawing.SystemColors.Control;
            this.ClientSize = new System.Drawing.Size(293, 273);
            this.Name = "123";
            this.ResumeLayout(false);
        }
}