กรอกตัวเลขใน textBox เก็บค่าให้อยู่ใน Array แล้วทำการวน loop

private List<string> array = new List<string>();

public void button_onclick(object sender, eventarg e) {
   string str = ((textbox) sender).text;

   if (!array.contains(str)) {
      array.add(str);
   }
}

private List<string> array = new List<string>();
        string C = DateTime.Now.ToString("dd/MM/yyyy");
        private void textBox1_KeyDown_1(object sender,KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter)
            {
                if (textBox1.Text.Length >= 15 || textBox1.Text.Length < 5)
                {
                    MessageBox.Show("Number Invalid");
                    textBox1.Text = "";
                    return;
                }
                string str = ((TextBox) sender).Text;
                if (!array.Contains(str))
                {
                    array.Add(str);
                    int Number = Convert.ToInt32(textBox3.Text.ToString());
                    string nb = (++Number).ToString();
                    textBox3.Text = nb;
                    textBox2.Text = textBox1.Text.Substring(0, 4);
                    label5.Text = textBox1.Text;
                    string log = "CheckStock.txt";
                    this.textBox1.Multiline = true;
                    if (File.Exists(log) == true)
                    {
                        f = new FileInfo(log);
                        StreamWriter sw = f.AppendText();
                        sw.WriteLine(C + "\t" + A + "\t" + B + "\t" + textBox1.Text + "\t" + textBox2.Text + "\t" + "1");
                        textBox1.Text = "";
                        sw.Close();
                    }
                    else if (File.Exists(log) == false)
                    {
                        f = new FileInfo(log);
                        StreamWriter sw = f.CreateText();
                        sw.WriteLine(C + "\t" + A + "\t" + B + "\t" + textBox1.Text + "\t" + textBox2.Text + "\t" + "1");
                        textBox1.Text = "";
                        sw.Close();
                    }
                }
                else
                { 
                    MessageBox.Show("aaaa");
                }                
            }         
        }