Как создать вложенный цикл для буквенных комбо?

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Я создаю графический интерфейс телефонного номера, используя объект StreamWriter для записи в файл всех возможных семибуквенных слов, соответствующих этому номеру. 2= " ABC "3=" DEF "4=" GHI " и т. д. Я не уверен, как создать цикл, который будет писать различные комбинации букв?

- Спасибо!

Что я уже пробовал:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace TeleNumLetters
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        int[] numberInput = new int[8] { 2, 3, 4, 5, 6, 7, 8, 9 };
        int maxLength = 7;
        char[] phoneArray = new char[7];

        char[] number2 = { 'A', 'B', 'C' };
        char[] number3 = { 'D', 'E', 'F' };
        char[] number4 = { 'G', 'H', 'I' };
        char[] number5 = { 'J', 'K', 'L' };
        char[] number6 = { 'M', 'N', 'O' };
        char[] number7 = { 'P', 'Q', 'R', 'S' };
        char[] number8 = { 'T', 'U', 'V' };
        char[] number9 = { 'W', 'X', 'Y', 'Z' };
        private string phoneNumber;

        public void ConvertNumber()
        {
           //**This is the loop I attempted but I'm confused//
            string phoneNumber = textBox1.Text;
            for (int i = 0; i < 7; i++)
            {
                phoneArray[i] = phoneNumber.ElementAt(i);

                for (int i1 = 0; i1 < phoneArray[0]; i1++)
                {
                    for (int i2 = 0; i2 < phoneArray[1]; i2++)
                    {
                    }
                }
            }
        }  

        private void generateButton_Click(object sender, EventArgs e)
            {
                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine(phoneArray);
                }
            }


            private void button2_Click(object sender, EventArgs e)
            {
                int maxLength = 7;
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[0]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }

            private void button3_Click(object sender, EventArgs e)
            {
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[1]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }

            private void button4_Click(object sender, EventArgs e)
            {
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[2]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }

            private void button5_Click(object sender, EventArgs e)
            {
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[3]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }

            private void button6_Click(object sender, EventArgs e)
            {
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[4]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }

            private void button7_Click(object sender, EventArgs e)
            {
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[5]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }

            private void button8_Click(object sender, EventArgs e)
            {
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[6]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }

            private void button9_Click(object sender, EventArgs e)
            {
                if (textBox1.TextLength < maxLength)
                {
                    textBox1.Text += Convert.ToString(numberInput[7]);
                }
                else { }

                using (System.IO.StreamWriter writer = new System.IO.StreamWriter("WriteLines.txt", true))
                {
                    writer.WriteLine();
                }
            }
        }
    }

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