5. Email組合程式

namespace Q5
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            List<string> numbers = new List<string>();
            if (textBox1.Text.Length != 0)
                for (int i = int.Parse(textBox1.Text), n = int.Parse(textBox2.Text); i <= n; i++)
                    numbers.Add($"{i:0}");

            string last = "";
            foreach (var c in textBox3.Text)
            {
                if (c == ' ' || c == ',') continue;
                last += c;
                if (last.Length == 8)
                {
                    numbers.Add(last);
                    last = "";
                }
            }

            for (int i = 0; i < numbers.Count; i++)
            {
                var k = numbers[i];
                int sum = 0;
                for (int j = 1; j <= 8; j++)
                {
                    sum += (k[j - 1] - '0') * j;
                }
                numbers[i] = k + (sum % 10) + "@antu.edu.tw";
            }

            textBox4.Text = string.Join("; ", numbers);
        }
    }
}