using System.Text;


string[] numbers;
using (FileStream fstream = File.OpenRead("/var/result/data.txt"))
{
    byte[] buffer = new byte[fstream.Length];
    await fstream.ReadAsync(buffer, 0, buffer.Length);
    string text = Encoding.Default.GetString(buffer);
    text = text.Replace("\r\n", ";");
    numbers = text.Split(new char[] { ';' }, StringSplitOptions.RemoveEmptyEntries);
}

int maxNum = int.MinValue;
foreach(var number in numbers)
{
    int currentNum = Convert.ToInt32(number);
    if(currentNum > maxNum)
    {
        maxNum = currentNum;
    }
}

int maxNumSqr = maxNum * maxNum;

using (FileStream fstream = new FileStream("/var/result/result.txt", FileMode.OpenOrCreate))
{
    byte[] buffer = Encoding.Default.GetBytes(maxNumSqr.ToString());
    await fstream.WriteAsync(buffer, 0, buffer.Length);
}

Console.WriteLine("Максимальное число: " + maxNum);
Console.WriteLine("Возведение в квадрат: " + maxNumSqr);