1. Use File.ReadAllLines to read 1.txt and 2.txt.
2. StreamWriter will be used in writing the output file.

using System;

public class Calculator
{
    // Modify these paths if needed
    // These terms are set to 'public' and 'static' to make them accessible for the Unit Tests
    public static string f1Path = "C:\\Users\\Public\\Documents\\1.txt";
    public static string f2Path = "C:\\Users\\Public\\Documents\\2.txt";
    public static string outputPath = "C:\\Users\\Public\\Documents\\result.txt";

    public static void Main(string[] args)
    {
        char opr;

        while (true)
        {
            Console.Write("Enter an operator (+, -, /, *): ");

            // Simplified code using TryParse
            if (char.TryParse(Console.ReadLine(), out opr) && "/-+*".Contains(opr))
                break;

            Console.WriteLine("Invalid operator.");
        }

        // Write code below this line

        string[] numbers_1 = File.ReadAllLines(f1Path);
        string[] numbers_2 = File.ReadAllLines(f2Path);

        float result = 0.0f, n1, n2;
        using (StreamWriter writer = new StreamWriter(outputPath))
        {
            for (int i = 0; i < numbers_1.Length; i++)
            {
                n1 = float.Parse(numbers_1[i]);
                n2 = float.Parse(numbers_2[i]);

                if (opr == '+')
                    result = n1 + n2;
                else if (opr == '-')
                    result = n1 - n2;
                else if (opr == '/')
                    result = n1 / n2;
                else if (opr == '*')
                    result = n1 * n2;

                writer.WriteLine(result);
            }
        }

        Console.WriteLine($"Results saved in {outputPath}");
        // Write code above this line
    }
}