Prime numbers

using System;
using System.Collections.Generic;
using System.IO;

class AllPrimesToNumber
{
    public static long inputNumber = 0;
    public static long currentNumber;
    public static bool isNotPrime = false;
    public static List<long> primes = new List<long>();

    public static void checkIfPrime()
    {
        foreach (long prime in primes)
        {
            isNotPrime = currentNumber % prime == 0;
            if (isNotPrime || prime >= Math.Sqrt(currentNumber))
            {
                break;
            }
        }
        if (!isNotPrime) primes.Add(currentNumber);
        currentNumber = currentNumber + 2;
    }

    static void Main()
    {
        if (!File.Exists("primes.txt"))
        {
            StreamWriter createFile = new StreamWriter("primes.txt");
            createFile.Write("2 3 5 7 ");
            createFile.Close();
        }

        string primesFile = File.ReadAllText("primes.txt");
        string[] primesString = primesFile.Split(' ','\n', '\t');

        long number;
        for (long i = 0; i < primesString.Length - 1; i++)
        {
            number = long.Parse(primesString[i]);
            primes.Add(number);
        }

        int numOfPrimesInFile = primesString.Length - 1;
        long valueOfLastPrimeInFile = long.Parse(primesString[numOfPrimesInFile - 1]);

        currentNumber = valueOfLastPrimeInFile + 2;


        Console.WriteLine("\nNumber of primes in file: " + numOfPrimesInFile);
        Console.WriteLine("Last prime in file: " + valueOfLastPrimeInFile + "\n");

        Console.WriteLine("1 - Check if number is prime");
        Console.WriteLine("2 - Find next prime numbers and add them to the file");
        Console.WriteLine("3 - Show prime numbers in range (between 1 and " + valueOfLastPrimeInFile + ")");
        Console.Write("\nYour choice: ");
        int choice = int.Parse(Console.ReadLine());
        Console.WriteLine();
        switch (choice)
        {
            case 1:
                {
                    Console.Write("Enter the number you want to check: ");
                    inputNumber = long.Parse(Console.ReadLine());
                    if (inputNumber <= valueOfLastPrimeInFile)
                    {
                        if (primes.Contains(inputNumber))
                            Console.WriteLine(inputNumber + " is prime");
                        else
                            Console.WriteLine(inputNumber + " is not prime");
                    }
                    else
                    {
                        while (currentNumber <= inputNumber)
                        {
                            checkIfPrime();
                        }
                        if (primes.Contains(inputNumber))
                            Console.WriteLine(inputNumber + " is prime");
                        else
                            Console.WriteLine(inputNumber + " is not prime");
                    }
                    break;
                }
            case 2:
                {
                    Console.Write("How many prime numbers you want to add to the file?: ");
                    int numOfprimesToFind = int.Parse(Console.ReadLine());
                    while (primes.Count <  numOfprimesToFind + numOfPrimesInFile)
                    {
                        checkIfPrime();
                    }
                    break;
                }

            case 3:
                {
                    Console.Write("First number: ");
                    long startRangeInput = long.Parse(Console.ReadLine());
                    Console.Write("Second number: ");
                    long endRange = long.Parse(Console.ReadLine());
                    long startRange = startRangeInput;
                    while (!primes.Contains(startRange))
                    {
                        startRange--;
                    }
                    if (startRangeInput!=startRange)
                        startRange = primes.IndexOf(startRange) + 1;
                    else
                        startRange = primes.IndexOf(startRange);

                    while (!primes.Contains(endRange))
                    {
                        endRange--;
                    }
                    endRange = primes.IndexOf(endRange);

                    for (int i = (int)startRange; i <= (int)endRange; i++)
                    {
                        Console.WriteLine(primes[i]);
                    }
                    break;
                }
        }

        if (numOfPrimesInFile < primes.Count)
        {
            StreamWriter outputFile = new StreamWriter("primes.txt", true);
            for (int i = numOfPrimesInFile; i < primes.Count; i++)
            {
                outputFile.Write(primes[i] + " ");
            }
            outputFile.Close();
            Console.WriteLine((primes.Count - numOfPrimesInFile - 1) + " were added to the file");
        }
    }
}