Quadratic primes test

using System;
using System.Collections.Generic;
using System.Text;
using System.Diagnostics;
using System.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static Primes primes;
        class Primes
        {
            public HashSet<int> all_numbers = new HashSet<int>();
            public HashSet<int> list_of_primes = new HashSet<int>();
            public HashSet<int> list_of_nonprimes = new HashSet<int>();


            public Primes(int n)
            {
                all_numbers = new HashSet<int>(Enumerable.Range(1, n));
                for (int i = 2; i < Math.Sqrt(n) + 1; i++)
                {
                    for (int j = 3; j <= n / i; j++)
                        list_of_nonprimes.Add(i * j);
                }
                list_of_primes = new HashSet<int>(all_numbers.Except(list_of_nonprimes));
            }

        }

        static void Main(string[] args)
        {
            Stopwatch t = new Stopwatch();
            t.Reset();
            t.Start();
            Console.WriteLine(FindMaxCoeff());
            t.Stop();
            Console.WriteLine(t.ElapsedMilliseconds);
            t.Start();
            primes = new Primes(1000);
            Console.WriteLine(FindMaxCoeff2());
            t.Stop();
            Console.WriteLine(t.ElapsedMilliseconds);
            Console.ReadKey();
        }

        static int FindMaxCoeff()
        {
            int maxConsec = 0;
            int maxCoeff = 0;
            for (int a = -999; a < 1000; a++)
            {
                for (int b = -999; b < 1000; b++)
                {
                    int currentConsec = FindMaxConsecutive(a, b);
                    if (currentConsec > maxConsec)
                    {
                        maxConsec = currentConsec;
                        maxCoeff = a * b;
                    }
                }
            }

            return maxCoeff;
        }

        static int FindMaxConsecutive(int a, int b)
        {
            int n = 0;

            for (n = 0; IsPrime(n * n + a * n + b); n++) ;

            return n;
        }

        static bool IsPrime(int n)
        {
            n = Math.Abs(n);
            if (n == 1 || n == 2 || n == 3)
            {
                return true;
            }

            if (n % 2 == 0 || n % 3 == 0)
            {
                return false;
            }

            for (int x = 6; x - 1 <= Math.Sqrt(n); x += 6)
            {
                if (n % (x - 1) == 0 || n % (x + 1) == 0)
                {
                    return false;
                }
            }

            return true;
        }

        static int FindMaxCoeff2()
        {
            int maxConsec = 0;
            int maxCoeff = 0;
            for (int a = -999; a < 1000; a++)
            {
                for (int b = -999; b < 1000; b++)
                {
                    int currentConsec = FindMaxConsecutive2(a, b);
                    if (currentConsec > maxConsec)
                    {
                        maxConsec = currentConsec;
                        maxCoeff = a * b;
                    }
                }
            }

            return maxCoeff;
        }

        static int FindMaxConsecutive2(int a, int b)
        {
            int n = 0;

            for (n = 0; primes.list_of_primes.Contains(n * n + a * n + b); n++) ;

            return n;
        }

    }
}