I made this so I can feed it integers and return an array with all the divisors of that integer. I put checks in in case the integer is less than 2. Order of integers in the array must be smallest to largest. The code works. What I need is to optimize this code to be as fast as possible. Right now on an online IDE I am getting around 40ms combined against the given test. I need to trim this down as much as possible.

using System.Collections.Generic;

public class Divisors
{
    public static bool IsPrime(int n)
    {
        if (n == 2) return true;
        if (n % 2 == 0) return false;

        for (int x = 3; x * x <= n; x += 2)
            if (n % x == 0)
                return false;

        return true;
    }

    public static int[] GetDivisors(int n)
    {
        List<int> divisors = new List<int>();

        if (n < 2)
        {
            return null;
        }
        else if (IsPrime(n))
        {
            return null;
        }
        else
        {
            for (int i = 2; i < n; i++)
                if (n % i == 0)
                    divisors.Add(i);
        }

        return divisors.ToArray();
    }
}

namespace Solution 
{
  using NUnit.Framework;
  using System;

  [TestFixture]
  public class SolutionTest
  {
    [Test]
    public void SampleTest()
    {
      Assert.AreEqual(new int[] {3, 5}, Divisors.Divisors(15));
      Assert.AreEqual(new int[] {2, 4, 8}, Divisors.Divisors(16));
      Assert.AreEqual(new int[] {11, 23}, Divisors.Divisors(253));
      Assert.AreEqual(new int[] {2, 3, 4, 6, 8, 12}, Divisors.Divisors(24));
    }
  }
}