Can't create huge arrays

Question

Like many other programmers, I went into primes, and as many of them, what I like is the challenge, so I'm not looking for comment like Atkin did this faster than you dude, but just a solution - or at least an hint - to my issue.

I need to create big arrays (like size > int.MaxValue). So I went to a lot of web pages and found the gcAllowVeryLargeObjects Element one. I thought I was saved, add the following magic to my App.config:

<configuration>
  <runtime>
    <gcAllowVeryLargeObjects enabled="true" />
  </runtime>
</configuration>

But it didn't work. Here's the code I use :

void go(object sender, EventArgs eventArgs)
{
    t.Stop();
    ulong maxprime = 10;
    Stopwatch stopwatch = new Stopwatch();
    string s = String.Empty;
    while (maxprime < ulong.MaxValue)
    {
        stopwatch.Restart();
        richTextBox2.Text += Environment.NewLine + ("Max \t= " + maxprime.ToString("N0"));
        try
        {
            richTextBox2.Text += Environment.NewLine + ("Count \t= " + GetAllPrimesLessThan(maxprime).Count);
            richTextBox2.Text += Environment.NewLine + ("Time \t= " + stopwatch.Elapsed);
            richTextBox2.Text += Environment.NewLine + ("--------------------------------");
            maxprime *= 10;
            richTextBox2.Refresh();
        }
        catch (Exception exception)
        {
            s = exception.Message + "; Allocation size: " + (maxprime + 1).ToString("N0");
            break;
        }
        
    }
    if (!string.IsNullOrEmpty(s))
    {
        richTextBox2.Text += Environment.NewLine + s;
    }
    richTextBox2.Text += Environment.NewLine + ("Done.");
}

private static List<ulong> GetAllPrimesLessThan(ulong maxPrime)
{
    var primes = new List<ulong>() { 2 };
    var maxSquareRoot = Math.Sqrt(maxPrime);
    var eliminated = new bool[maxPrime + 1];

    for (ulong i = 3; i <= maxPrime; i += 2)
    {
        if (!eliminated[i])
        {
            primes.Add(i);
            if (i < maxSquareRoot)
            {
                for (ulong j = i * i; j <= maxPrime; j += 2 * i)
                {
                    eliminated[j] = true;
                }
            }
        }
    }
    return primes;
}

Which output this:

[...]
Max     = 1 000 000 000
Count   = 50847534
Time    = 00:00:15.3355367
--------------------------------
Max     = 10 000 000 000
Array dimensions exceeded supported range.; Allocation size: 10 000 000 001
Done.

How can I get rid of this error?

---

FYI: I've got

• 16GB ram;
• 32GB memory mapped(/paged?) on SSD;
• 64bits enabled

Answer 1

From your link:

Using this element in your application configuration file enables arrays that are larger than 2 GB in size, but does not change other limits on object size or array size:

The maximum index in any single dimension is 2,147,483,591 (0x7FFFFFC7) for byte arrays and arrays of single-byte structures, and 2,146,435,071 (0X7FEFFFFF) for other types.

See also What is the maximum length of an array in .NET on 64-bit Windows:

An array could theoretically have at most 2,147,483,647 elements, since it uses an int for indexing.

Answer 2

If you hit the bounds of the integer max range, you can opt to use a long-index-based array.

The problem is that this isn't supported by the C# indexer properties, which uses int. You can build them by hand though by using Array.CreateInstance(Type, long[]).

Note you have to get the values using Array.GetValue(long).