PrimeSieveCS

using System;
using System.Buffers;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.CompilerServices;

using static System.Int64;

Int32      passes = 0;
PrimeSieve sieve  = default;

Stopwatch watch = Stopwatch.StartNew();

while (watch.ElapsedMilliseconds < 10000)
{
    sieve = new PrimeSieve(1000000);
    sieve.RunSieve();
    passes++;
}

watch.Stop();

sieve.PrintResults(false, watch.Elapsed.TotalSeconds, passes);

public readonly ref struct PrimeSieve
{
    private readonly Int32  size;

    private readonly Span<Int64> bits;
    private readonly Int64[]     data;

    private static readonly Dictionary<Int32, Int32> Factors = new()
    {
        { 10, 1 },
        { 100, 25 },
        { 1000, 168 },
        { 10000, 1229 },
        { 100000, 9592 },
        { 1000000, 78498 },
        { 10000000, 664579 },
        { 100000000, 5761455 }
    };

    public PrimeSieve(Int32 size)
    {
        this.size = size;

        data = ArrayPool<Int64>.Shared.Rent(size >> 6);
        bits = data.AsSpan();
        bits.Fill(MaxValue);
    }

    public Int32 CountPrimes()
    {
        Int32 count = 1;

        for (Int32 index = 3; index <= size; index++)
        {
            if (GetBit(index)) count++;
        }

        return count;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)] private Boolean GetBit(Int32 index) => index % 2 != 0 && (bits[index >> 6] & (1 << (index >> 1))) != 0;
    [MethodImpl(MethodImplOptions.AggressiveInlining)] private void ClearBit(Int32 index) => bits[index >> 6] &= ~(1 << (index >> 1));

    public void RunSieve()
    {
        Int32 factor = 3;
        Int32 q      = (Int32) Math.Sqrt(size);

        while (factor < q)
        {
            for (Int32 num = factor; num <= size; num++)
            {
                if (GetBit(num))
                {
                    factor = num;
                    break;
                }
            }

            Int32 factorTimes2 = factor << 1;

            for (Int32 num = factor + factorTimes2; num <= size; num += factorTimes2) ClearBit(num);

            factor += 2;
        }

        ArrayPool<Int64>.Shared.Return(data);
    }

    public Boolean ValidateResults()
    {
        if (Factors.ContainsKey(size)) return Factors[size] == CountPrimes();

        return false;
    }

    public void PrintResults(Boolean showResults, Double duration, Int32 passes)
    {
        if (showResults) Console.Write("2, ");

        Int32 count = 1;

        for (Int32 num = 3; num <= size; num++)
        {
            if (GetBit(num))
            {
                if (showResults) Console.Write(num + ", ");
                count++;
            }
        }

        if (showResults) Console.WriteLine("");

        Console.WriteLine("Passes: " + passes + ", Time: " + duration + ", Avg: " + (duration/passes) + ", Limit: " + size + ", Count: " + count + ", Valid: " + ValidateResults());
        Console.ReadKey();
    }
}