ArrayAccessBenchmark50.cs

namespace ArrayAccessBenchmark50
{
    using System.Runtime.CompilerServices;
    using System.Runtime.InteropServices;

    using BenchmarkDotNet.Attributes;
    using BenchmarkDotNet.Configs;
    using BenchmarkDotNet.Diagnosers;
    using BenchmarkDotNet.Exporters;
    using BenchmarkDotNet.Jobs;
    using BenchmarkDotNet.Running;

    public static class Program
    {
        public static void Main()
        {
            BenchmarkRunner.Run<Benchmark>();
        }
    }

    public class BenchmarkConfig : ManualConfig
    {
        public BenchmarkConfig()
        {
            AddExporter(MarkdownExporter.Default, MarkdownExporter.GitHub);
            AddDiagnoser(MemoryDiagnoser.Default);
            AddJob(Job.MediumRun);
        }
    }

    [Config(typeof(BenchmarkConfig))]
    public class Benchmark
    {
        private int[] array = new int[16];

        [Benchmark]
        public int Single() => array[0];

        [Benchmark]
        public int SingleByHelper() => ArrayHelper.GetItemReference(array, 0);

        [Benchmark]
        public int Loop()
        {
            var total = 0;
            for (var i = 0; i < array.Length; i++)
            {
                total += array[i];
            }
            return total;
        }

        [Benchmark]
        public int LoopWithCheck()
        {
            var total = 0;
            for (var i = 0; i < 16; i++)
            {
                total += array[i];
            }
            return total;
        }

        [Benchmark]
        public int LoopByHelper()
        {
            var total = 0;
            for (var i = 0; i < 16; i++)
            {
                total += ArrayHelper.GetItemReference(array, i);
            }
            return total;
        }

        [Benchmark]
        public unsafe int LoopByPointer()
        {
            var total = 0;
            fixed (int* ptr = &array[0])
            {
                for (var i = 0; i < 16; i++)
                {
                    total += *(ptr + i);
                }
            }
            return total;
        }
    }

    public static class ArrayHelper
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ref T GetItemReference<T>(T[] array, int index)
        {
            ref var data = ref MemoryMarshal.GetArrayDataReference(array);
            return ref Unsafe.Add(ref data, index);
        }
    }
}

Result.md

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19041.508 (2004/?/20H1)
Intel Core i5-9500 CPU 3.00GHz, 1 CPU, 6 logical and 6 physical cores
.NET Core SDK=5.0.100-rc.1.20452.10
  [Host]    : .NET Core 5.0.0 (CoreCLR 5.0.20.45114, CoreFX 5.0.20.45114), X64 RyuJIT
  MediumRun : .NET Core 5.0.0 (CoreCLR 5.0.20.45114, CoreFX 5.0.20.45114), X64 RyuJIT

Job=MediumRun  IterationCount=15  LaunchCount=2  
WarmupCount=10  

Method	Mean	Error	StdDev	Median	Gen 0	Gen 1	Gen 2	Allocated
Single	0.1809 ns	0.0080 ns	0.0115 ns	0.1774 ns	-	-	-	-
SingleByHelper	0.0000 ns	0.0000 ns	0.0000 ns	0.0000 ns	-	-	-	-
Loop	7.3754 ns	0.0186 ns	0.0272 ns	7.3710 ns	-	-	-	-
LoopWithCheck	14.1764 ns	0.1524 ns	0.2282 ns	14.0926 ns	-	-	-	-
LoopByHelper	10.6515 ns	0.0229 ns	0.0322 ns	10.6445 ns	-	-	-	-
LoopByPointer	6.4811 ns	1.0807 ns	1.4792 ns	7.7960 ns	-	-	-	-