Small ImmutableArray vs Hashset

Benchmark Results

|  Method | DesiredContainer | NumFetches |           Mean |         Error |        StdDev |
|-------- |----------------- |----------- |---------------:|--------------:|--------------:|
| HashSet |                1 |          1 |      26.149 ns |     0.4213 ns |     0.3941 ns |
|   Array |                1 |          1 |       3.068 ns |     0.0386 ns |     0.0361 ns |
| HashSet |                1 |        100 |   2,374.146 ns |     4.8138 ns |     4.5028 ns |
|   Array |                1 |        100 |     275.267 ns |     1.3763 ns |     1.2201 ns |
| HashSet |                1 |      10000 | 243,664.632 ns |   341.7355 ns |   319.6596 ns |
|   Array |                1 |      10000 |  27,245.051 ns |   143.4551 ns |   134.1880 ns |
| HashSet |                2 |          1 |      25.953 ns |     0.0330 ns |     0.0275 ns |
|   Array |                2 |          1 |       7.641 ns |     0.0147 ns |     0.0123 ns |
| HashSet |                2 |        100 |   2,315.760 ns |     2.9523 ns |     2.4653 ns |
|   Array |                2 |        100 |     801.805 ns |     2.4706 ns |     2.3110 ns |
| HashSet |                2 |      10000 | 232,675.796 ns | 2,138.9253 ns | 2,000.7521 ns |
|   Array |                2 |      10000 |  82,425.552 ns |   253.1002 ns |   211.3501 ns |
| HashSet |                3 |          1 |      23.327 ns |     0.1635 ns |     0.1450 ns |
|   Array |                3 |          1 |      12.608 ns |     0.0739 ns |     0.0655 ns |
| HashSet |                3 |        100 |   2,565.776 ns |    45.3442 ns |    79.4167 ns |
|   Array |                3 |        100 |   1,246.539 ns |    20.6464 ns |    21.2023 ns |
| HashSet |                3 |      10000 | 236,579.193 ns | 1,071.8413 ns | 1,002.6010 ns |
|   Array |                3 |      10000 | 125,318.257 ns | 1,422.8662 ns | 1,330.9500 ns |

Benchmark.cs

using System.Collections.Generic;
using System.Collections.Immutable;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace MyBenchmarks
{
    public class Container
    {
        public string Str { get; }
        public Container(string s) => Str = s;
    }

    public class ImmutableArrayVsHashSet
    {
        private static KeyValuePair<string, Container> kvp(string s) => new KeyValuePair<string, Container>(s, new Container(s));
        private ImmutableDictionary<string, Container> _containerHashSet = ImmutableDictionary.CreateRange(new[] { kvp("1"), kvp("2"), kvp("3") });
        private ImmutableArray<Container> _containerArray = ImmutableArray.CreateRange(new[] { new Container("1"), new Container("2"), new Container("3") });

        [Params("1", "2", "3")]
        public string DesiredContainer;

        [Params(1, 100, 10000)]
        public int NumFetches;

        [Benchmark]
        public Container HashSet()
        {
            Container c = null;
            for (int i = 0; i < NumFetches; i++)
            {
                c = _containerHashSet[DesiredContainer];
            }

            return c;
        }

        [Benchmark]
        public Container Array()
        {
            Container c = null;
            for (int i = 0; i < NumFetches; i++)
            {
                foreach (Container v in _containerArray)
                {
                    if (v.Str == DesiredContainer)
                    {
                        c = v;
                        break;
                    }
                }
            }

            return c;
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            var summary = BenchmarkRunner.Run<ImmutableArrayVsHashSet>();
        }
    }
}