Updated Dictionary Benchmarks

Program.cs

using System;
using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Collections.ObjectModel;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Engines;
using BenchmarkDotNet.Jobs;
using BenchmarkDotNet.Running;

namespace Bench_Dictionaries
{
    // See https://stackoverflow.com/questions/16612936/immutable-dictionary-vs-dictionary-vs-c5
    // for original source of these benchmarks.

    class Program
    {
        static void Main(string[] args)
        {
            BenchmarkRunner.Run<Bench>();
        }
    }

    [SimpleJob(RuntimeMoniker.Net60)]
    [SimpleJob(RuntimeMoniker.NetCoreApp31)]
    public class Bench
    {
        private const int RandSeed = 826528;
        private const int BenchmarkAccessCount = 1000;

        [Params(100, 1000, 10000, 100000)]
        public int MaxItems { get; set; }

        private ReadOnlyDictionary<string, object> _dictionary = null!;
        private ReadOnlyDictionary<string, object> _concurrentDictionary = null!;
        private ImmutableDictionary<string, object> _immutableDictionary = null!;
        private string[] _randomKeys = null!;
        
        
        [GlobalSetup]
        public void GlobalSetup()
        {
            var keyValuePairs = Enumerable.Range(0, MaxItems)
                .Select(i => new KeyValuePair<string, object>(i.ToString(), i))
                .ToList();

            // Init Dictionaries
            _immutableDictionary = keyValuePairs.ToImmutableDictionary();
            
            // ImmutableDictionary.CreateRange<string, object>();

            _dictionary = new ReadOnlyDictionary<string, object>(
                new Dictionary<string, object>(keyValuePairs));

            _concurrentDictionary = new ReadOnlyDictionary<string, object>(
                new ConcurrentDictionary<string, object>(keyValuePairs));

            // Init keys to access in benchmarks
            var r = new Random(RandSeed);
            _randomKeys = Enumerable.Range(0, BenchmarkAccessCount)
                .Select(i => r.Next(MaxItems).ToString())
                .ToArray();
        }

        [Benchmark]
        public void BaseDictionary()
        {
            foreach (var key in _randomKeys) {
                _dictionary.TryGetValue(key, out var value);
            }
        }

        [Benchmark]
        public void ConcurrentDictionary()
        {
            foreach (var key in _randomKeys) {
                _concurrentDictionary.TryGetValue(key, out var value);
            }
        }

        [Benchmark]
        public void ImmutableDictionaryT()
        {
            foreach (var key in _randomKeys) {
                _immutableDictionary.TryGetValue(key, out var value);
            }
        }

    }
}

Results.md

See Charts

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19042.1466 (20H2/October2020Update)
AMD Ryzen Threadripper 2920X, 1 CPU, 24 logical and 12 physical cores
.NET SDK=6.0.100
  [Host]        : .NET Core 3.1.10 (CoreCLR 4.700.20.51601, CoreFX 4.700.20.51901), X64 RyuJIT
  .NET 6.0      : .NET 6.0.0 (6.0.21.52210), X64 RyuJIT
  .NET Core 3.1 : .NET Core 3.1.10 (CoreCLR 4.700.20.51601, CoreFX 4.700.20.51901), X64 RyuJIT

Method	Job	Runtime	MaxItems	Mean	Error	StdDev
BaseDictionary	.NET 6.0	.NET 6.0	100	28.17 us	0.319 us	0.283 us
ConcurrentDictionary	.NET 6.0	.NET 6.0	100	19.56 us	0.289 us	0.270 us
ImmutableDictionaryT	.NET 6.0	.NET 6.0	100	49.82 us	0.115 us	0.090 us
BaseDictionary	.NET Core 3.1	.NET Core 3.1	100	36.88 us	0.135 us	0.120 us
ConcurrentDictionary	.NET Core 3.1	.NET Core 3.1	100	25.57 us	0.222 us	0.185 us
ImmutableDictionaryT	.NET Core 3.1	.NET Core 3.1	100	58.79 us	0.249 us	0.233 us
BaseDictionary	.NET 6.0	.NET 6.0	1000	24.44 us	0.481 us	0.625 us
ConcurrentDictionary	.NET 6.0	.NET 6.0	1000	26.47 us	0.489 us	0.458 us
ImmutableDictionaryT	.NET 6.0	.NET 6.0	1000	66.43 us	0.248 us	0.194 us
BaseDictionary	.NET Core 3.1	.NET Core 3.1	1000	32.54 us	0.181 us	0.169 us
ConcurrentDictionary	.NET Core 3.1	.NET Core 3.1	1000	31.40 us	0.243 us	0.190 us
ImmutableDictionaryT	.NET Core 3.1	.NET Core 3.1	1000	77.94 us	0.431 us	0.404 us
BaseDictionary	.NET 6.0	.NET 6.0	10000	30.68 us	0.595 us	0.685 us
ConcurrentDictionary	.NET 6.0	.NET 6.0	10000	26.26 us	0.256 us	0.239 us
ImmutableDictionaryT	.NET 6.0	.NET 6.0	10000	100.70 us	0.724 us	0.604 us
BaseDictionary	.NET Core 3.1	.NET Core 3.1	10000	35.45 us	0.205 us	0.181 us
ConcurrentDictionary	.NET Core 3.1	.NET Core 3.1	10000	36.78 us	0.672 us	0.628 us
ImmutableDictionaryT	.NET Core 3.1	.NET Core 3.1	10000	106.51 us	0.708 us	0.628 us
BaseDictionary	.NET 6.0	.NET 6.0	100000	36.74 us	0.253 us	0.224 us
ConcurrentDictionary	.NET 6.0	.NET 6.0	100000	49.94 us	0.352 us	0.329 us
ImmutableDictionaryT	.NET 6.0	.NET 6.0	100000	161.21 us	0.714 us	0.633 us
BaseDictionary	.NET Core 3.1	.NET Core 3.1	100000	42.55 us	0.458 us	0.428 us
ConcurrentDictionary	.NET Core 3.1	.NET Core 3.1	100000	54.18 us	0.371 us	0.310 us
ImmutableDictionaryT	.NET Core 3.1	.NET Core 3.1	100000	176.20 us	2.040 us	1.808 us