Use Dictionary instead of Hashtable in EventMap

[stephentoub]

Description

It's currently using a Hashtable with Int32 keys, which means every operation on the Hashtable boxes the int. Switching to a Dictionary removes those costs. It also lets use use TryGetValue to perform a single lookup to get a value rather than first calling ContainsKey and then using the indexer, and avoiding multiple virtual calls (most APIs on Hashtable are virtual) when we're only using the base class.

Customer Impact

Less unnecessary boxing means less pauses due to GC.

Regression

No

Testing

Just CI

Risk

Minimal. In general we prefer Dictionary over Hashtable, and the only places we're generally keeping Hashtable is when we benefit from its thread-safety guarantees, but all accesses here are protected by a lock.

You can see some of the associated costs with a microbenchmark like this:

Method	Count	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
WithHashtable	1	326.98 ns	0.813 ns	0.635 ns	0.0114	-	-	72 B
WithDictionary	1	21.29 ns	0.108 ns	0.096 ns	-	-	-	-
WithHashtable	3	456.96 ns	4.039 ns	3.778 ns	0.0343	-	-	216 B
WithDictionary	3	49.60 ns	0.220 ns	0.195 ns	-	-	-	-
WithHashtable	5	573.64 ns	1.306 ns	1.091 ns	0.0572	-	-	360 B
WithDictionary	5	71.76 ns	0.280 ns	0.248 ns	-	-	-	-
WithHashtable	7	735.64 ns	7.836 ns	6.946 ns	0.0801	-	-	504 B
WithDictionary	7	91.98 ns	0.594 ns	0.527 ns	-	-	-	-
WithHashtable	9	811.44 ns	6.010 ns	5.622 ns	0.1030	-	-	648 B
WithDictionary	9	117.27 ns	0.813 ns	0.720 ns	-	-	-	-
WithHashtable	20	1,417.64 ns	10.259 ns	9.596 ns	0.2289	-	-	1,440 B
WithDictionary	20	290.11 ns	3.202 ns	2.838 ns	-	-	-	-

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

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    private readonly Hashtable _table = new Hashtable(20, .1f);
    private readonly Dictionary<int, object> _dictionary = new Dictionary<int, object>(20);
    private readonly object[] _objects = Enumerable.Range(0, 20).Select(_ => new object()).ToArray();

    [Params(1, 3, 5, 7, 9, 20)]
    public int Count { get; set; }

    [Benchmark]
    public void WithHashtable()
    {
        for (int i = 0; i < Count; i++)
        {
            _table.Add(i, _objects[i]);
        }
        for (int i = 0; i < Count; i++)
        {
            if (_table.ContainsKey(i))
            {
                object o = _table[i];
            }
        }
        _table.Clear();
    }

    [Benchmark]
    public void WithDictionary()
    {
        for (int i = 0; i < Count; i++)
        {
            _dictionary.Add(i, _objects[i]);
        }
        for (int i = 0; i < Count; i++)
        {
            if (_dictionary.TryGetValue(i, out object o))
            {
            }
        }
        _dictionary.Clear();
    }
}