.net 为什么具有可为空值的 struct 的 HashSet 非常慢

我调查了性能下降情况，并将其追踪到速度较慢的HashSets.
我有一些 struct ，它们的值可以为空，用作主键.例如:

public struct NullableLongWrapper
{
    private readonly long? _value;

    public NullableLongWrapper(long? value)
    {
        _value = value;
    }
}

我注意到创建HashSet<NullableLongWrapper>的速度非常慢.

下面是一个使用BenchmarkDotNet:(Install-Package BenchmarkDotNet)的例子

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

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

public class Config : ManualConfig
{
    public Config()
    {
        Add(Job.Dry.WithWarmupCount(1).WithLaunchCount(3).WithTargetCount(20));
    }
}

public struct NullableLongWrapper
{
    private readonly long? _value;

    public NullableLongWrapper(long? value)
    {
        _value = value;
    }

    public long? Value => _value;
}

public struct LongWrapper
{
    private readonly long _value;

    public LongWrapper(long value)
    {
        _value = value;
    }

    public long Value => _value;
}

[Config(typeof (Config))]
public class HashSets
{
    private const int ListSize = 1000;

    private readonly List<long?> _nullables;
    private readonly List<long> _longs;
    private readonly List<NullableLongWrapper> _nullableWrappers;
    private readonly List<LongWrapper> _wrappers;

    public HashSets()
    {
        _nullables = Enumerable.Range(1, ListSize).Select(i => (long?) i).ToList();
        _longs = Enumerable.Range(1, ListSize).Select(i => (long) i).ToList();
        _nullableWrappers = Enumerable.Range(1, ListSize).Select(i => new NullableLongWrapper(i)).ToList();
        _wrappers = Enumerable.Range(1, ListSize).Select(i => new LongWrapper(i)).ToList();
    }

    [Benchmark]
    public void Longs() => new HashSet<long>(_longs);

    [Benchmark]
    public void NullableLongs() => new HashSet<long?>(_nullables);

    [Benchmark(Baseline = true)]
    public void Wrappers() => new HashSet<LongWrapper>(_wrappers);

    [Benchmark]
    public void NullableWrappers() => new HashSet<NullableLongWrapper>(_nullableWrappers);
}

结果:

           Method |          Median |   Scaled
----------------- |---------------- |---------
            Longs |      22.8682 us |     0.42
    NullableLongs |      39.0337 us |     0.62
         Wrappers |      62.8877 us |     1.00
 NullableWrappers | 231,993.7278 us | 3,540.34

使用带Nullable<long>的 struct 比使用带long的 struct 慢3540倍

以下是来自BenchmarkDotNet的环境信息:

操作系统=Microsoft Windows NT 6.1.7601 Service Pack 1
处理器=英特尔(R)酷睿(TM)i7-5600U CPU 2.60 GHz，ProcessorCount=4
频率=2536269刻度，分辨率=394.2799 ns，计时器=Tsc
clr=ms.net 4.0.30319.42000，Arch=64位版本[RyuJIT]
GC=并发 workstation
JitModules=clrjit-v4.6.1076.0

性能如此差的原因是什么？