Benchmarking OfType() vs. custom WhereNotNull() to discard null values

Benchmark.cs

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

public static class EnumerableExtensions
{
    static readonly Func<object, bool> notNullTest = x => x is not null;

    public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> source)
        where T : class =>
            source.Where((Func<T?, bool>)notNullTest)!;

    public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> source)
        where T : struct =>
            source.Where(x => x.HasValue).Select(x => x!.Value);
}

public class BenchmarkClass
{
    const int dataLength = 100_000;
    int?[] intData = new int?[dataLength];
    string?[] stringData = new string?[dataLength];

    public BenchmarkClass()
    {
        var random = new Random();

        for (int idx = 0; idx < dataLength; ++idx)
        {
            var value = random.Next(100);
            if (value != 0)
            {
                intData[idx] = value;
                stringData[idx] = value.ToString();
            }
        }
    }

    [Benchmark]
    public void Integer_OfType() => intData.OfType<int>().Consume(new Consumer());

    [Benchmark]
    public void Integer_WhereNotNull() => intData.WhereNotNull().Consume(new Consumer());

    [Benchmark]
    public void String_OfType() => stringData.OfType<string>().Consume(new Consumer());

    [Benchmark]
    public void String_WhereNotNull() => stringData.WhereNotNull().Consume(new Consumer());
}

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

Benchmark.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="BenchmarkDotNet" Version="0.13.12" />
  </ItemGroup>

</Project>

BenchmarkResults.md

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3007/23H2/2023Update/SunValley3)
AMD Ryzen 9 5900X, 1 CPU, 24 logical and 12 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

Method	Mean	Error	StdDev
Integer_OfType	3,809.1 us	13.63 us	12.08 us
Integer_WhereNotNull	141.0 us	1.43 us	1.34 us
String_OfType	808.7 us	10.92 us	10.22 us
String_WhereNotNull	472.3 us	0.75 us	0.59 us