Net48Tester - standalone program to reproduce https://github.com/dotnet/coreclr/issues/26990

Net48Tester.sln

Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio Version 16
VisualStudioVersion = 16.0.28803.156
MinimumVisualStudioVersion = 10.0.40219.1
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Tester", "Tester/Tester.csproj", "{84C8A065-2217-4B26-B3F4-2E74743B7BD4}"
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		{84C8A065-2217-4B26-B3F4-2E74743B7BD4}.Release|x64.Build.0 = Release|x64
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Program.cs

using System;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;

namespace Net48Tester
{
    internal class Program
    {
        public static int Main(string[] args)
        {
            try
            {
                using (var current = Process.GetCurrentProcess())
                {
                    if (args.Contains("--child"))
                    {
                        Child(current);
                    }
                    else
                    {
                        Launcher(current);
                    }
                }

                return 0;
            }
            catch (Exception exception)
            {
                var color = Console.ForegroundColor;

                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("ERROR: " + exception);
                Console.ForegroundColor = color;

                return 1;
            }
        }

        /// <summary>
        /// Launcher process: set CPU affinity to 1 core, then run sub-processes in a loop.
        /// </summary>
        private static void Launcher(Process current)
        {
            // Environment and runtime information.
            var system = typeof(int).Assembly;
            Console.WriteLine("Framework: Location: " + system.Location);
            Console.WriteLine("Framework: ImageRuntimeVersion: " + system.ImageRuntimeVersion);
            Console.WriteLine("Framework: full version: " + system.GetName().Version);
            FileVersionInfo myFileVersionInfo = FileVersionInfo.GetVersionInfo(Assembly.GetAssembly(typeof(int)).GetFiles()[0].Name);
            Console.WriteLine("Framework: ProductVersion: " + myFileVersionInfo.ProductVersion);

            foreach (var module in current.Modules.Cast<ProcessModule>().Where(m => m.ModuleName.Contains("jit")))
            {
                Console.WriteLine("JIT Compiler: '{0}' {1}", module.ModuleName, module.FileVersionInfo.FileVersion);
            }

            // Change affinity mask.
            Console.WriteLine($"Environment.ProcessorCount: {Environment.ProcessorCount}");
            Console.WriteLine($"CPU affinity: {current.ProcessorAffinity.ToInt64():X16}");
            current.ProcessorAffinity = new IntPtr(0x0000_0000_0000_0002);
            Console.WriteLine($"New affinity: {current.ProcessorAffinity.ToInt64():X16}");
            Console.WriteLine();

            // Launch sub-processes.
            for (var i = 0; i != 50; ++i)
            {
                var color = Console.ForegroundColor;

                Console.ForegroundColor = ConsoleColor.Cyan;
                Console.WriteLine("Launching subprocess...");
                Console.ForegroundColor = color;

                using (var process = new Process
                {
                    StartInfo =
                    {
#if NETCOREAPP2_1
                        FileName = "dotnet",
                        Arguments = "Net48Tester.dll --child",
#else
                        FileName = "Net48Tester.exe",
                        Arguments = "--child",
#endif
                        CreateNoWindow = true,
                        RedirectStandardOutput = true,
                        RedirectStandardError = true,
                        UseShellExecute = false
                    }
                })
                {
                    process.OutputDataReceived += ProcessOnOutputDataReceived;
                    process.ErrorDataReceived += ProcessOnOutputDataReceived;

                    process.Start();
                    process.BeginOutputReadLine();
                    process.BeginErrorReadLine();
                    process.WaitForExit();
                }

                Console.WriteLine();
            }

            Console.WriteLine("Launcher completed");
        }

        private static void ProcessOnOutputDataReceived(object sender, DataReceivedEventArgs dataReceivedEventArgs)
        {
            if (dataReceivedEventArgs.Data != null)
            {
                Console.WriteLine(dataReceivedEventArgs.Data);
            }
        }

        /// <summary>
        /// Child process: set CPU affinity to N cores (from 1 core) and launch some Tasks.
        /// </summary>
        /// <param name="current"></param>
        private static void Child(Process current)
        {
            Console.WriteLine($"CPU affinity: {current.ProcessorAffinity.ToInt64():X16}");
            current.ProcessorAffinity = new IntPtr(0x0000_0000_0000_0FFF);
            Console.WriteLine($"New affinity: {current.ProcessorAffinity.ToInt64():X16}");

            Parallel.For(0, 10, i => Thread.Sleep(500));

            Console.WriteLine("Child completed.");
        }
    }
}

Tester.csproj

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

  <PropertyGroup>
    <TargetFrameworks>net48;netcoreapp3.0;netcoreapp2.1</TargetFrameworks>
    <LangVersion>7.3</LangVersion>
    <AssemblyName>Net48Tester</AssemblyName>
    <RootNamespace>Net48Tester</RootNamespace>
    <Platforms>x64</Platforms>
    <PlatformTarget>x64</PlatformTarget>
    <OutputType>Exe</OutputType>
  </PropertyGroup>

</Project>