https://github.com/dotnet/coreclr/pull/25883

comparer-serialization.csproj

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net472;netcoreapp3.0</TargetFrameworks>
  </PropertyGroup>

</Project>

output.cmd

PS C:\code\tmp\comparer-serialization> .\bin\Debug\netcoreapp3.0\win10-x64\publish\comparer-serialization.exe v
_comparer: System.Collections.Generic.NonRandomizedStringEqualityComparer
Comparer: System.OrdinalCaseSensitiveComparer
PS C:\code\tmp\comparer-serialization> dotnet run -f net472 s
comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> dotnet run -f net472 d
comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> dotnet run -f netcoreapp3.0 d
_comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> .\bin\Debug\netcoreapp3.0\win10-x64\publish\comparer-serialization.exe d
_comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> dotnet run -f netcoreapp3.0 s
_comparer: System.Collections.Generic.NonRandomizedStringEqualityComparer
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> dotnet run -f net472 d
comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> dotnet run -f netcoreapp3.0 d
_comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> .\bin\Debug\netcoreapp3.0\win10-x64\publish\comparer-serialization.exe d
_comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> .\bin\Debug\netcoreapp3.0\win10-x64\publish\comparer-serialization.exe s
_comparer: System.Collections.Generic.NonRandomizedStringEqualityComparer
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> dotnet run -f net472 d
comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> dotnet run -f netcoreapp3.0 d
_comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
PS C:\code\tmp\comparer-serialization> .\bin\Debug\netcoreapp3.0\win10-x64\publish\comparer-serialization.exe d
_comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]
Comparer: System.Collections.Generic.GenericEqualityComparer`1[System.String]

Program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization.Formatters.Binary;
using static System.Reflection.BindingFlags;

class Program
{
    static void Main(string[] args)
    {
        string path = "dict.bin";

        Dictionary<string, string> dict;

        string arg = args.Single();

        if (arg == "v")
        {
            // used to verify we're actually running modified code
            dict = new Dictionary<string, string>(StringComparer.Ordinal);
        }
        else if (arg == "s")
        {
            dict = new Dictionary<string, string>();

            using (var file = File.OpenWrite(path))
            {
                var formatter = new BinaryFormatter();
                formatter.Serialize(file, dict);
            }
        }
        else if (arg == "d")
        {
            using (var file = File.OpenRead(path))
            {
                var formatter = new BinaryFormatter();
                dict = (Dictionary<string, string>)formatter.Deserialize(file);
            }
        }
        else
        {
            throw new Exception();
        }

        string comparerFieldName = "comparer";
#if NETCOREAPP3_0
        comparerFieldName = "_" + comparerFieldName;
#endif

        var comparerFieldValue = dict.GetType().GetField(comparerFieldName, NonPublic | Instance).GetValue(dict);

        Console.WriteLine($"{comparerFieldName}: {comparerFieldValue}");
        Console.WriteLine($"Comparer: {dict.Comparer}");        
    }
}