Mention how non-public fields of inherited classes are serialized

[adamsitnik]

Repro:

using System.Formats.Nrbf;
using System.Runtime.Serialization.Formatters.Binary;

namespace NrbfBestPractices
{
    internal class Program
    {
        static void Main(string[] args)
        {
            using MemoryStream payload = new();
#pragma warning disable SYSLIB0011 // Type or member is obsolete
            BinaryFormatter formatter = new();
#pragma warning restore SYSLIB0011 // Type or member is obsolete
            formatter.Serialize(payload, new Derived(42)
            {
                Text = "Hello, World!",
            });
            payload.Position = 0;

            ClassRecord rootRecord = NrbfDecoder.DecodeClassRecord(payload);
            Derived decoded = new(rootRecord.GetInt32("Base+integer")) // HERE
            {
                Text = rootRecord.GetString(nameof(Derived.Text)),
            };
            Console.WriteLine($"Integer: {decoded.Integer}, Text: {decoded.Text}");
        }
    }

    [Serializable]
    public class Base
    {
        private int integer;

        public int Integer => integer;

        public Base(int _)
        {
            integer = _;
        }
    }

    [Serializable]
    public class Derived : Base
    {
        public string Text;
        public Derived(int integer) : base(integer)
        {
        }
    }
}

Source of confusion:

https://github.com/dotnet/runtime/blob/513ff1acc981118e4b981e965dce614c3b8770f5/src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/FormatterServices.cs#L42-L44

https://github.com/dotnet/runtime/blob/513ff1acc981118e4b981e965dce614c3b8770f5/src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/FormatterServices.cs#L68

https://github.com/dotnet/runtime/blob/513ff1acc981118e4b981e965dce614c3b8770f5/src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/SerializationFieldInfo.cs#L21

---

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docs/standard/serialization/binaryformatter-migration-guide/functionality-reference.md	BinaryFormatter functionality reference

[GrabYourPitchforks]

Feels like this deserves a code sample showing how things work.

For example:

[Serializable]
public class BaseClass {
    public int PublicIntField;
    private int NonPublicIntField;

    public BaseClass(int publicIntFieldValue, nonPublicIntFieldValue) {
        PublicIntField = publicIntFieldValue;
        NonPublicIntField = nonPublicIntFieldValue;
    }
}

[Serializable]
public class DerivedClass : BaseClass {
    public string PublicStringField;
    private string NonPublicStringField;

    public DerivedClass(int publicIntFieldValue, int nonPublicIntFieldValue)
        : base(publicIntFieldValue, nonPublicIntFieldValue) {
        PublicStringField = PublicIntField.ToString();
        NonPublicStringField = NonPublicIntField.ToString();
    }
}

Then show an example of serializing both a BaseClass and a DerivedClass, including what values are contained within each payload.