Cannot serialize type inheriting from Dictionary about orleans

[ReubenBond]

@defilerc I've opened #8993 with a fix. I assume you will need this backported to v7.x

[ReubenBond]

This does seem like a bug to me. Thanks for reporting. I'm investigating.
For now, here are some workarounds:

1. Make the constructor private and add a static method to create instances. This hides the ctor from the serializer.

using Orleans;
using Orleans.Serialization;
using Microsoft.Extensions.DependencyInjection;
using System.Collections.Generic;
using NUnit.Framework.Legacy;

var serviceProvider = new ServiceCollection()
    .AddSerializer()
    .BuildServiceProvider();

var serializer = serviceProvider.GetRequiredService<Serializer>();

var instance = MatchStatistics.Create();
var bytes = serializer.SerializeToArray(instance);
var deserialized = serializer.Deserialize<MatchStatistics>(bytes);

CollectionAssert.AreEquivalent(instance, deserialized);

[GenerateSerializer]
public class MatchStatistics : Dictionary<IncidentType, int[]>
{
    public static MatchStatistics Create() => new();

    private MatchStatistics()
    {
        this[IncidentType.Score] = new[] { 0, 0 };
    }
}

[GenerateSerializer]
public enum IncidentType { Unknown = 0, Score = 1 }

2. Define a custom activator for the MatchStatistics class which avoids the dictionary mutation and add the [UseActivator] attribute to the MatchStatistics class

using Orleans;
using Orleans.Serialization;
using Microsoft.Extensions.DependencyInjection;
using System.Collections.Generic;
using NUnit.Framework.Legacy;
using Orleans.Serialization.Activators;

var serviceProvider = new ServiceCollection()
    .AddSerializer()
    .BuildServiceProvider();

var serializer = serviceProvider.GetRequiredService<Serializer>();

var instance = new MatchStatistics();
var bytes = serializer.SerializeToArray(instance);
var deserialized = serializer.Deserialize<MatchStatistics>(bytes);

CollectionAssert.AreEquivalent(instance, deserialized);

[GenerateSerializer]
[UseActivator]
public class MatchStatistics : Dictionary<IncidentType, int[]>
{
    public MatchStatistics() : this(true)
    {
    }

    internal MatchStatistics(bool addDefaults)
    {
        if (addDefaults)
        {
            this[IncidentType.Score] = new[] { 0, 0 };
        }
    }
}

[RegisterActivator]
public sealed class MatchStatisticsActivator : IActivator<MatchStatistics>
{
    public MatchStatistics Create() => new MatchStatistics(addDefaults: false);
}

[GenerateSerializer]
public enum IncidentType { Unknown = 0, Score = 1 }

Option 1 is less code. Option 2 does not require you to change call sites or break binary compatibility.

[defilerc]

@ReubenBond thx a lot for your prompt reply and fix! If that's possible, that would be great, as we're migrating to Orleans v7.x at the moment and will migrate to .NET8/Orleans 8.X at a later stage.