gistfile1.txt

[ProtoContract]
class Foo
{
    public JToken? Bar { get; set; }

    [ProtoMember(1)]
    [SuppressMessage("CodeQuality", "IDE0051:Remove unused private members", Justification = "Used for serialization")]
    private string? BarSerialized
    {
        get => Bar?.ToString();
        set => Bar = value is null ? null : JToken.Parse(value);
    }
}