UseAutomataDictionary.cs

// AutomataDictionaryを使う＆むしろType無視してIValueだけで済ましてしまうパターン(こら
// reader.ReadObjectSegment/ReadArraySegmentがあって、小ブロックを取得できるようにすれば解決しそう（追加します

public class Container
{
    public string Type { get; set; }
    public IValue Value { get; set; }
}


[JsonFormatter(typeof(InterfaceFormatter))]
public interface IValue
{

}

public class InterfaceFormatter : IJsonFormatter<IValue>
{
    static readonly AutomataDictionary automata = new AutomataDictionary();

    static InterfaceFormatter()
    {
        automata.Add("Foo", 0);
        automata.Add("Hoge", 1);
    }

    public IValue Deserialize(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
    {
        if (reader.ReadIsNull()) return null;

        var count = 0;
        IValue result = null;
        while (reader.ReadIsInObject(ref count))
        {
            var propName = reader.ReadPropertyNameSegmentRaw();
            var i = -1;
            automata.TryGetValue(propName, out i);
            switch (i)
            {
                case 0:
                    result = new ValueTypeA { Foo = reader.ReadInt32() };
                    break;
                case 1:
                    result = new ValueTypeB { Hoge = reader.ReadString() };
                    break;
                default:
                    reader.ReadNextBlock();
                    break;
            }
        }
        return result;
    }

    public void Serialize(ref JsonWriter writer, IValue value, IJsonFormatterResolver formatterResolver)
    {
        throw new NotImplementedException();
    }
}

public class ValueTypeA : IValue
{
    public int Foo { get; set; }
}
public class ValueTypeB : IValue
{
    public string Hoge { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var json = @"
    {
        ""Type"": ""TypeA"",
        ""Value"": {
            ""Foo"": 10
        }
    }
    ";
        JsonSerializer.Deserialize<Container>(json).Dump();

        json = @"
    {
        ""Value"": {
            ""Hoge"": ""mogemoge""
        },
        ""Type"": ""TypeB""
    }
    ";
        JsonSerializer.Deserialize<Container>(json).Dump();

        var container = JsonSerializer.Deserialize<Container>(json);
        if (container.Type == "TypeA")
        {
            var value = (ValueTypeA)container.Value;
            value.Foo.Dump();
        }
        else if (container.Type == "TypeB")
        {
            var value = (ValueTypeB)container.Value;
            value.Hoge.Dump();
        }
    }
}

Utf8Json.cs

// dynamic使えばJSON.NET likeですよパターン

[JsonFormatter(typeof(ContainerFormatter))]
public class Container
{
    public string Type { get; set; }
    public IValue Value { get; set; }


    public class ContainerFormatter : IJsonFormatter<Container>
    {
        public Container Deserialize(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
        {
            if (reader.ReadIsNull()) return null;
            var obj = formatterResolver.GetFormatterWithVerify<dynamic>().Deserialize(ref reader, formatterResolver);

            var type = obj["Type"] as string;
            var value = (type == "TypeA") ? new ValueTypeA { Foo = (int)obj["Value"]["Foo"] }
                      : (type == "TypeB") ? new ValueTypeB { Hoge = (string)obj["Value"]["Hoge"] }
                      : (IValue)null;

            return new Container()
            {
                Type = type,
                Value = value
            };
        }

        public void Serialize(ref JsonWriter writer, Container value, IJsonFormatterResolver formatterResolver)
        {
            throw new NotImplementedException();
        }
    }
}



public interface IValue
{

}


public class ValueTypeA : IValue
{
    public int Foo { get; set; }
}
public class ValueTypeB : IValue
{
    public string Hoge { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var json = @"
    {
        ""Type"": ""TypeA"",
        ""Value"": {
            ""Foo"": 10
        }
    }
    ";
        JsonSerializer.Deserialize<Container>(json).Dump();

        json = @"
    {
        ""Value"": {
            ""Hoge"": ""mogemoge""
        },
        ""Type"": ""TypeB""
    }
    ";
        JsonSerializer.Deserialize<Container>(json).Dump();

        var container = JsonSerializer.Deserialize<Container>(json);
        if (container.Type == "TypeA")
        {
            var value = (ValueTypeA)container.Value;
            value.Foo.Dump();
        }
        else if (container.Type == "TypeB")
        {
            var value = (ValueTypeB)container.Value;
            value.Hoge.Dump();
        }
    }
}