Mapping flat JSON/Dictionary to model (containing sub classes)

You can make a JsonConverter that does this in a generic way, using a ContractResolver to group and populate properties in the class being deserialized or its contained classes as appropriate.

You didn't ask for serialization, only deserialization, so that's what this does:

public class JsonFlatteningConverter : JsonConverter{    readonly IContractResolver resolver;    public JsonFlatteningConverter(IContractResolver resolver)    {        if (resolver == null)            throw new ArgumentNullException();        this.resolver = resolver;    }    public override bool CanConvert(Type objectType)    {        return resolver.ResolveContract(objectType) is JsonObjectContract;    }    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)    {        if (reader.TokenType == JsonToken.Null)            return null;        JObject jObject = JObject.Load(reader);        var contract = (JsonObjectContract)resolver.ResolveContract(objectType); // Throw an InvalidCastException if this object does not map to a JObject.        existingValue = existingValue ?? contract.DefaultCreator();        if (jObject.Count == 0)            return existingValue;        var groups = jObject.Properties().GroupBy(p => p.Name.Contains('.') ? p.Name.Split('.').FirstOrDefault() : null).ToArray();        foreach (var group in groups)        {            if (string.IsNullOrEmpty(group.Key))            {                var subObj = new JObject(group);                using (var subReader = subObj.CreateReader())                    serializer.Populate(subReader, existingValue);            }            else            {                var jsonProperty = contract.Properties[group.Key];                if (jsonProperty == null || !jsonProperty.Writable)                    continue;                if (jsonProperty != null)                {                    var subObj = new JObject(group.Select(p => new JProperty(p.Name.Substring(group.Key.Length + 1), p.Value)));                    using (var subReader = subObj.CreateReader())                    {                        var propertyValue = serializer.Deserialize(subReader, jsonProperty.PropertyType);                        jsonProperty.ValueProvider.SetValue(existingValue, propertyValue);                    }                }            }        }        return existingValue;    }    public override bool CanWrite { get { return false; } }    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)    {        throw new NotImplementedException();    }}

And then use it thusly:

        var resolver = new DefaultContractResolver();        var settings = new JsonSerializerSettings { ContractResolver = resolver, Converters = new JsonConverter[] { new JsonFlatteningConverter(resolver) } };        var person = JsonConvert.DeserializeObject<Person>(json, settings);

Prototype fiddle.