Deserialize a value serialized in two different ways with Json.Net

Question

I have a class similar to this one:

public class Values
{
    public int BestValue { get; set; }
    public List<string> AllValues { get; set; }
}

This class could be (because of history) be stored either as

"MyFirstValue"  /* if only one value */

or

{ "BestValue" : 0, "AllValues" : ["DefaultValue", "OtherValue"] }

How can I deserialize this automatically using a JsonConverter? (Or any other solution.)

Answer 1

You will need to create a custom JsonConverter for this purpose:

internal class ValuesConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return typeof(Values).IsAssignableFrom(objectType);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var tokenType = reader.SkipComments().TokenType;
        if (tokenType == JsonToken.Null)
            return null;
        var value = existingValue as Values ?? (Values)serializer.ContractResolver.ResolveContract(objectType).DefaultCreator();
        if (tokenType == JsonToken.Date)
        {
            // Avoid annoying bug that converts date strings to local format described in 
            // https://stackoverflow.com/questions/35166060/json-net-get-specific-json-date-value
            value.AllValues = new List<string> { JToken.Load(reader).ToString(Formatting.None).Trim('"') };
        }
        else if (tokenType.IsPrimitive())
        {
            value.AllValues = new List<string> { (string)JToken.Load(reader) };
        }
        else
        {
            serializer.Populate(reader, value);
        }
        return value;
    }

    public override bool CanWrite { get { return false; } }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

public static partial class JsonExtensions
{
    public static JsonReader SkipComments(this JsonReader reader)
    {
        while (reader.TokenType == JsonToken.Comment && reader.Read())
            ;
        return reader;
    }

    public static bool IsPrimitive(this JsonToken tokenType)
    {
        switch (tokenType)
        {
            case JsonToken.Integer:
            case JsonToken.Float:
            case JsonToken.String:
            case JsonToken.Boolean:
            case JsonToken.Undefined:
            case JsonToken.Null:
            case JsonToken.Date:
            case JsonToken.Bytes:
                return true;
            default:
                return false;
        }
    }   
}

Then apply it to your type as follows:

[JsonConverter(typeof(ValuesConverter))]
public class Values
{
    public int BestValue { get; set; }
    public List<string> AllValues { get; set; }
}

Or, add it to JsonSerializerSettings.Converters then modify your settings as shown in JsonSerializerSettings and Asp.Net Core.

Notes -

• By setting CanWrite to false one can use default serialization when writing to JSON.

• When the JSON value is not a string, JsonSerializer.Populate() is used to fill in the contents of Values via default deserialization.

• By using ContractResolver.ResolveContract(objectType).DefaultCreator() to construct the Values object, deserialization of subclasses of Values (if any) can be supported.

Working sample .Net fiddle here.