Give Name to Array of Arrays

Question

I am using Newtonsoft to parse some JSon into a .Net type. The json contains an array of arrays called 'data'. I would like to make each array within the data array it's own type, but am unsure how to do this.

Hopefully the code below demonstrates this.

    public class TheData
    {
        [JsonProperty(PropertyName = "data")]
        public List<object> dataItems { get; set; }
    }

Usage:

        string json =
            "{\"data\":[[\"20180511\",1094391],[\"20180504\",1097315],[\"20180427\",1100221],[\"20180420\",1094455],[\"20180413\",1093023]]}";

        var myObj = JsonConvert.DeserializeObject<TheData>(json);

This works ok, however, I would like to change the type of dataItems from List to List as below:

    public class TheData
    {
        [JsonProperty(PropertyName = "data")]
        public List<DataItem> dataItems { get; set; }
    }

public class DataItem
{
    public string deldate { get; set; }
    public int value { get; set; }
}

However, this results in an exception:

Newtonsoft.Json.JsonSerializationException occurred
  HResult=-2146233088
  Message=Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'CE.FOTools.Feeds2.EIA.DataItem' because the type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly.
To fix this error either change the JSON to a JSON object (e.g. {"name":"value"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List<T> that can be deserialized from a JSON array. JsonArrayAttribute can also be added to the type to force it to deserialize from a JSON array.
Path 'data[0]', line 1, position 10.

The error message suggests my desired outcome may not be possible, but can anyone suggest how to correct this? I have no control over the JSON format (unles I operate on the string once it is retrieved). I'm using .Net 4.5 if that makes any difference.

Evk · Accepted Answer · 2018-05-22 11:13:21Z

2

I think the least obtrusive way is to use custom converter. For example:

class DataItemConverter : JsonConverter<DataItem> {
    public override void WriteJson(JsonWriter writer, DataItem value, JsonSerializer serializer) {
        // if item can be null - handle that
        writer.WriteStartArray();
        writer.WriteValue(value.deldate);
        writer.WriteValue(value.value);
        writer.WriteEndArray();
    }

    public override DataItem ReadJson(JsonReader reader, Type objectType, DataItem existingValue, bool hasExistingValue, JsonSerializer serializer) {
        var ar = serializer.Deserialize<List<object>>(reader);
        // perform some checks for length and data types, omitted here
        var result = new DataItem();
        result.deldate = (string) ar[0];
        result.value = Convert.ToInt32(ar[1]);
        return result;
    }
}

Then specify that this converted should be used by decorating your type:

[JsonConverter(typeof(DataItemConverter))]
public class DataItem {
    public string deldate { get; set; }
    public int value { get; set; }
}

And after that it should work as you expect.

If generic JsonConverter<> is not available in your Json.NET version - use non-generic one:

class DataItemConverter : JsonConverter {
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {
        var item = (DataItem) value;
        // if null is possible - handle that
        writer.WriteStartArray();
        if (item != null) {
            writer.WriteValue(item.deldate);
            writer.WriteValue(item.value);
        }

        writer.WriteEndArray();
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {
        var ar = serializer.Deserialize<List<object>>(reader);
        // perform some checks for length and data types, omitted here
        var result = new DataItem();
        result.deldate = (string) ar[0];
        result.value = Convert.ToInt32(ar[1]);
        return result;
    }

    public override bool CanConvert(Type objectType) {
        return objectType == typeof(DataItem);
    }
}

edited May 22, 2018 at 11:13

answered May 22, 2018 at 11:00

Evk

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2 Comments

GinjaNinja Over a year ago

Many thanks for your answer. I think I may be using an older version of Newtonsoft as this does not compile. I will spend a bit of time upgrading and report back.

Evk Over a year ago

@GinjaNinja I've updated answer with version which should work in older versions.

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