Deserializing Json array with variable names first using C# Json.NET

Question

I'm getting an irregular JSON array from the Census Bureau's public api. The variable names are all in the first element, and I'm having trouble deserializing it.

http://api.census.gov/data/2014/pep/agesex?get=AGE,POP,SEX&for=us:*&DATE=7

gives me JSON like this:

[["AGE","POP","SEX","DATE","us"],
["0","3948350","0","7","1"],
["1","3962123","0","7","1"],
["2","3957772","0","7","1"],
["3","4005190","0","7","1"],
["4","4003448","0","7","1"],
["5","4004858","0","7","1"],
["6","4134352","0","7","1"],
["7","4154000","0","7","1"]]

I can successfully deserialize this using:

var test1 = JsonConvert.DeserializeObject<String[][]>(jsonStr);

However, I'm trying to deserialize it to a class like this:

public class TestClass
{
    public string AGE { get; set; }
    public string POP { get; set; }
    public string SEX { get; set; }
    public string DATE { get; set; }
    public string us { get; set; }
}

I'm trying to do this:

var test2 = JsonConvert.DeserializeObject<TestClass[]>(jsonStr);

But I'm getting the following exception:

An exception of type 'Newtonsoft.Json.JsonSerializationException' occurred in Newtonsoft.Json.dll but was not handled in user code

Additional information: Cannot create and populate list type TestClass. Path '[0]', line 1, position 2.

Answer 1

There's two parts to this.

First is turning the JSON in to data usable in C#, and the second is turning that data in to nice objects.

Here's a working dotNetFiddle.net example of the following code: https://dotnetfiddle.net/Cr0aRL

Each row in your JSON is made up of an array of strings. So that's an array of an array of strings. In C# that can be written as string[][].

So to turn the JSON in to usable data with JSON.Net you can do:

    var json = "[[\"AGE\",\"POP\",\"SEX\",\"DATE\",\"us\"],[\"0\",\"3948350\",\"0\",\"7\",\"1\"],[\"1\",\"3962123\",\"0\",\"7\",\"1\"],[\"2\",\"3957772\",\"0\",\"7\",\"1\"],[\"3\",\"4005190\",\"0\",\"7\",\"1\"],[\"4\",\"4003448\",\"0\",\"7\",\"1\"],[\"5\",\"4004858\",\"0\",\"7\",\"1\"],[\"6\",\"4134352\",\"0\",\"7\",\"1\"],[\"7\",\"4154000\",\"0\",\"7\",\"1\"]]";
    var rawData = JsonConvert.DeserializeObject<string[][]>(json);

Next up is is turning that data in to objects.

The first row is the header, containing the column names, so we want to grab that, and then figure out the column index for each column name.

    var headerRow = rawData.First();    

    var ageIndex = Array.IndexOf(headerRow, "AGE");
    var popIndex = Array.IndexOf(headerRow, "POP");
    var sexIndex = Array.IndexOf(headerRow, "SEX");
    var dateIndex = Array.IndexOf(headerRow, "DATE");
    var usIndex = Array.IndexOf(headerRow, "us");

Now we have the indexes, we need to take each row, and convert it in to the appropriate object. I've used LINQ for this as it's very good at representing data processing in a clear way.

    var testData = rawData
        .Skip(1) //The first row is a header, not data
        .Select(dataRow => new TestClass()
        {
            AGE = dataRow[ageIndex],
            POP = dataRow[popIndex],
            SEX = dataRow[sexIndex],
            DATE = dataRow[dateIndex],
            us = dataRow[usIndex]
        });

Finally a bit of testing, to make sure you have the data you're expecting.

    //Get the second data row as an example
    var example = testData.Skip(1).First();

    //Output example POP to check value
    Console.WriteLine(example.POP);

Everything above is very manual.

You have to know what headers you expect, then you manually find the indexes, then you manually map the rows to objects.

It's quite possible for a simple use case that doing that is fine. But in larger and/or more complex systems you might want/need to automate those steps.

Automating those steps is possible, but is beyond the scope of this answer as how you approach it can depend on a lot of different factors.

Answer 2

You could make a custom JsonConverter to handle this conversion during deserialization. The conversion code is really not much different than other answers here, except that it is encapsulated into a separate class so that you don't muddy up your main code with the conversion details. From the point of view of your main code it "just works".

Here is how to write the converter:

public class TestClassArrayConverter : JsonConverter 
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(TestClass[]));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JArray table = JArray.Load(reader);
        TestClass[] items = new TestClass[table.Count - 1];
        for (int i = 1; i < table.Count; i++)
        {
            JArray row = (JArray)table[i];
            items[i - 1] = new TestClass
            {
                AGE = (string)row[0],
                POP = (string)row[1],
                SEX = (string)row[2],
                DATE = (string)row[3],
                us = (string)row[4]
            };
        }
        return items;
    }

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

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

And here is how you would use it:

var test2 = JsonConvert.DeserializeObject<TestClass[]>(jsonStr, new TestClassArrayConverter());

Fiddle: https://dotnetfiddle.net/68Q0KT