Json Deserialization from reader (Utf8JsonReader)

Question

I'm calling JsonSerializer.Deserialize(ref reader, options), but for some reason it isn't working.

public class WeatherForecastJsonConverter : JsonConverter<WeatherForecast>
{
    public override WeatherForecast? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        // Returns an empty WeatherForecastData
        var data = JsonSerializer.Deserialize<WeatherForecastData>(ref reader, options);

        return new WeatherForecast
        {
            Date = data.Date,
            TemperatureCelsius = data.TC,
            Summary = data.Summary == 1 ? "Hot" : data.Summary == -1 ? "Cold" : string.Empty,
        };
    }

    public override void Write(Utf8JsonWriter writer, WeatherForecast value, JsonSerializerOptions options)
    {
        var data = new WeatherForecastData(value.Date, value.TemperatureCelsius, value.Summary ?? string.Empty);
        JsonSerializer.Serialize(writer, data, options);
    }
}

Below are the data types I use. The idea is that calling JsonSerializer.Serialize<WeatherForecast>(...) and JsonSerializer.Deserialize<WeatherForecast>(...) will generate an optimized json all on its own.

[JsonConverter(typeof(WeatherForecastJsonConverter))]
public class WeatherForecast
{
    public DateTime Date { get; set; }
    public int TemperatureCelsius { get; set; }
    public string? Summary { get; set; }
}

internal readonly struct WeatherForecastData
{
    public DateTime Date { get; }
    public int TC { get; }
    public int Summary { get; }

    public WeatherForecastData(DateTime date, int temperatureCelsius, string summary)
    {
        this.Date = date;
        this.TC = temperatureCelsius;
        this.Summary = summary == "Hot" ? 1 : summary == "Cold" ? -1 : 0;
    }
}

My test code is:

// This works
[Fact]
public void TestSerialize()
{
    var weatherForecast = new WeatherForecast
    {
        Date = System.DateTime.Parse("1731-02-08T00:00:00"),
        TemperatureCelsius = 2,
        Summary = "Cold"
    };
    var json = JsonSerializer.Serialize(weatherForecast);
}

// This doesn't
[Fact]
public void TestDeserialize()
{
    var json = "{\"Date\":\"2019-08-01T00:00:00\",\"TC\":25,\"Summary\":1}";
    var weatherForecast = JsonSerializer.Deserialize<WeatherForecast>(json);
}

I was expecting this to work, why it's failing me I have no idea. I have read the documentation on UTF8JsonReader usage but see no information on JsonSerializer.Deserialize(ref reader). I'm kindof stumped.

Answer 1

The problem is because your target class is readonly, I changed and it works.

internal struct WeatherForecastData
 {
     public DateTime Date { get; set; }
     public int TC { get; set; }
     public int Summary { get; set; }

     public WeatherForecastData(DateTime date, int temperatureCelsius, string summary)
     {
         this.Date = date;
         this.TC = temperatureCelsius;
         this.Summary = summary == "Hot" ? 1 : summary == "Cold" ? -1 : 0;
     }
 }