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D. Albarracín
Asked: 2020-10-16 07:30:56 +0800 CST 2020-10-16 07:30:56 +0800 CST

How do I Deserialize a .json File?

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I'm doing some LINQ exercises, where I have to do some queries on a .json file . I need it to show Manufacturers and Models that are the color "Fuscia".

Json format:

{"id":1,"Maker":"Mercedes-Benz","Model":"SL-Class","Year":null,"Color":null,"Location":{"Latitude":null,"Longitude":null}}

I have created a class to be able to pass to List<>this .json, looking like this:

class Cars
{
    public string Maker { get; set; }
    public string Model { get; set; }
    public int Year { get; set; }
    public string Color { get; set; }
    public double Location { get; set; }
    public double Latitude { get; set; }
    public double Longitude { get; set; }
}

In the mainI call and create the List<>as follows:

        string archivoJ = File.ReadAllText("Cars.json"); //Cargamos el archivo.

        List<Cars> coches = JsonConvert.DeserializeObject<List<Cars>>(archivoJ); //Creamos una lista con el json.

And in my method where I must execute the Query:

    static void Ejer4(List<Cars> lc)
    {
        var fm = lc.Where(x => x.Color == "Fuscia").Select( y => new { Fabricante = y.Maker, Modelo = y.Model });

        foreach(var i in fm)
        {
            Console.WriteLine($"Fabricante: {i.Fabricante} Modelo: {i.Modelo}");
        }
    }

But when executing, I get the following exception, marking the position of JasonConvert:

Newtonsoft.Json.JsonSerializationException: 'Error converting value {null} to type 'System.Int32'. Path '[0].Year', line 1, position 63.'

In the class CarsI have tried to change the data types to stringall, but it is still the same. How can I prevent it from converting the types null?

c#

1 Answers

  1. Best Answer

    The error is clear and concise, maybe you don't understand it because it is in English, I translate it:

    Newtonsoft.Json.JsonSerializationException: 'Error converting value {null} to type 'System.Int32'. Path '[0].Year', line 1, position 63.'


    Newtonsoft.Json.JsonSerializationException: 'Error convirtiendo el valor {null} a 'System.Int32'. Path '[0].Year', línea 1, posición 63.'

    Your JSON has only one line, position 63 is:

    {"id":1,"Maker":"Mercedes-Benz","Model":"SL-Class","Year":null,"Color":null,"Location":{"Latitude":null,"Longitude":null}}
                                                             ><

    The error is obvious: Cannot convert value null(which is in position 63) to an integer (which is what a year is supposed to be).

    How can I prevent it from converting the types null?

    You can configure the Newtonsoft deserializer to ignore null values ​​by creating a configuration object:

    var configuracion = new JsonSerializerSettings
                    {
/* ignorar nulos */     NullValueHandling = NullValueHandling.Ignore,
                    };

    And provide it to the deserializer:

    List<Cars> coches = JsonConvert.DeserializeObject<List<Cars>>(archivoJ, configuracion);

    Optionally, you could have made the year nullable :

    class Cars
{
    public string Maker { get; set; }
    public string Model { get; set; }
    public int? Year { get; set; }
//         ^^^^ <--- Year puede ser null
    public string Color { get; set; }
    public double Location { get; set; }
    public double Latitude { get; set; }
    public double Longitude { get; set; }
}

    But the most appropriate thing is that you sanitize your input data: A year cannot be null in this context.

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