Unable To Find A Constructor

Question

ScreenShot Of Error i have a json file which contains some data and i want to parse it but every time when i want to Deserialize Object it give me this error "Unable to find a constructor. A Class should either have a default constructor, one constructor with arguments or a constructor marked with json constructor i dont know what to do because i did not work with json before it is my first time working with json. here is my json :

{
   "Addition":
   {
       "Easy": [

                "New York Bulls",
                "Los Angeles Kings",
                "Golden State Warriros",
                "Huston Rocket"
            ],
      "Medium":[
                "New York Bulls",
                "Los Angeles Kings",
                "Golden State Warriros",
                "Huston Rocket"
      ],
       "Difficult": [
               "New York Bulls",
               "Los Angeles Kings",
               "Golden State Warriros",
               "Huston Rocket"
      ] 
   }
}

here is my model class

public class Addition
{

     public List<string> Easy { get; set; }
        public List<string> Medium { get; set; }
        public List<string> Difficult { get; set; }

    public Addition() { }

}

here is my function in which i am Deserialize object

private void ReadJson()
    {
        var assembly = typeof(WordProblemsScreen).GetTypeInfo().Assembly;
        Stream stream = 
assembly.GetManifestResourceStream("MathRandomizer.demo.json");

        using (var reader = new System.IO.StreamReader(stream))
        {

            string json = reader.ReadToEnd();

            JObject jObject = JObject.Parse(json);
            JToken jUser = jObject["Addition"];


            var addition = JsonConvert.DeserializeObject<Addition>(json);




        }
    }

Answer 1

The exception occurs because you try to convert your JSON content into the wrong object. As your JSON contains an attribute "Addition" in top, it is considered as a property of some parent class. So it expects some class like this:

namespace JsonStuff
{
    public class SomeParentClass
    {
        public Addition AdditionObject { get; set; }

        public SomeParentClass()
        {

        }
    }
}

If you try it the other way serializing an object from type "Addition" and have a look on the outcoming json string, you will see the difference:

var filledObject = new Addition()
{
    Easy = new List<string>()
    {
        "New York Bulls",
        "Los Angeles Kings",
        "Golden State Warriros",
        "Huston Rocket"
    },
    Medium = new List<string>()
    {
        "New York Bulls",
        "Los Angeles Kings",
        "Golden State Warriros",
        "Huston Rocket"
    },
    Difficult = new List<string>()
    {
        "New York Bulls",
        "Los Angeles Kings",
        "Golden State Warriros",
        "Huston Rocket"
    }
};

var serializedObject = JsonConvert.SerializeObject(filledObject);

The result will be:

{
    "Easy":
    [
        "New York Bulls",
        "Los Angeles Kings",
        "Golden State Warriros",
        "Huston Rocket"
    ],
    "Medium":
    [
        "New York Bulls",
        "Los Angeles Kings",
        "Golden State Warriros",
        "Huston Rocket"
    ],
    "Difficult":
    [
        "New York Bulls",
        "Los Angeles Kings",
        "Golden State Warriros",
        "Huston Rocket"
    ]
}

and there you see the difference: There is no "Addition" property on top.

Answer 2

You need a root object:

public class RootObject
{
    public Addition Addition { get; set; }
}

var result = JsonConvert.DeserializeObject<RootObject>(json);
Console.WriteLine(result.Addition.Easy.First());

Try it online

And it even works with your old code with the [JsonConstructor] attribute.

Simply put: your C# classes must match the JSON you hope to deserialize.

Imagine you have the following JSON:

{
    "person": {
        "name": "John"
    }
}

You cannot simply deserialize it into the following class:

public class Person
{
    public string Name { get; set; }
}

Why? Because the "person" object is not your root object. The above class serialized as JSON simply looks like this:

{
    "name": "John"
}

So to deserialize the first sample above, we need a root object:

public class RootObject
{
    public Person Person { get; set; }
}

public class Person
{
    public string Name { get; set; }
}

And now we can deserialize that JSON (the one that contains the "person" key) to RootObject

JSON is really simple - things just have to match.

Answer 3

Paste your Json as C# using Edit - Paste Special - Paste JSON as C# or use one of the online JSON to C# converters and you will see the issue:

public class Addition
{
    public List<string> Easy { get; set; }
    public List<string> Medium { get; set; }
    public List<string> Difficult { get; set; }
}

public class RootObject
{ 
   public Addition Addition { get; set; }
}