Deserialize json into C# object for class which has default private constructor

Question

I need to deserialize json for following class.

public class Test
{
    public string Property { get; set; }

    private Test()
    {
        //NOTHING TO I         


        

Answer 1

This is unfortunately not an option for new System.Text.Json.JsonSerializer (core version of Newtonsoft.Json) according to this and this.

System.Text.Json supports only parameterless constructor

Answer 2

No need to create a Serializer setting and give assign ConstructorHandling here. Please remember to define the [JsonConstructor] attribute to the private constructor. I have similar case with abstract BaseNode.cs and its concrete ComputerNode.cs implementation. You can create the classes, copy/paste the code below and do some experiment.

    public abstract class BaseNode
{
    [JsonConstructor] // ctor used when Json Deserializing
    protected BaseNode(string Owner, string Name, string Identifier)
    {
        this.Name = Name;
        this.Identifier = Identifier;
    }

    // ctor called by concrete class.
    protected BaseNode(string [] specifications)
    {
        if (specifications == null)
        {
            throw new ArgumentNullException();
        }
        if (specifications.Length == 0)
        {
            throw new ArgumentException();
        }

        Name = specifications[0];
        Identifier = specifications[1];

    }

    public string Name{ get; protected set; }
    public string Identifier { get; protected set; }

}


public class ComputerNode: BaseNode
{
    public string Owner { get; private set; }

    [JsonConstructor] // not visible while creating object from outside and only used during Json Deserialization.
    private ComputerNode(string Owner, string Name, string Identifier):base(Owner, Name, Identifier)
    {
        this.Owner = Owner;
    }

    public ComputerNode(string[] specifications):base(specifications)
    {
        Owner = specifications[2];
    }
}

For JSon Read and Write following code helps -

    public class Operation<T>
{
    public string path;

    public Operation()
    {
        var path = Path.Combine(Directory.GetCurrentDirectory(), "nodes.txt");

        if (File.Exists(path) == false)
        {
            using (File.Create(path))
            {
            }
        }
        this.path = path;
    }

    public void Write(string path, List<T> nodes)
    {
        var ser = JsonConvert.SerializeObject(nodes, Formatting.Indented);

        File.WriteAllText(path, ser);
    }

    public List<T> Read(string path)
    {
        var text = File.ReadAllText(path);

        var res =  JsonConvert.DeserializeObject<List<T>>(text);
        return res;
    }

}

All the best!

Answer 3

You can make Json.Net call the private constructor by marking it with a [JsonConstructor] attribute:

[JsonConstructor]
private Test()
{
    //NOTHING TO INITIALIZE
}

Note that the serializer will still use the public setters to populate the object after calling the constructor.

EDIT

Another possible option is to use the ConstructorHandling setting:

JsonSerializerSettings settings = new JsonSerializerSettings
{
    ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor
};

Test t = JsonConvert.DeserializeObject<Test>(json, settings);

Answer 4

It doesn't seem like you need to take any extra steps.

Using Json.NET v6.0.8, the following C# program works inside LINQPad:

void Main()
{   
    var o = JsonConvert.DeserializeObject<Test>("{\"Property\":\"Instance\"}");

    Debug.Assert(o.Property == "Instance",
        "Property value not set when deserializing.");
}

public class Test
{
    public string Property { get; set; }

    private Test()
    {
    }

    public Test(string propertyValue)
    {
        Property = propertyValue;
    }
}