Unable to deserialize classes with multiple constructors with Json.NET [duplicate]

14
votes

I have a type that I don't control with multiple constructors, equivalent to this one:

    public class MyClass
    {
        private readonly string _property;

        private MyClass()
        {
            Console.WriteLine("We don't want this one to be called.");
        }

        public MyClass(string property)
        {
            _property = property;
        }

        public MyClass(object obj) : this(obj.ToString()) {}

        public string Property
        {
            get { return _property; }
        }

    }

Now when I try to deserialize it, the private parameterless constuctor is called and the property is never set. The test:

    [Test]
    public void MyClassSerializes()
    {
        MyClass expected = new MyClass("test");
        string output = JsonConvert.SerializeObject(expected);
        MyClass actual = JsonConvert.DeserializeObject<MyClass>(output);
        Assert.AreEqual(expected.Property, actual.Property);
    }

gives the following output:

We don't want this one to be called.

  Expected: "test"
  But was:  null

How can I fix it, without changing the definition of MyClass? Also, this type is a key deep in the definition of the objects that I really need to serialize.

1
c#.netjsonjson.net

1 Answers

12
votes

Try adding the [JsonConstructor] attribute to the constructor you want to use when deserializing.

Change this property in your class:

[JsonConstructor]
public MyClass(string property)
{
    _property = property;
}

I have just tried it and your test passes :-)

If you can't make this change then I guess you'd need to create a CustomJsonConverter. http://james.newtonking.com/json/help/index.html?topic=html/CustomJsonConverter.htm and How to implement custom JsonConverter in JSON.NET to deserialize a List of base class objects? might help.

Here is a useful link for creating a CustomJsonConverter: https://stackoverflow.com/a/8312048/234415