Json.net also tricked me

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Recently, I was tricked by Roslyn, today by Json.NET. My bloody luck ;) Let's look at the following two very simple classes. Class A has one readonly property and I had to define a special constructor to allow Json.NET to set this property. B is also simple. It has one property, this time of type A with some default value.

public class A
{
   public string Name { get; }

   [JsonConstructor]
   public A(string name) { Name = name; }
}

public class B
{
   public A Test { get; set; } = new A("Default");

}

Now let's serialize an instance of B in the following way:

var serializedText = JsonConvert.SerializeObject(new B() { Test = new A("Fun") { } });

In the result, as expected, we will get the following text:

{"Test":{"Name":"Fun"}}

Now let's deserialize this string:

var result = (B)JsonConvert.DeserializeObject(serializedText, typeof(B));

Now question for 1 million dollars. What will be the value of A.Test.Name in a deserialized object. If you say Fun you are wrong! It will be Default!

I was surprised and I spent some time investigating this issue. Why did it happen? Well, it seems to me that when Json.NET creates a new object during deserialization and it notices that some property of this object is not null, them this default value will not be overriden. If it's a problem, there are at least 2 solutions:

• Remove a default value for this property. It doesn't matter if it is set in a constructor of via a property initializer.
• Add a special constructor. For instance, for class B it'll look as below. This constructor instructs Json.NET that Test property must be set even if a default value exists.

public class B
{
   public A Test { get; set; } = new A("Default");

   public B() {}

   [JsonConstructor]
   public B(A test) { Test = test; }
}

^*The picture at the beginning of the post comes from own resources and shows elephants from Warsaw zoo.