C# Newtonsoft: Override Constructor Default Value When Deserializing

Question

I have some fairly complex JSON and need to use Newtonsoft to deserialize, as System.Text.Json is not as extensible as Newtonsoft.

My problem is as follows: I have a class, let's call "A", which I use a few places in my app including saving to a DB using EFCore and converting to a binary file. I populate the class using a JSON File, but due to requirements beyond my control I need to keep the default value setter in the class. However, IF the property does NOT exist in the Json we are deserializing, I'd like to use a custom default value.

Example Class:

public class A
{
  public int Id { get; set; } = 0;
  public bool IsRequired { get; set; } = true;
}

And if this is my Json:

[{
  "id": 4,
  "isRequired": true;
},
{
  "id": 7
}]

I'd like to override the isRequired default to false if the key is not in the json.

var list = JsonConvert.DeserializeObject<List<A>>( -- some settings --);
// list[0]
//  - Id = 4
//  - IsRequired = true
//
// list[1]
//  - Id = 6
//  - IsRequired = false

I've been able to get a basic version of this working as described in this SO post, but the deserialization is very simple and breaks immediately with nested properties and complex types + my custom contract resolver I have.

Note that I am using .NET 6, and my models are separated from my deserializers.

Answer 1

You just can add a json constructor to your class. You don't need to put all properties in the constructor, only those properties that behavier the special way

public class A
{
    public int Id { get; set;} = 0;
    public bool IsRequired { get; set; } = true;
   
    [JsonConstructor]
    public A (bool? isRequired) 
    {
      this.IsRequired = isRequired ?? false;
    }
}

test

var list = JsonConvert.DeserializeObject<List<A>>(json);

json=JsonConvert.SerializeObject(list, Newtonsoft.Json.Formatting.Indented);

result

[
  {
    "Id": 4,
    "IsRequired": true
  },
  {
    "Id": 7,
    "IsRequired": false
  }
]