How Can I Control Which Constructor JSON.NET Uses During Deserialization?

Managing Constructor Usage in JSON.NET Deserialization

.NET's JSON.NET library typically uses the default constructor when deserializing JSON data into objects. However, if a class has both a default and an overloaded constructor, JSON.NET might default to the parameterless constructor, even if you intend to use a different one.

To specify which constructor JSON.NET should use, employ the [JsonConstructor] attribute. This attribute designates the marked constructor for deserialization.

[JsonConstructor]
public Result(int? code, string format, Dictionary<string, string> details = null)
{
    // ... constructor logic ...
}

The constructor parameters must match the JSON property names (case-insensitive). It's not mandatory to map every property to a constructor parameter; JSON.NET will attempt to populate remaining properties using public setters or attributes like [JsonProperty].

If attributes are unsuitable or you can't modify the class being deserialized, create a custom JsonConverter. This gives you complete control over object instantiation and population.

Here's a custom converter example:

class ResultConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(Result));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        // ... custom deserialization logic ...
    }

    public override bool CanWrite => false;

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

To use this custom converter, add it to your serializer settings:

JsonSerializerSettings settings = new JsonSerializerSettings();
settings.Converters.Add(new ResultConverter());
Result result = JsonConvert.DeserializeObject<Result>(jsontext, settings);

These methods ensure precise control over constructor selection during JSON.NET deserialization, even with a default constructor present, guaranteeing correct object initialization.

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