JSON Serialization attribute to expand object and serialize its fields directly

Question

I have some class with following structure

public class A {
  [JsonProperty(PropertyName = "prop_b")]
  public B PropB {get; set;}
}

public class B {
  [JsonProperty(PropertyName = "val1")]
  public int Val1 {get; set;}

  [JsonProperty(PropertyName = "val2")]
  public int Val2 {get; set;}
}

Which would be serialized to a JSON of the following scheme:

{
  "prop_b": { "val1": X, "val2": Y }
}

Is there any way for me to skip B and serialize the fields directly, without changing the class structure? I would assume there would be some attribute, or that I could implement one.

{
   "val1": X,
   "val2": Y
}

Can you pass the PropB property of your A class directly to your Serialize method? — Anthony McGrath
– Anthony McGrath, Commented Sep 3, 2017 at 5:56
I could, but that's not a generic enough solution, especially if A has more than one field — Mugen
– Mugen, Commented Sep 3, 2017 at 5:57
You might need a custom solution. Maybe this could help: stackoverflow.com/questions/11934487/… — Anthony McGrath
– Anthony McGrath, Commented Sep 3, 2017 at 6:00
Can you use reflection and LINQ to JSON in your project? newtonsoft.com/json/help/html/LINQtoJSON.htm — Sorashi
– Sorashi, Commented Sep 3, 2017 at 7:53

Anders Carstensen · Accepted Answer · 2017-09-03 12:15:41Z

2

One possibility could be to create a JsonConverter.

  [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false)]
  public sealed class ImportChildAttribute : Attribute
  {
  }

  class ImportChildJsonConverter : JsonConverter
  {
    public override bool CanConvert(Type objectType)
    {
      var attr = CustomAttributeExtensions.GetCustomAttribute(objectType.GetTypeInfo(), typeof(ImportChildAttribute), true);
      if (attr != null)
      {
        var props = objectType.GetProperties();
        if (props.Length != 1)
          throw new NotSupportedException($"Only supports {nameof(ImportChildAttribute)} on classes with one property.");
        return true;
      }

      return false;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
      // Deserialize the object into the first property.
      var props = objectType.GetProperties();
      var obj = Activator.CreateInstance(objectType);
      var val = serializer.Deserialize(reader, props[0].PropertyType);
      props[0].SetValue(obj, val);
      return obj;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
      // Find the only property and serialize it.
      var props = value.GetType().GetProperties();
      serializer.Serialize(writer, props[0].GetValue(value));
    }
  }

Then you can put the ImportChild attribute on all the classes you want to have this behaviour.

  [ImportChild]
  public class A
  {
    [JsonProperty(PropertyName = "prop_b")]
    public B PropB { get; set; }
  }

  public class B
  {
    [JsonProperty(PropertyName = "val1")]
    public int Val1 { get; set; }

    [JsonProperty(PropertyName = "val2")]
    public int Val2 { get; set; }
  }

Finally, try it out:

  var settings = new JsonSerializerSettings
  {
    Converters = new[] { new ImportChildJsonConverter() },
    Formatting = Formatting.Indented
  };

  var obj = new A { PropB = new B { Val1 = 1, Val2 = 2 } };
  string json = JsonConvert.SerializeObject(obj, settings);
  Console.WriteLine(json);
  /* Outputs:
     {
       "val1": 1,
       "val2": 2
     }
  */

  var originalObj = JsonConvert.DeserializeObject<A>(json, settings);
  // originalObj and obj are now identical.

edited Sep 3, 2017 at 12:15

answered Sep 3, 2017 at 11:47

Anders Carstensen

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6 Comments

dbc Over a year ago

Rather than on the type itself, I'd put the [ImportChild] attribute on the property to be serialized. That way other get-only properties won't interfere.

Anders Carstensen Over a year ago

@dbc But JsonConverters don't work on properties - they work on types. So, to my knowledge, it is not possible to detect whether the property being serialized has a specific attribute. If you know how to do it, please post another answer with that solution. I'd love to see it!

dbc Over a year ago

You could put the converter explicitly on the type, with [JsonConverter(typeof(ImportChildJsonConverter))]. That's going to improve performance by avoiding calls to CanConvert for every type including primitive types. Or, you could do it with two attributes: an [ImportChild] attribute and an [ImportChildProperty] attribute.

Anders Carstensen Over a year ago

Your other approach could work, but the code will get more complicated since there might be name clashes. I just provided a simple example that shows it can be done - it can of course be improved a lot as you suggest, but if we added those complexities to the code, it may be more difficult for the original poster to understand the answer.

dbc Over a year ago

I meand, put the converter on A like so: [JsonConverter(typeof(ImportChildConverter))] public class A { [ImportChild] public B PropB { get; set; } }. But, I agree the simplest answer is often best, and the current works and is simple.

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