How do you modify the Json serialization of just one field using Json.net?

Question

Say for example I\'m trying to convert an object with 10 fields to Json, however I need to modify the process of serializing 1 of these fields. At the moment, I\'d have to use m

Answer 1

If you have access to the class (and you always need it to be serialized the same way) you could modify the class to your needs. Suppose this.is the class:

public class MyClass
{
    public string Value4 {get; set;}
}

If you want value 4 to be serialized as an int you could do this:

public class MyClass
{
    [JsonIgnore()]
    public string Value4 {get; set;}

    public int Value4AsInt
    {
        return Convert.ToInt32(Value4);
    }
}

Answer 2

You might use System.Reflection, however it's slow but you don't have to modify the class

public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
    writer.WriteStartObject();

    Type vType = value.GetType();
    MemberInfo[] properties = vType.GetProperties(BindingFlags.Public
                                        | BindingFlags.Instance);

    foreach (PropertyInfo property in properties)
    {
        object serValue = null;
        if (property.Name == "Field4")
        {
            serValue = Convert.ToInt32(property.GetValue(value, null));
        }
        else
        {
            serValue = property.GetValue(value, null);
        }
        writer.WritePropertyName(property.Name);
        serializer.Serialize(writer, serValue);
    }

    writer.WriteEndObject();
}

Answer 3

You can try by decorating the property you need to modify manually with JsonConverterAttribute and pass the appropriate JsonConverter type.

For example, using OP's original example:

public class IntegerConverter : JsonConverter
{
  public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
  {
    serializer.Serialize(writer, Convert.ToInt32(value));
  }

  public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
  {
    throw new NotImplementedException();
  }

  public override bool CanConvert(Type objectType)
  {
    return objectType == typeof(string);
  }
}

class TestJson
{
  public string Field1 { get; set; }
  public string Field2 { get; set; }
  public string Field3 { get; set; }

  [JsonConverter(typeof(IntegerConverter))]
  public string Field4 { get; set; }        
}

You can then serialize the object as usual using JsonConvert:

var test = new TestJson {Field1 = "1", Field2 = "2", Field3 = "3", Field4 = "4"};

var jsonString = JsonConvert.SerializeObject(test);