C# How can I deserialize a List<T> with condition using Newtonsoft.Json

Question

I have this classes:

public class CTest
{
  public int Id { get; set; }
  public string Name { get; set; }
  public List<Class2> Details { get; set; }
}

public class Class2
{
  public int Id { get; set; }
  public int Data1 { get; set; }
}

I know that I can deserialize using JsonConvert.DeserializeObject<T>(myObject); even Lists, but in Class2, I'd like to know how to filter if Id > 0.

For example, I'd like to get this result:

{
  "id":5,
  "name":"test",
  "details":[
    {
      "id":1,
      "data1":0.25
    }
  ]
}

Instead of:

{
  "id":5,
  "name":"test",
  "details":[
    {
      "id":12,
      "data1":0.25
    },
    {
      "id":0,
      "data1":0.0
    },
  ]
}

Answer 1

You can use Custom JsonConverter

class ListClass2Converter : JsonConverter<List<Class2>>
{
    public override List<Class2> ReadJson(JsonReader reader, Type objectType,
        [AllowNull] List<Class2> existingValue, bool hasExistingValue,
        JsonSerializer serializer)
    {            
        var list = new List<Class2>();

        while (reader.Read() && reader.TokenType != JsonToken.EndArray)
        {
            if (reader.TokenType == JsonToken.StartObject)
            {
                var item = serializer.Deserialize<Class2>(reader);
                if (item.Id != 0)
                    list.Add(item);
            }
        }
        return list;
    }

    public override void WriteJson(JsonWriter writer, [AllowNull] List<Class2> value,
        JsonSerializer serializer)
    {            
        serializer.Serialize(writer, ((List<Class2>)value).Where(c => c.Id != 0).ToArray());
    }
}

Read code

var json = File.ReadAllText("test.json");
var сTest = JsonConvert.DeserializeObject<CTest>(json, new ListClass2Converter());

Write code

var json = JsonConvert.SerializeObject(
    cTest, Formatting.Indented, new ListClass2Converter());

You can use as many converters for different types of collections as you need.