Json.net deserializing list gives duplicate items
c#, json, json.net
Solution
That is because you are adding items in the constructor. A common approach in deserializers when processing a list is basically:
- read the list via the getter
- if the list is null: create a new list and assign via the property setter, if one
- deserialize each item in turn, and append (`Add`) to the list
this is because most list members don't have setters, i.e.
public List<Foo> Items {get {...}} // <=== no set
Contrast to arrays, which must have a setter to be useful; hence the approach is usually:
- deserialize each item in turn, and append (`Add`) to a temporary list
- convert the list to an array (`ToArray`), and assign via the setter
Some serializers give you options to control this behavior (others don't); and some serializers give you the ability to bypass the constructor completely (others don't).
Problem
I have just started using Newtonsoft.Json (Json.net). In my first simple test, I ran into a problem when deserializing generic lists. In my code sample below I serialize an object, containing three types of simple integer lists (property, member var and array). The resulting json looks fine (the lists are converted into json-arrays). However, when I deserialize the json back to a new object of the same type, all list items are duplicated, expect for the array. I've illustrated that by serializing it a second time. From searching around, I've read that there may be a "private" backing field to the lists that the deserializer also fills. So my question is: Is there a (preferably simple) way to avoid duplicate items in following case? Code ``` using System; using System.Collections.Generic; using Newtonsoft.Json; namespace JsonSerializeExample { public class Program { static void Main() { var data = new SomeData(); var json = JsonConvert.SerializeObject(data); Console.WriteLine("First : {0}", json); var data2 = JsonConvert.DeserializeObject<SomeData>(json); var json2 = JsonConvert.SerializeObject(data2); Console.WriteLine("Second: {0}", json2); } } public class SomeData { public string SimpleField; public int[] IntArray; public IList<int> IntListProperty { get; set; } public IList<int> IntListMember; public SomeData() { SimpleField = "Some data"; IntArray = new[] { 7, 8, 9 }; IntListProperty = new List<int> { 1, 2, 3 }; IntListMember = new List<int> { 4, 5, 6 }; } } } ``` Resulting output ``` First : {"SimpleField":"Some data","IntArray":[7,8,9],"IntListMember":[4,5,6],"IntListProperty":[1,2,3]} Second: {"SimpleField":"Some data","IntArray":[7,8,9],"IntListMember":[4,5,6,4,5,6],"IntListProperty":[1,2,3,1,2,3]} ``` There may be some overlap here with Json.Net duplicates private list items. However, I think my problem is even simpler, and I still haven't figured it out.