JsonConvert.Deserializer indexing issues

Question

While playing around Stack collection in C# I encountered the following issue. Exactly I am not sure why it is happening. Please put some light on the reason and alternative

Answer 1

It seems like the stack is being serialized as a List. The problem is that this does not preserve the proper order when deconstructing the stack (the items are actually pushed in the reverse order). Here's a quick workaround to this issue:

using System;
using static System.Console;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using Newtonsoft.Json;

namespace StackCollection
{
    class Program
    {
        static void Main(string[] args)
        {
            Progress progress = new Progress();

            progress.Items.Push(new Item { PlanID = null, PlanName = "Plan A" });

            var jsonString = JsonConvert.SerializeObject(progress);
            var temp = JsonConvert.DeserializeObject<Progress>(jsonString);

            temp.Items.Push(new Item { PlanID = null, PlanName = "Plan B" });

            jsonString = JsonConvert.SerializeObject(temp);
            temp = JsonConvert.DeserializeObject<Progress>(jsonString);

            temp.Items.Push(new Item { PlanID = null, PlanName = "Plan C" });

            jsonString = JsonConvert.SerializeObject(temp);
            temp = JsonConvert.DeserializeObject<Progress>(jsonString);

            WriteLine(temp.Items.Peek().PlanName);

            ReadLine();
        }
    }

    class Progress
    {
        [JsonIgnore]
        public Stack<Item> Items { get; set; }

        public List<Item> ItemList { get; set; }

        [OnSerializing]
        internal void OnSerializing(StreamingContext context)
        {
            ItemList = Items?.ToList();
        }

        [OnDeserialized]
        internal void OnDeserialized(StreamingContext context)
        {
            ItemList?.Reverse();
            Items = new Stack<Item>(ItemList ?? Enumerable.Empty<Item>());
        }

        public Progress()
        {
            Items = new Stack<Item>();
        }
    }

    class Item
    {
        public string PlanID { get; set; }

        public string PlanName { get; set; }
    }
}

Answer 2

If you try to debug it then you will notice that items order is broken after Stack deserialization.

The same question has been asked on JSON.NET GitHub issue tracker a month ago.

The answer from JamesNK:

I'm afraid this is a limition of a Stack. The results returned when it is serialized and the opposite order for when it is deserialized.

Answer 3

In Visual Studio 2019, this C# works:

List<string> ls = null;
Stack<string> ss = null;
if (json != null)
{
    ls = JsonConvert.DeserializeObject<List<string>>(json);
    ss = new Stack<string>(ls);
}

(This is an edit of the answer from here, which originally had an errant Reverse method call on the list that caused the opposite of the desired result.)

Answer 4

Since this is a known behavior of Json.NET, as noted by this answer, a custom JsonConverter can be used when deserializing a stack that pushes items on in the correct order.

The following universal converter can be used with Stack<T> for any T:

/// <summary>
/// Converter for any Stack<T> that prevents Json.NET from reversing its order when deserializing.
/// </summary>
public class StackConverter : JsonConverter
{
    // Prevent Json.NET from reversing the order of a Stack<T> when deserializing.
    // https://github.com/JamesNK/Newtonsoft.Json/issues/971
    static Type StackParameterType(Type objectType)
    {
        while (objectType != null)
        {
            if (objectType.IsGenericType)
            {
                var genericType = objectType.GetGenericTypeDefinition();
                if (genericType == typeof(Stack<>))
                    return objectType.GetGenericArguments()[0];
            }
            objectType = objectType.BaseType;
        }
        return null;
    }

    public override bool CanConvert(Type objectType)
    {
        return StackParameterType(objectType) != null;
    }

    object ReadJsonGeneric<T>(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;
        var list = serializer.Deserialize<List<T>>(reader);
        var stack = existingValue as Stack<T> ?? (Stack<T>)serializer.ContractResolver.ResolveContract(objectType).DefaultCreator();
        for (int i = list.Count - 1; i >= 0; i--)
            stack.Push(list[i]);
        return stack;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;
        try
        {
            var parameterType = StackParameterType(objectType);
            var method = GetType().GetMethod("ReadJsonGeneric", BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Public);
            var genericMethod = method.MakeGenericMethod(new[] { parameterType });
            return genericMethod.Invoke(this, new object[] { reader, objectType, existingValue, serializer });
        }
        catch (TargetInvocationException ex)
        {
            // Wrap the TargetInvocationException in a JsonSerializerException
            throw new JsonSerializationException("Failed to deserialize " + objectType, ex);
        }
    }

    public override bool CanWrite { get { return false; } }

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

Then add it to JsonSerializerSettings to correct the ordering of stacks when deserializing:

var settings = new JsonSerializerSettings { Converters = new[] { new StackConverter() } };
var jsonString = JsonConvert.SerializeObject(progress, settings);
var temp = JsonConvert.DeserializeObject<Progress>(jsonString, settings);

Or mark the Stack<T> property directly with [JsonConverter(typeof(StackConverter))]:

class Progress
{
    [JsonConverter(typeof(StackConverter))]
    public Stack<Item> Items { get; set; }

    public Progress()
    {
        Items = new Stack<Item>();
    }
}