How can I serialize a Stack<T> to JSON using System.Text.Json without reversing the stack?

Question

If I have a Stack<T> for some T, and round-trip it to JSON using the new System.Text.Json.JsonSerializer, the order of the items in the stack will be reversed after deserialization. How can I serialize and deserialize a stack to JSON using this serializer without this happening?

Details as follows. I have a Stack<int> and push 3 values 1, 2, 3 onto it. I then serialize it to JSON using JsonSerializer, which results in

[3,2,1]

However, when I deserialize the JSON to a new stack, the integers in the stack are reversed, and a later assert that the stacks are sequentially equals fails:

var stack = new Stack<int>(new [] { 1, 2, 3 });

var json = JsonSerializer.Serialize(stack);

var stack2 = JsonSerializer.Deserialize<Stack<int>>(json);

var json2 = JsonSerializer.Serialize(stack2);

Console.WriteLine("Serialized {0}:", stack);
Console.WriteLine(json); // Prints [3,2,1]

Console.WriteLine("Round-tripped {0}:", stack);
Console.WriteLine(json2); // Prints [1,2,3]

Assert.IsTrue(stack.SequenceEqual(stack2)); // Fails
Assert.IsTrue(json == json2);               // Also fails

How can I prevent the serializer from reversing the stack during serialization?

Demo fiddle here.

Answer 1

This seems to be a bug in the serializer. In .NET Core 3.1 there is some code in CreateDerivedEnumerableInstance(ref ReadStack state, JsonPropertyInfo collectionPropertyInfo, IList sourceList) to create a stack from a deserialized list:

else if (instance is Stack<TDeclaredProperty> instanceOfStack)
{
    foreach (TDeclaredProperty item in sourceList)
    {
        instanceOfStack.Push(item);
    }

    return instanceOfStack;
}

However, it pushes them on in the wrong order. Thus a custom JsonConverter<Stack<T>> will be required to correctly deserialize a Stack<T>. In addition, a JsonConverterFactory can be used to manufacture an appropriate converter for every stack type Stack<T>:

public class StackConverterFactory : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert)
    {
        return GetStackItemType(typeToConvert) != null;
    }

    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
    {
        var itemType = GetStackItemType(typeToConvert);
        var converterType = typeof(StackConverter<,>).MakeGenericType(typeToConvert, itemType);
        return (JsonConverter)Activator.CreateInstance(converterType);
    }

    static Type GetStackItemType(Type type)
    {
        while (type != null)
        {
            if (type.IsGenericType)
            {
                var genType = type.GetGenericTypeDefinition();
                if (genType == typeof(Stack<>))
                    return type.GetGenericArguments()[0];
            }
            type = type.BaseType;
        }
        return null;
    }
}

public class StackConverter<TItem> : StackConverter<Stack<TItem>, TItem>
{
}

public class StackConverter<TStack, TItem> : JsonConverter<TStack> where TStack : Stack<TItem>, new()
{
    public override TStack Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var list = JsonSerializer.Deserialize<List<TItem>>(ref reader, options);
        if (list == null)
            return null;
        var stack = typeToConvert == typeof(Stack<TItem>) ? (TStack)new Stack<TItem>(list.Count) : new TStack();
        for (int i = list.Count - 1; i >= 0; i--)
            stack.Push(list[i]);
        return stack;
    }

    public override void Write(Utf8JsonWriter writer, TStack value, JsonSerializerOptions options)
    {
        writer.WriteStartArray();
        foreach (var item in value)
            JsonSerializer.Serialize(writer, item, options);
        writer.WriteEndArray();
    }
}

Then use it in JsonSerializerOptions as follows:

var stack = new Stack<int>(new [] { 1, 2, 3 });

var options = new JsonSerializerOptions
{
    Converters = { new StackConverterFactory() },
};

var json = JsonSerializer.Serialize(stack, options);

var stack2 = JsonSerializer.Deserialize<Stack<int>>(json, options);

var json2 = JsonSerializer.Serialize(stack2, options);

Assert.IsTrue(stack.SequenceEqual(stack2)); // Passes
Assert.IsTrue(json == json2);  // Passes

The converter could also be applied directly to some data model using JsonConverterAttribute

public class Model
{
    [JsonConverter(typeof(StackConverter<int>))]
    public Stack<int> Stack { get; set; }
}

Demo fiddle here.

Update: Looks round-tripping of Stack<T> will not be built into JsonSerializer. See (De)serializing stacks with JsonSerializer should round-trip #41887 (Closed):

We shouldn't do this. There's no standard on which side to reverse the items (serialization or deserialization) in order to roundtrip, so it is a non-starter as a breaking change candidate. The current behavior is compatible with Newtonsoft.Json behavior.

There's a work item to provide a sample converter showing how to roundtrip in the JSON docs which I think should suffice as a resolution for this issue: dotnet/docs#16690. Here's what this converter could look like - dotnet/docs#16225 (comment).