如何在不反转堆栈的情况下使用 System.Text.Json 将 Stack<T> 序列化为 JSON？

Question

如果我有一个Stack<T> for some T, and round-trip it to JSON using the new System.Text.Json.JsonSerializer，反序列化后堆栈中项目的顺序将颠倒。如何在不发生这种情况的情况下使用此序列化程序将堆栈序列化和反序列化为 JSON？

详情如下。我有一个 Stack<int> 并将 3 个值 1, 2, 3 推到它上面。然后我使用 JsonSerializer 将它序列化为 JSON，结果是

[3,2,1]

然而，当我将 JSON 反序列化到一个新堆栈时，堆栈中的整数被反转，并且后来断言堆栈顺序等于失败：

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

如何防止序列化程序在序列化期间反转堆栈？

演示 fiddle here.

Answer 1

这似乎是序列化程序中的错误。在 .NET Core 3.1 中，CreateDerivedEnumerableInstance(ref ReadStack state, JsonPropertyInfo collectionPropertyInfo, IList sourceList) 中有一些代码用于从反序列化列表创建堆栈：

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

    return instanceOfStack;
}

但是，它以错误的顺序推送它们。因此 custom JsonConverter<Stack<T>> will be required to correctly deserialize a Stack<T>. In addition, a JsonConverterFactory 可用于为每种堆栈类型制造适当的转换器 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();
    }
}

然后在JsonSerializerOptions中使用如下：

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

也可以使用 JsonConverterAttribute

将转换器直接应用于某些数据模型

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

演示 fiddle here.

更新：Stack<T> 的循环看起来不会内置到JsonSerializer 中。见 (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).

如何在不反转堆栈的情况下使用 System.Text.Json 将 Stack<T> 序列化为 JSON？

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

c#

json

.net-core-3.1

system.text.json