c# - 如何使用 System.Text.Json 将 Stack 序列化为 JSON，而不反转堆栈？

Question

如果我有一个 Stack<T> 对于一些T ，并使用新的 System.Text.Json.JsonSerializer 将其往返为 JSON ，反序列化后堆栈中项目的顺序将相反。如何使用此序列化器将堆栈序列化和反序列化为 JSON，而不发生这种情况？

详情如下。我有一个Stack<int>并推送 3 个值 1, 2, 3到它上面。然后我使用 JsonSerializer 将其序列化为 JSON ，结果是

[3,2,1]

但是，当我将 JSON 反序列化到新堆栈时，堆栈中的整数会反转，并且稍后断言堆栈顺序相等会失败:

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

如何防止序列化器在序列化过程中反转堆栈？

演示 fiddle here .

Answer 1

这似乎是序列化器中的一个错误。在.NET Core 3.1中， CreateDerivedEnumerableInstance(ref ReadStack state, JsonPropertyInfo collectionPropertyInfo, IList sourceList) 中有一些代码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，它以错误的顺序插入它们。因此custom JsonConverter<Stack<T>> 将需要正确反序列化 Stack<T> 。另外，一个 JsonConverterFactory 可用于为每种堆栈类型制造合适的转换器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 中使用它如下:

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

转换器还可以使用 JsonConverterAttribute 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:看起来是 Stack<T> 的往返不会被内置到JsonSerializer中。请参阅(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).