I need to deserialize this json:

我需要反序列化这个json：

[null]

[空]

to a List<JsonNode>

添加到列表

unfortunately instead, I get this exception:

不幸的是，我得到了这样的例外：

System.Text.Json.JsonException: 'The JSON value could not be converted to System.Text.Json.Nodes.JsonNode. Path: $[0] | LineNumber: 0 | BytePositionInLine: 5.'

How to deserialize to a list with a single null value in it?

如何反序列化到一个只有一个空值的列表？

This code

此代码

var toJson = JsonSerializer.Serialize<List<JsonNode>>(new List<JsonNode>{ null });

Gives this json:

给出了这个json：

[null]

[空]

But this code

但是这个代码

var fromJson = JsonSerializer.Deserialize<List<JsonNode>>("[null]");

Throws above

在上方抛出

JsonNode is an abstract class and a base class for JsonObject and for a JsonArray. But JsonArray and JsonObject[] are completely different. I don't know why do you need it but you can not deserialize a json string directly. But there is a trick, you can use LINQ Select and ToList

JsonNode是一个抽象类，也是JsonObject和Json数组的基类。但是Json数组和JsonObject[]是完全不同的。我不知道您为什么需要它，但是您不能直接反序列化一个json字符串。但有一个诀窍，你可以使用LINQ选择和ToList

string toJson = JsonSerializer.Serialize(new List<JsonNode>{ null });

List<JsonNode> fromJson = JsonArray.Parse(toJson)
                            .AsArray()
                            .Select(v => v.Deserialize<JsonNode>())
                            .ToList();
//or

List<JsonNode> fromJson = JsonSerializer.Deserialize<List<JsonElement>>(toJson)
                                           .Select(v => v.Deserialize<JsonNode>() )
                                           .ToList();

Update this is a bug in System.Text.Json which has been fixed in .NET 8 as of Preview 5. See:

更新这是System.Text.Json中的错误，该错误已在.NET 8预览版5中修复。请参阅：

• Deserialising null to JsonNode in nested structures fails in System.Text.Json #85172.


• fix JsonNode deserialising from null #85463.


• System.Text.Json: Exception thrown trying to deserialize a List<JsonNode> or JsonNode [] from a JSON array containing a null entry, e.g. [null]. #92902/

A failing .NET 7 demo fiddle was created here.

这里创建了一个失败的.NET7演示小提琴。

As a workaround in earlier versions, you could create a custom converter for List<JsonNode> like the following:

作为早期版本中的一种解决办法，您可以为List 创建一个自定义转换器，如下所示：

public class JsonNodeListConverter : JsonConverter<IEnumerable<JsonNode?>>
{
    public override bool CanConvert(Type objectType) => 
        objectType == typeof(List<JsonNode>) || objectType == typeof(IReadOnlyList<JsonNode>) || objectType == typeof(IReadOnlyCollection<JsonNode>) || objectType == typeof(JsonNode [])  || objectType == typeof(IEnumerable<JsonNode>);
    public override void Write(Utf8JsonWriter writer, IEnumerable<JsonNode?> value, JsonSerializerOptions options)
    {
        writer.WriteStartArray();
        foreach (var node in value)
            JsonSerializer.Serialize(writer, node, options);
        writer.WriteEndArray();
    }
    
    public override IEnumerable<JsonNode?> Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) 
    {
        if (reader.TokenType != JsonTokenType.StartArray)
            throw new JsonException();
        var list = new List<JsonNode?>();
        while (reader.ReadAndAssert().TokenType != JsonTokenType.EndArray)
        {
            list.Add(JsonSerializer.Deserialize<JsonNode>(ref reader, options));
        }
        return typeToConvert.IsArray ? list.ToArray() : list;
    }
}

public static class JsonExtensions
{
    public static ref Utf8JsonReader ReadAndAssert(ref this Utf8JsonReader reader) { if (!reader.Read()) { throw new JsonException(); } return ref reader; }
}

Then use as follows:

然后按如下方式使用：

var options = new JsonSerializerOptions
{
    Converters = { new JsonNodeListConverter() },
};
JsonSerializer.Deserialize<List<JsonNode>>("""[null]""", options)

And you will be able to deserialize lists and arrays of JsonNode that include null values.

并且您将能够反序列化包含空值的JsonNode的列表和数组。

Demo fiddle #2 here.

演示小提琴2号在这里。

Alternatively, you could deserialize to an intermediate IEnumerable<object> with the option JsonSerializerOptions.UnknownTypeHandling = JsonUnknownTypeHandling.JsonNode and your non-null JSON array values will be deserialized as JsonNode subtypes:

或者，您可以使用选项JsonSerializerOptions.UnnownTypeHandling=JsonUnnownTypeHandling.JsonNode将非空的JSON数组值反序列化为JsonNode子类型，并将其反序列化为中间IEumable

var options = new JsonSerializerOptions
{
    UnknownTypeHandling = JsonUnknownTypeHandling.JsonNode,
};
var list = JsonSerializer.Deserialize<IEnumerable<object>>("""[null, [null], 1, {}, "a"]""",
                                                           options)
     ?.Cast<JsonNode?>().ToList();

Demo fiddle #3 here.

演示小提琴#3在这里。