How to deserialize array with null values?

Question

I need to deserialize this json:

[null]

to a List<JsonNode>

unfortunately instead, I get this exception:

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?

This code

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

Gives this json:

[null]

But this code

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

Throws above

Answer 1

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

• 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.

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

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:

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.

Demo fiddle #2 here.

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:

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.