c# - Serialising BigInteger using System.Text.Json

Question

I'm serialising a BigInteger to JSON using System.Text.Json:

JsonSerializer.Serialize(new {foo = new BigInteger(ulong.MaxValue) + 1})

This results in the following output:

{"foo":{"IsPowerOfTwo":true,"IsZero":false,"IsOne":false,"IsEven":true,"Sign":1}}

If I add a converter that casts the BigInteger value to a ulong, it of course fails because the BigInteger value is too big:

var options = new JsonSerializerOptions();
options.Converters.Add(new BigIntegerConverter());
JsonSerializer.Serialize(new {foo = new BigInteger(ulong.MaxValue) + 1}, options);

This is the converter:

public class BigIntegerConverter : JsonConverter<BigInteger>
{
    public override BigInteger Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => throw new NotImplementedException();

    public override void Write(Utf8JsonWriter writer, BigInteger value, JsonSerializerOptions options) => writer.WriteNumberValue((ulong)value);
}

The output I want is:

{"foo":18446744073709551616}

I know this can be achieved with JsonWriter.WriteRawValue in Json.NET, but I am restricted to using System.Text.Json.

Is there any way to do this without manually hacking the serialised string?

Answer 1

Writing a converter for BigInteger is a little awkward because, as you note, Utf8JsonReader and Utf8JsonWriter do not provide the ability to read and write raw JSON as of .NET 5.

JsonDocument, however, does provide access to the raw JSON via RootElement.GetRawText(), so you can write your converter by reading from and writing to an intermediate document like so:

public class BigIntegerConverter : JsonConverter<BigInteger>
{
    public override BigInteger Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if (reader.TokenType != JsonTokenType.Number)
            throw new JsonException(string.Format("Found token {0} but expected token {1}", reader.TokenType, JsonTokenType.Number ));
        using var doc = JsonDocument.ParseValue(ref reader);
        return BigInteger.Parse(doc.RootElement.GetRawText(), NumberFormatInfo.InvariantInfo);
    }

    public override void Write(Utf8JsonWriter writer, BigInteger value, JsonSerializerOptions options)
    {
        var s = value.ToString(NumberFormatInfo.InvariantInfo);
        using var doc = JsonDocument.Parse(s);
        doc.WriteTo(writer);
    }
}

Demo fiddle here.