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?

dbc · Answer 1 · 2022-08-24T21:32:50.117

In .NET 6 methods to write raw values directly have been added: Utf8JsonWriter.WriteRawValue(). Thus BigIntegerConverter may now be written as follows:

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) =>
        writer.WriteRawValue(value.ToString(NumberFormatInfo.InvariantInfo), false);
}

Notes:

The converter takes advantage of the fact that BigInteger.ToString(NumberFormatInfo.InvariantInfo) returns a value formatted consistently with the JSON standard's formatting for numbers.
There is no directly equivalent method Utf8JsonReader.GetRawText() that can be used during parsing. Instead you can parse to JsonDocument and call RootElement.GetRawText() to get the complete raw JSON token value as a string.

Demo fiddle here.

In .NET 5 and earlier, 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. Instead, an intermediate JsonDocument can be used for both reading and writing an arbitrary JSON value 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.

Serialising BigInteger using System.Text.Json

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