Auto-sized number parsing with System.Text.Json

Program.cs

using System.Numerics;
using System.Text;
using System.Text.Json;
using System.Text.Unicode;

var json = """
    {
        "anInt": 123,
        "negativeInt": -123,
        "biggerInt": 1234567890,
        "hugeInt": 123456789012345678901234567890,
        "massiveInt": 123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890,
        "float": 123.456,
        "negativeFloat": -123.456,
        "exponential": 2.99792458e8,
        "negativeExponential": -2.99792458e8,
        "bigFloat": 123456789012345678901234567890123456789012345678901234567890.123456789012345678901234567890,
        "negativeBigFloat": -123456789012345678901234567890123456789012345678901234567890.123456789012345678901234567890
    }
    """u8;

var dict = new Dictionary<string, object>();

var jsonReader = new Utf8JsonReader(json);

while (jsonReader.Read())
{
    if (jsonReader.CurrentDepth == 1) // top-level only
    {
        if (jsonReader.TokenType == JsonTokenType.PropertyName)
        {
            var key = jsonReader.GetString();
            if (key is not null && jsonReader.Read())
            {
                if (jsonReader.TokenType == JsonTokenType.Number)
                {
                    var value = jsonReader.GetNumber();
                    dict[key] = value;
                }
            }
            else
            {
                break;
            }
        }
    }
}

foreach (var key in dict.Keys)
{
    Console.WriteLine($"{key} = {dict[key]} ({dict[key].GetType()})");
}

public static class JsonReaderExtensions
{
    public static object GetNumber(this Utf8JsonReader jsonReader)
    {
        return TryGetNumber(jsonReader, out var number)
            ? number!
            : throw new FormatException($"Could not read JSON number value '{Encoding.UTF8.GetString(jsonReader.ValueSpan)}' as a .NET number type.");
    }

    public static bool TryGetNumber(this Utf8JsonReader jsonReader, out object? number)
    {
        // TODO: Optimize by reusing common boxed values
        var value = jsonReader.ValueSpan;
        if (value.Contains("."u8[0]))
        {
            // Floating point number
            if (jsonReader.TryGetDouble(out var dblValue))
            {
                number = dblValue switch
                {
                    >= float.MinValue and <= float.MaxValue => (object)(float)dblValue,
                    _ => dblValue
                };
                return true;
            }
        }
        else
        {
            if (jsonReader.TryGetInt64(out var longValue))
            {
                number = longValue switch
                {
                    >= short.MinValue and <= short.MaxValue => (object)(short)longValue,
                    >= int.MinValue and <= int.MaxValue => (object)(int)longValue,
                    _ => longValue
                };
                return true;
            }
            else
            {
                // Possibly too big for int64, try parsing as int128
                if (Int128.TryParse(value, out var int128Value))
                {
                    number = int128Value;
                    return true;
                }

                if (value.Length <= 256)
                {
                    // Too big for Int128, try BigInteger
                    // TODO: Handle case where length of digits is more than we want to stackalloc
                    Span<char> charSpan = stackalloc char[256];
                    var status = Utf8.ToUtf16(value, charSpan, out var bytesRead, out var charsWritten);
                    if (status == System.Buffers.OperationStatus.Done && BigInteger.TryParse(charSpan[..charsWritten], out var bigInt))
                    {
                        number = bigInt;
                        return true;
                    }
                }
            }
        }

        number = null;
        return false;
    }
}

ProgramOutput.txt

anInt = 123 (System.Int16)
negativeInt = -123 (System.Int16)
biggerInt = 1234567890 (System.Int32)
hugeInt = 123456789012345678901234567890 (System.Int128)
massiveInt = 123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890 (System.Numerics.BigInteger)
float = 123.456 (System.Single)
negativeFloat = -123.456 (System.Single)
exponential = 299792450 (System.Single)
negativeExponential = -299792450 (System.Single)
bigFloat = 1.2345678901234567E+59 (System.Double)
negativeBigFloat = -1.2345678901234567E+59 (System.Double)