Issue with System.Text.Json Serialization of Floating Point Numbers in C#

Question

I'm encountering an issue with System.Text.Json in C# related to the serialization of floating-point numbers. When serializing a float with a decimal part (e.g., 4.0), the library seems to omit the decimal part, resulting in the JSON representation being just 4 instead of 4.0.

I've tried using various configurations, including setting NumberHandling = JsonNumberHandling.AllowNamedFloatingPointLiterals and NumberHandling = JsonNumberHandling.WriteAsString, but the behavior remains the same.

Is there a way to preserve the complete representation (e.g., 4.0) during serialization, or is there a known limitation with the current version of System.Text.Json?

Thank you for any insights or suggestions.

Answer 1

JSON does not distinguish between float and int, it only has a concept of Number so in general you should avoid relying on the particular number formatting.

If currently you can't fix the app which uses the JSON you can use custom converter which will use specific number format. For example:

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

    public override void Write(Utf8JsonWriter writer, float value, JsonSerializerOptions options)
    {
        // at least one decimal place, add reasonable amount of places with #
        writer.WriteRawValue(value.ToString("0.0########", CultureInfo.InvariantCulture));
    }
}

And usage:

var serialize = JsonSerializer.Serialize(new float[] { 1, 1.1f, 4.5555f }, new JsonSerializerOptions
{
    Converters = { new FloatConverter() }
});
Console.WriteLine(serialize); // prints "[1.0,1.1,4.5555]"