Inconsistency of System.Text.Json behavior between .NET Framework and .NET Core

[DenSmoke]

JsonSerializer.Serialize works differently with double depending on target.
I wrote simple test to check System.Text.Json behavior comparing to Newtonsoft.Json

public struct TestStruct
{
    public TestStruct(double number)
    {
        DoubleField = number;
    }

    public double DoubleField { get; set; }
}

public class JsonTest
{

    [Theory]
    [InlineData(0.99999999999999)]
    [InlineData(0.999999999999999)]
    [InlineData(0.9999999999999999)]
    [InlineData(0.99999999999999999)]
    public void SystemTextJsonTest(double number)
    {
        var data = new TestStruct(number);
        var datastring = JsonSerializer.Serialize(data);
        var deserializedData = JsonSerializer.Deserialize<TestStruct>(datastring);
        Assert.Equal(data.DoubleField, deserializedData.DoubleField);
    }

    [Theory]
    [InlineData(0.99999999999999)]
    [InlineData(0.999999999999999)]
    [InlineData(0.9999999999999999)]
    [InlineData(0.99999999999999999)]
    public void NewtonsoftJsonTest(double number)
    {
        var data = new TestStruct(number);
        var dataString = Newtonsoft.Json.JsonConvert.SerializeObject(data);
        var deserializedData = Newtonsoft.Json.JsonConvert.DeserializeObject<TestStruct>(dataString);
        Assert.Equal(data.DoubleField, deserializedData.DoubleField);
    }
}

Results

For some reason JsonSerializer.Serialize rounds double when it exceeds 17 digits, but JsonConvert.Serialize doesn't.

I have ASP.NET Core Web API which is sending good JSON provided by System.Text.Json and this JSON is being deserialized on .NET Framework without problems so the problem is only with JsonSerializer.Serialize

[stephentoub]

cc: @tannergooding

[Wraith2]

Probably a consequence of https://devblogs.microsoft.com/dotnet/floating-point-parsing-and-formatting-improvements-in-net-core-3-0/

[ahsonkhan]

As I understand it, we use "G17" on full framework as part of this change dotnet/corefx#38322 since the defaults produce fewer strings that round-trip.

I believe the inconsistency you observed is expected due to the limitations in netfx (https://github.com/dotnet/corefx/issues/40406#issuecomment-522552006).

https://github.com/dotnet/corefx/blob/3d72b56b8e7d1e4e44262be6c45c2aac580bbbc4/src/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Double.cs#L98-L142

[ericstj]

@tannergooding can you confirm this is by-design?

[tannergooding]

Yes, this is by design; as per my explanation in the comment(s) @ahsonkhan linked to above.