Double serialization in net481 accuracy problems

[gregsdennis]

Description

Using System.Text.Json serialization for double values serializes inaccurately in .Net Framework versions.

Note that it works fine in .Net Core+ versions.

Reproduction Steps

[Test]
public void RawValue()
{
    var expected = "4.2";

    var actual = JsonSerializer.Serialize(4.2, _options);

    Assert.That(actual, Is.EqualTo(expected)); // actual: 4.2000000000000002
}

Expected behavior

I expect that it would serialize to 4.2 as that's the original value, and as it works in .Net Core+ versions.

Actual behavior

double serializes to an approximation.

Regression?

Unknown, guessing not.

Known Workarounds

None.

Configuration

Built and run under .Net 9.0.100 on Windows 11 x86
Using System.Text.Json 9.0.0

Other information

No response

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[tannergooding]

The behavior of floating-point parsing and formatting in .NET Framework has essentially always been buggy, but it can't be fixed due to the much higher compatibility bar that exists.

This is covered in more detail as part of https://devblogs.microsoft.com/dotnet/floating-point-parsing-and-formatting-improvements-in-net-core-3-0/, which details the breaking change that was made to fix this behavior in .NET Core 3.0 and later.