Double.Parse rounding bug when there are trailing zeroes in the input string

[jfd16]

Description

Code to reproduce:

using System;

class Program {
    public static void Main(string[] args) {
        Console.WriteLine("{0:R}", double.Parse("9007199254740997"));
        Console.WriteLine("{0:R}", double.Parse("900719925474099.7e+1"));
        Console.WriteLine("{0:R}", double.Parse("9007199254740997.0"));
        Console.WriteLine("{0:R}", double.Parse("90071992547409970e-1"));
    }
}

Actual output:

9007199254740996
9007199254740996
9007199254740998
9007199254740998

Expected: All four outputs should be the same. (9007199254740996 if rounding to even)

Configuration

• Which version of .NET is the code running on? .NET 5
• What OS and version, and what distro if applicable? Windows 10
• What is the architecture (x64, x86, ARM, ARM64)? x64
• Do you know whether it is specific to that configuration? No

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Issue Details

---

Description

Code to reproduce:

using System;

class Program {
    public static void Main(string[] args) {
        Console.WriteLine("{0:R}", double.Parse("9007199254740997"));
        Console.WriteLine("{0:R}", double.Parse("900719925474099.7e+1"));
        Console.WriteLine("{0:R}", double.Parse("9007199254740997.0"));
        Console.WriteLine("{0:R}", double.Parse("90071992547409970e-1"));
    }
}

Actual output:

9007199254740996
9007199254740996
9007199254740998
9007199254740998

Expected: All four outputs should be the same. (9007199254740996 if rounding to even)

Configuration

• Which version of .NET is the code running on? .NET 5
• What OS and version, and what distro if applicable? Windows 10
• What is the architecture (x64, x86, ARM, ARM64)? x64
• Do you know whether it is specific to that configuration? No

Author:	jfd16
Assignees:	-
Labels:	area-System.Numerics, area-System.Runtime, untriaged
Milestone:	-

[pgovind]

Looks like a bug. I'll investigate this as I work on #46874 next week!

[pgovind]

Yup, reproduced it locally. Will look into this further this week.

This happens because of rounding due to the call here. The .0 is (correctly) interpreted as having a fractional part (=> HasZeroTail is false) that results in blind rounding. I'm not sure what the fix should be yet. Investigating...

Meanwhile, how did you find this bug?