Round-trip Double ToString/Parse failure with some cultures and exponential notation

[paulirwin]

Found via random test in Lucene.NET. J2N.Numerics.Double has a round-trip failure parsing its own output for cultures with Unicode minus sign as its NegativeSign and exponential notation values where the exponent is negative.

using J2N.Numerics;
using System.Globalization;

var sv = CultureInfo.GetCultureInfo("sv-FI");
var d = 6.815791e-4d;

Console.WriteLine(Double.ToString(d, sv)); // prints: "6,815791E-4"
Double.Parse(Double.ToString(d, sv), sv); // fails with FormatException: "Input string was not in a correct format. Value: '6,815791E-4'."

Note that this works with cultures with "-" (int value 45) as the NegativeSign, such as en-US.

[paulirwin]

Found the root cause, investigating a possible solution. The sv-FI culture has as its NegativeSign the character with integer value 8722, which is a Unicode minus sign, but the formatted string uses the regular '-' character for the exponent, which is integer value 45. Because of this mismatch, it does not parse the exponent correctly and fails. I am guessing it's just neglecting to use the NegativeSign for the exponent formatting.