How to know if a double string is round-trip safe?

Question

I have a text representation of a double and want to know if it's safe to round-trip it to double and back. How do I know this if I also want to accept any kind of number-style of the input? Or how do I know if any precision is lost when a double-string is parsed with Double.Parse? Or how do I ToString a double to match the same format as another double-string? An answer to any of these questions would be a solution I think.

Answer 1

Use the R format specifier to convert the double to a string:

myDouble.ToString("R")

See The Round-trip ("R") Format Specifier on MSDN.

The round-trip ("R") format specifier guarantees that a numeric value that is converted to a string will be parsed back into the same numeric value. This format is supported only for the Single, Double, and BigInteger types.

(emphasis mine)

Answer 2

Of course, this is not round-trip safe for several reasons:

1. The number format gets lost when parsing a String to a double, since the double does not contain any information about its visual representation.

2. While the string representation will normally be decimal, double is a binary floating point number. So 0.1 in double will not be exactly 0.1, since the binary representation of 0.1 does not have finitely many digits. You can parse to decimal instead of double in order to avoid this problem.

I would suggest building a struct that stores both the number and the string representation.