Fixed-point number format specifier rounding of doubles?

Question

I have a pretty decent understanding of IEEE 754 so this is not one of those "why does adding number a and number b result in..."-type of questions.

Rather I want to ask if I've understood the fixed-point number-format specifier correctly because it's not behaving as I would expect for some double values.

For example:

double d = 0x3FffffFFFFfffe * (1.0 / 0x3FffffFFFFffff);
Console.WriteLine(d.ToString("R"));
Console.WriteLine(d.ToString("G20"));
Console.WriteLine(d.ToString("F20"));

Both the "R" and "G" specifier prints out the same thing - the correct value of: 0.99999999999999989 but the "F" specifier always rounds up to 1.0 no matter how many decimals I tell it to include. Even if I tell it to print the maximum number of 99 decimals ( "F99" ) it still only outputs "1."-followed by 99 zeroes.

So is my understanding broken, and can someone point me to the relevant section in the spec, or is this behavior broken? (It's no deal-breaker for me, I just want to know.)

Here is what I've looked at, but I see nothing explaining this.

(This is .Net4.0)

Answer 1

User "wb" linked to another question which I suspect has the best possible answer available for this question... although there was some fuzzy-ness on the details and documentation. (Unfortunately the comment was removed.)

The linked question was Formatting doubles for output in C# .

And in short it appears to state that unless you use the G or R specifier the output is always reduced to 15 decimal-places before custom formatting. Best doc anyone was able to link to in that question was this MSDN page . The details and wording isn't as crystal clear as I would wish, but as stated I think this is the best I'm going to find.