Double ToString gives rounded result

Question

Why does this simple Console App Output 100000000000000 instead of 99999999999999.99 or atleast 99999999999999.984 as the debugger is showing?

static void Main(string[] args)
{
    double bigNum = 99999999999999.99;
    Console.WriteLine(bigNum); //Console.WriteLine will internally ToString, but you can add what ever ToString() you want here the result will always be undesired
}

Console Output:

Nope

Debug Output:

For clarification:

Yes, I know of the value type decimal and its superiour precision over double, but I never heard of that ToString() of double could give wrong results.

Possible duplicate of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Robert Columbia, Apr 26 '18 at 14:49
`double` has limited precision, so it will only store the first 15-16 digits of your number. For more precision, use `System.Numerics.BigInteger` - it doesn't support non-integers though, so you must add the decimal points yourself (if you have a fixed number of decimals). — Didix, Apr 26 '18 at 14:51
Try using `bigNum.ToString("R")` or `Console.WriteLine("{0:R}", bigNum);` — Matthew Watson, Apr 26 '18 at 14:51
@RobertColumbia I think the question is different - "why debugger does not match `double.ToString("G")`" rather than generic explanation of double's behavior. (Note that https://learn.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings#GFormatString tells you to use `"G17"` if you want to keep all digits, by default `"G"` is `"G15"`) — Alexei Levenkov, Apr 26 '18 at 14:52

Stefan · Accepted Answer · 2018-04-26T15:05:38.497

2

See: MSDN for the standard ToString conversion:

If format is null or an empty string, the return value is formatted with the general numeric format specifier ("G").

This will show you, that the defaulted number format specifier "G", will default to max 15 digits if possible: G-format specifier

As requested:

double bigNum = 99999999999999.99;

Console.WriteLine(bigNum);                 //default
//100000000000000
Console.WriteLine(bigNum.ToString("G"));   //explicit default
//100000000000000
Console.WriteLine(bigNum.ToString("G17")); //G17
//99999999999999.984

edited Apr 26 '18 at 15:05

answered Apr 26 '18 at 14:55

Stefan

17,448
11
60
79

Thx, do you know how I could do this with the thousands seperator in use? So, how to fix my issue with `.ToString("N2")` ? Dont think I can combine G and N ?!? – Rand Random Apr 26 '18 at 14:59
You can try: `Console.WriteLine(bigNum.ToString("G17"));` I'll run a test to see if it works. – Stefan Apr 26 '18 at 15:01
Nope, this results in the correct value (as the debugger) but without the thousand seperators. – Rand Random Apr 26 '18 at 15:04
Ah, sorry, I misunderstood, I thought you meant the decimal sepperator. – Stefan Apr 26 '18 at 15:06
I'll need to check that, one moment – Stefan Apr 26 '18 at 15:10
No problem, take your time. – Rand Random Apr 26 '18 at 15:16
Sorry, no luck. I tried a custom `NumberInfoFormat` and some `###.###...etc` styles but they all tend to round the number to 1000000.. – Stefan Apr 26 '18 at 15:24
To bad :( - seems like I have to do my own stuff - gona try "G17" and than add the thousand seperator. – Rand Random Apr 26 '18 at 15:30

Double ToString gives rounded result

1 Answers1