c# how to String.Format decimal with unlimited decimal places?

Question

I need to convert a decimal number to formatted string with thousand groups and unlimited (variable) decimal numbers:

1234 -> "1,234"
1234.567 -> "1,234.567"
1234.1234567890123456789 -> "1,234.1234567890123456789"

I tried String.Format("{0:#,#.#}", decimal), but it trims any number to max 1 decimal place.

Answer 1

You can use # multiple times (see Custom Numeric Format Strings):

string.Format("{0:#,#.#############################}", decimalValue)

Or, if you're just formatting a number directly, you can also just use decimal.ToString with the format string.

However, there is no way to include "unlimited decimal numbers". Without a library supporting arbitrary precision floating point numbers (for example, using something like BigFloat from Extreme Numerics), you'll run into precision issues eventually. Even the decimal type has a limit to its precision (28-29 significant digits). Beyond that, you'll run into other issues.

Answer 2

As I've said, the decimal type has a precision of 28-29 digits.

decimal mon = 1234.12345678901234567890123M;
var monStr = mon.ToString("#,0.##############################");
var monStr2 = String.Format("{0:#,0.##############################}", mon);

Here there are 30x# after the decimal separator :-)

I've changed one # with 0 so that 0.15 isn't written as .15 .