How do I format a number in C# with commas and decimals?

Not sure (and unable to test right now) but would something like this work?

"#,##0.################"

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

will give you up to 12 decimal places.

There is not a custom format specifier for "all following digits", so something like this will be closest to what you want.

Note too that you're limited by the precision of your variable. A double only has 15-16 digits of precision, so as your left-hand side gets bigger the number of decimal places will fall off.

UPDATE: Looking at the MSDN documentation on the System.Double type, I see this:

By default, a Double value contains 15 decimal digits of precision, although a maximum of 17 digits is maintained internally.

So I think pdr's on to something, actually. Just do this:

// As long as you've got at least 15 #s after the decimal point,
// you should be good.
value.ToString("#,#.###############");

---

Here's an idea:

static string Format(double value)
{
    double wholePart = Math.Truncate(value);
    double decimalPart = Math.Abs(value - wholePart);
    return wholePart.ToString("N0") + decimalPart.ToString().TrimStart('0');
}

Example:

Console.WriteLine(Format(42023212.0092343234));

Output:

42,023,212.00923432409763336

Ha, well, as you can see, this gives imperfect results, due (I think) to floating point math issues. Oh well; it's an option, anyway.