Format a number with commas, but keep decimals

Question

I'd like to group the digits in a double by thousands, but also output however number of decimals are actually in the number. I cannot figure out the format string.

 1000 => 1,000
 100000 => 100,000
 123.456 => 123.456
 100000.21 => 100,000.21
 100200.123456 => 100,200.123456

Disclaimers (it's not as straight forward as you think):

• Please do not point me to this question it does not answer my question
• Please do not point me to MSDN, the articles have not helped me. (If you believe I'm reading them incorrectly, read them yourself and post the correct answer)
• .ToString("n") does not work, it rounds the digits

Answer 1

This appears to do exactly what you want:

public void Code(params string[] args)
{
    Print(1000);
    Print(100000);
    Print(123.456);
    Print(100000.21 );
    Print(100200.123456);
}

void Print(double n)
{
    Console.WriteLine("{0:###,###.#######}", n);
}

1,000
100,000
123.456
100,000.21
100,200.123456

Answer 2

An old thread, but none of the answers looks entirely satisfactory to me. Most of them turn zero into the empty string, including the one with the most upvotes.

I think a better solution is "#,##0.#################", which does at least show zeroes. It's still ugly though. There must be a better way.

double[] vals = new double[] { 0.0, 0.1234, -0.1234, 1.0, 
                               123456789.123456, 123456789.0, 
                               0.123456789123456 };
foreach (double val in vals) 
{ 
    Console.WriteLine(val + ": " + val.ToString("#,##0.#################")); 
}