Decimal group seperator for the fractional part

Question

I wonder what would be the best way to format numbers so that the NumberGroupSeparator would work not only on the integer part to the left of the comma, but also on

Answer 1

A little clunky, and it won't work for scientific numbers but here is a try:

class Program
{
    static void Main(string[] args)
    {
        var π=Math.PI*10000;

        Debug.WriteLine(Display(π));
        // 31,415.926,535,897,931,899
    }

    static string Display(double x)
    {
        int s=Math.Sign(x);
        x=Math.Abs(x);
        StringBuilder text=new StringBuilder();
        var y=Math.Truncate(x);
        text.Append((s*y).ToString("#,#"));
        x-=y;
        if (x>0)
        {
            // 15 decimal places is max reasonable precision
            y=Math.Truncate(x*Math.Pow(10, 15));
            text.Append(".");
            text.Append(y.ToString("#,#").TrimEnd('0'));
        }
        return text.ToString();
    }

}