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

It might be best to work with the string generated by your .ToString():

class Program
{
    static string InsertSeparators(string s)
    {
        string decSeparator = System.Threading.Thread.CurrentThread.CurrentCulture.NumberFormat.NumberDecimalSeparator;

        int separatorPos = s.IndexOf(decSeparator);
        if (separatorPos >= 0)
        {
            string decPart = s.Substring(separatorPos + decSeparator.Length);
            // split the string into parts of 3 or less characters
            List parts = new List();
            for (int i = 0; i < decPart.Length; i += 3)
            {
                string part = "";
                for (int j = 0; (j < 3) && (i + j < decPart.Length); j++)
                {
                    part += decPart[i + j];
                }
                parts.Add(part);
            }
            string groupSeparator = System.Threading.Thread.CurrentThread.CurrentCulture.NumberFormat.NumberGroupSeparator;
            s = s.Substring(0, separatorPos) + decSeparator + String.Join(groupSeparator, parts);
        }

        return s;

    }

    static void Main(string[] args)
    {

        for (int n = 0; n < 15; n++)
        {
            string s = Math.PI.ToString("0." + new string('#', n));
            Console.WriteLine(InsertSeparators(s));
        }

        Console.ReadLine();

    }
}

Outputs:

3
3.1
3.14
3.142
3.141,6
3.141,59
3.141,593
3.141,592,7
3.141,592,65
3.141,592,654
3.141,592,653,6
3.141,592,653,59
3.141,592,653,59
3.141,592,653,589,8
3.141,592,653,589,79