Adding comma after each two digits in String.Format

Question

I was trying to format a number 173910234.23 to something like 17,39,10,234.23 . Here only the first thousand separator is after three digits. But after that all separators ( , ) are after two digits. I have tried the following -

double d = 173910234.23;
Console.WriteLine(string.Format("{0:#,##,##,###.00}", d));

but it gives output with comma after every three digits, 173,910,234.23

How can I achieve the format 17,39,10,234.23 using string.Format ?

Answer 1

Number groups are defined by NumberGroupSizes property of NumberFormatInfo . So modify it accordingly and just use N format specifier.

double d = 173910234.23;
var culture = new CultureInfo("en-us", true)
{
    NumberFormat =
    {
        NumberGroupSizes = new int[] { 3, 2 }
    }
};
Console.WriteLine(d.ToString("N", culture));

This outputs

17,39,10,234.23

---

Thanks @Rawling and @Hamlet, for shedding light on this. Now OP gets expected output and me too learned something..

Answer 2

string output = null;    
string num = Number.ToString();
for(int i = 0; i < num.Length; ++i)
 {
        switch(i)
        case 2:
        case 4:
        case 6:
            output += ",".ToString() + num[i].ToString(); 
        break;
        default:
            output += num[i].ToString(); 
        break:
 }

If you want to use a different number, replace the numbers with the positions of the numbers where you want to insert the comma before (remember the first character of a string has the position of 0 )