Fun.cs

public static string Fun(int num)
{
    if (num == 0)
    {
        return "0";
    }

    var result = new StringBuilder();
    var count = 0;

    while (num > 0 && ++count > 0)
    {
        var digit = num % 10;
        num = (num - digit) / 10;
        result.Insert(0, digit);

        if (num > 0 && count % 3 == 0)
        {
            result.Insert(0, ",");
        } 
    }

    return result.ToString();
}