Number formatter

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("This program will take a number to be formatted and another number that\nspecifies the number of decimal places the formatted number should have.\n");
            double num = (double)GetNumber(0);
            int dPlaces = (int)GetDecimalPlaces(0);
            Console.WriteLine($"\nYour number, after formatting: {FormatNumber(num, dPlaces)}");
            Console.ReadKey();
            Console.Clear();
        }

        static object GetNumber(int attempts)
        {
            if (attempts > 0)
                Console.WriteLine("\nYour input is invalid. Try again.\n");
            Console.WriteLine("Enter a number...\n");
            double num;
            if (double.TryParse(Console.ReadLine(), out num))
                return num;
            return GetNumber(++attempts);
        }

        static object GetDecimalPlaces(int attempts)
        {
            if (attempts > 0)
                Console.WriteLine("\nYour input is invalid. Try again.");
            Console.WriteLine("\nEnter the number of decimal places...\n");
            int num;
            if (Int32.TryParse(Console.ReadLine(), out num))
                return num;
            return GetDecimalPlaces(++attempts);
        }

        static string FormatNumber(double num, int dPlaces)
        {
            string nStr = num.ToString();
            bool positive = nStr.IndexOf('-') == -1;
            if (!positive)
                nStr = nStr.Substring(1);
            int decimalSet = (int)Math.Pow(10, dPlaces);
            nStr = (Math.Round(double.Parse(nStr.Replace(",", "")) * decimalSet) / decimalSet).ToString();
            int decimalIndex = nStr.IndexOf('.');
            string decimalValue = decimalIndex != -1 ? nStr.Substring(decimalIndex) : "";
            nStr = decimalIndex == -1 ? nStr : nStr.Substring(0, decimalIndex);
            int numLength = nStr.Length;
            int leadingGroup = numLength % 3;
            List<string> returnNumList = new List<string>();
            string returnNum = "";
            if (leadingGroup > 0)
                returnNumList.Add(nStr.Substring(0, leadingGroup));
            for (int i = leadingGroup; i < numLength; i += 3)
                returnNumList.Add(nStr.Substring(i, 3));
            returnNum = String.Join(",", returnNumList);
            if (!string.IsNullOrWhiteSpace(decimalValue))
                returnNum += decimalValue;
            if (decimalValue.Length < (dPlaces + 1))
            {
                if (returnNum.IndexOf('.') == -1)
                    returnNum += ".";
                returnNum += new string('0', dPlaces - (decimalValue.Length - 1));
            }
            if (!positive)
                if (returnNum[0] == ',')
                    returnNum.Replace(returnNum[0], '-');
                else
                    returnNum = '-' + returnNum;
            if (double.IsNaN(double.Parse(returnNum.Replace(",", ""))))
                returnNum = "0.00";
            return returnNum;
        }
    }
}