using System;

public class Program

{

    public static void Main()

    {

​

        string a = "1000";

        string b = "1000.00";

        string c = "1000.0";

        string d = "1,000.5";

        Console.WriteLine(InsertThousandsSeparator(a));

        Console.WriteLine(InsertThousandsSeparator(b));

        Console.WriteLine(InsertThousandsSeparator(c));

        Console.WriteLine(InsertThousandsSeparator(d));

    }

    static string InsertThousandsSeparator(string input) {

        var dec = decimal.Parse(input);

        var bits = decimal.GetBits(dec);

        var prec = bits[3] >> 16 & 255;

        return dec.ToString("N" + prec);

    }

​

}