using System;

using System.Globalization;

using System.Text;

using System.Text.RegularExpressions;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        string[] tests = ["123,45", "1.234,56", "15.456.789,52000", "1,234,567.8900000000", "10.00000000", "10,00000000", "123456,897827", "12978312.10231"];

        foreach (var t in tests) {

            decimal? res = ParseDecimal(t);

            if (res != null) Console.WriteLine("RESULT: " + res);

        }

    }

​

    public static decimal? ParseDecimal(string input)

    {

        if (string.IsNullOrEmpty(input.Trim()))

            return null;

        decimal result = 0;

        int dotsCount = input.Count(c => c == '.');

        int commasCount = input.Count(c => c == ',');

        Console.WriteLine($"INPUT: {input}\tDOTS: {dotsCount}\tCOMMAS: {commasCount}");

        NumberFormatInfo dotSeparator = new NumberFormatInfo

        {

            NumberDecimalSeparator = ".",

            NumberGroupSeparator = ","

        };

        NumberFormatInfo commaSeparator = new NumberFormatInfo

        {

            NumberDecimalSeparator = ",",

            NumberGroupSeparator = "."

        };

        try

        {

            if (commasCount > 0 && dotsCount > commasCount)

            {

                result = decimal.Parse(input, commaSeparator);

                return result;

            }

​

            if (dotsCount > 0 && commasCount > dotsCount)

            {

                result = decimal.Parse(input, dotSeparator);

                return result;

            }

​

            if (dotsCount == 1 && commasCount == 1)

            {

                string[] commaSplit = input.Split(',');

                if (commaSplit[0].Length >= commaSplit[1].Length)

                {

                    result = decimal.Parse(input, commaSeparator);

                    return result;

                }

                else

                {

                    result = decimal.Parse(input, dotSeparator);

                    return result;

                }

            }

            if (dotsCount == 1 && commasCount == 0) {

                result = decimal.Parse(input);

            } else {

                result = decimal.Parse(input, new CultureInfo("es-ES"));

            }

​

            return result;

        }

        catch (Exception ex)

        {

            Console.WriteLine($"ERROR: {ex.Message}");

            return null;

        }

    }

}