Template service preview

using System;
using System.Globalization;

public class Program
{
    public static void Main()
    {
        double val = 123456.789;
        string format = "#,.##0.00";
        string culture = "RU";

        var formattedValue = GetFormattedValue(val, format, culture);

        Console.WriteLine(formattedValue);
    }

    private static string GetFormattedValue(double? val, string format, string culture)
    {
      var formatString = format ?? "0.0";
      var signFormat =
        !formatString.StartsWith("#") && !formatString.StartsWith("+") ? "" :
        val <= 0                                                       ? "" :
                                                                         "+";

      format = $"{signFormat}{formatString}";

      var ci = CultureInfo.GetCultureInfo(culture);
      var pos = format.IndexOf(',');
      if (pos != -1 && format.Length > pos + 1 && format[pos + 1] != '0')
      {
        var delimiter = format[pos + 1].ToString();
        ci = (CultureInfo) ci.Clone();
        ci.NumberFormat.CurrencyGroupSeparator = delimiter;
        ci.NumberFormat.PercentGroupSeparator = delimiter;
        ci.NumberFormat.NumberGroupSeparator = delimiter;
        format = format.Replace("," + delimiter, ",");
      }

      pos = format.IndexOf('.');
      if (pos != -1 && format.Length > pos + 1 && format[pos + 1] != '0')
      {
        var point = format[pos + 1].ToString();
        ci = (CultureInfo) ci.Clone();
        ci.NumberFormat.NumberDecimalSeparator = point;
        ci.NumberFormat.PercentDecimalSeparator = point;
        ci.NumberFormat.CurrencyDecimalSeparator = point;
        format = format.Replace("." + point, ".");
      }

      return val.Value.ToString(format, ci);
    }

}