using System;

using System.Globalization;

using System.Text;

​

//https://stackoverflow.com/questions/77982158/how-to-use-numberformatinfo

public class Program

{

    public static void Main()

    {

​

        var nfi = new NumberFormatInfo();

​

        // https://learn.microsoft.com/en-us/dotnet/api/system.globalization.numberformatinfo.numberdecimaldigits?view=net-8.0#remarks

        // The NumberDecimalDigits property is used with the "F" and "N" standard format strings without a precision specifier in numeric formatting operations.

        nfi.NumberDecimalDigits = 4;       //<----- for point 1

        //https://learn.microsoft.com/en-us/dotnet/api/system.globalization.numberformatinfo.numberdecimalseparator?view=net-8.0#remarks

        //The NumberDecimalSeparator property is used with the "E", "F", "G", "N", and "R" standard format strings to define the symbol that separates integral from fractional digits. 

        nfi.NumberDecimalSeparator = "@"; //<----- for point 2

        //https://learn.microsoft.com/en-us/dotnet/api/system.globalization.numberformatinfo.numbergroupseparator?view=net-8.0#remarks

        //The NumberGroupSeparator property is used with the "N" standard format string to define the symbol that separates groups of integral digits. For more information, see Standard Numeric Format Strings.

        nfi.NumberGroupSeparator = "^";   //<----- for point 3? or

        //https://learn.microsoft.com/en-us/dotnet/api/system.globalization.numberformatinfo.permillesymbol?view=net-8.0#remarks

        //The string assigned to the PerMilleSymbol property is included in the result string when a numeric value is formatted with a format string that includes the "‰" .

        //https://learn.microsoft.com/en-us/dotnet/standard/base-types/custom-numeric-format-strings#the--custom-specifier-4

        //A per mille character (‰ or \u2030) in a format string causes a number to be multiplied by 1000 before it is formatted. The appropriate per mille symbol is inserted in the returned string at the location where the ‰ symbol appears in the format string.

        nfi.PerMilleSymbol = "#";          //<----- for point 3?

        double aaa = -123456.789;

        foreach (var formatString in new [] { "G", "N", "F", "P", "#0.## " + '\u2030'})

        {

            string str = aaa.ToString(formatString, nfi);

            Console.WriteLine("aaa.ToString(\"{0}\") = {1}", formatString, str);

        }

    }

}