How to use NumberFormatInfo

I want to be able to format any decimal number as I want.
Specifically I want to be able to control:

1. the number of decimals
2. the decimal separator
3. the thousand separato

I have tried to do that

var nfi = new NumberFormatInfo();
nfi.NumberDecimalDigits= 4;       <----- for point 1
nfi.NumberDecimalSeparator = "@"; <----- for point 2
nfi.NumberGroupSeparator = "^";   <----- for point 3? or
nfi.PerMilleSymbol= "#";          <----- for point 3?

and then I have applied it:

but for some reasons I can’t get to have what I expect.

If I look here it seems that it applies to integers?

And also this answer is not helping

Thanks

Patrick

>Solution :

The formatting properties NumberDecimalDigits, NumberDecimalSeparator, NumberGroupSeparator and PerMilleSymbol do not apply to all format strings. To determine their applicability you need to check the docs:

• NumberDecimalDigits: The NumberDecimalDigits property is used with the "F" and "N" standard format strings without a precision specifier in numeric formatting operations.
• NumberDecimalSeparator: 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.
• NumberGroupSeparator: The NumberGroupSeparator property is used with the "N" standard format string to define the symbol that separates groups of integral digits.
• PerMilleSymbol: 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 "‰" .

Since, as noted in the doc remarks for ToString(IFormatProvider):

The ToString(IFormatProvider) method formats a Double value in the default ("G", or general) format of a specified culture.

Only NumberDecimalSeparator applies in this format. You need to pass formats for which you the other options are implemented to see their effect. For instance:

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}\", nfi) = {1}", formatString, str);
}

Results in:

aaa.ToString("G", nfi) = -123456@789
aaa.ToString("N", nfi) = -123^456@7890
aaa.ToString("F", nfi) = -123456@7890
aaa.ToString("P", nfi) = -12,345,678.90 %
aaa.ToString("#0.## ‰", nfi) = -123456789 #

Demo fiddle here: https://dotnetfiddle.net/81HXz9