String.Format - How can I format to x digits (regardless of decimal place)?

Question

I need to format a floating point number to x characters (6 in my case including the decimal point). My output also needs to include the sign of the number

So given the inputs, here are the expected outputs

1.23456   => +1.2345

-12.34567 => -12.345

-0.123456 => -0.1234

1234.567  => +1234.5

Please assume there is always a decimal place before the last character. I.e. there will be no 12345.6 number input - the input will always be less than or equal to 9999.9.

I'm thinking this has to be done conditionally.

Answer 1

Here's a transparent way to do it without format strings (except for "F"):

  static void Main()
  {
     double y = 1.23456;
     Console.WriteLine(FormatNumDigits(y,5));
     y= -12.34567;
     Console.WriteLine(FormatNumDigits(y,5));
     y = -0.123456;
     Console.WriteLine(FormatNumDigits(y,5));
     y = 1234.567;
     Console.WriteLine(FormatNumDigits(y,5));

     y = 0.00000234;
     Console.WriteLine(FormatNumDigits(y,5));

     y = 1.1;
     Console.WriteLine(FormatNumDigits(y,5));
  }


  public string FormatNumDigits(double number, int x) {
     string asString = (number >= 0? "+":"") + number.ToString("F50",System.Globalization.CultureInfo.InvariantCulture);

     if (asString.Contains('.')) {
        if (asString.Length > x + 2) {
           return asString.Substring(0, x + 2);
        } else {
           // Pad with zeros
           return asString.Insert(asString.Length, new String('0', x + 2 - asString.Length));
        }
     } else {
        if (asString.Length > x + 1) {
           return asString.Substring(0, x + 1);
        } else {
           // Pad with zeros
           return asString.Insert(1, new String('0', x + 1 - asString.Length));
        }
     }
  }

Output:

  +1.2345
  -12.345
  -0.1234
  +1234.5
  +0.0000
  +1.1000

EDIT

Notice that it does not chop off trailing zeros.