C# Convert string to double/decimal and back to st

Question

I am trying to get user input, parse it and then display with String.Format(), formatting thousands with comas.

So, if user provides
1000  I will display 1,000
1000.00 => 1,000.00
1000.0  => 1,000.0
1,000.5 => 1,000.5

Basically I want to keep all decimals(including trailing zeroes) that were provided and just add formatting for thousands. I tried:

String.Format("{0:#,0.######}" , Decimal.Parse(input));
String.Format("{0:#,0.######}" , Double.Parse(input);

Answer 1

double.Parse(input) is a no go, as double does not keep track of the number of decimals.

decimal.Parse(input).ToString() will show that decimal does keep track of that. Unfortunately, decimal.Parse(input).ToString() uses this precision and doesn't use a thousands separator, and decimal.Parse(input).ToString("N") ignores the precision but does use a thousands separator.

Manually extracting the precision from the decimal works though, and that allows you to build the correct format string:

static string InsertThousandsSeparator(string input) {
    var dec = decimal.Parse(input);
    var bits = decimal.GetBits(dec);
    var prec = bits[3] >> 16 & 255;
    return dec.ToString("N" + prec);
}

This is based on the layout of decimal as described on MSDN:

Bits 16 to 23 must contain an exponent between 0 and 28, which indicates the power of 10 to divide the integer number.

See it working on .NET Fiddle. (courtesy of @Alisson)

Answer 2

You can use regex for this:

var input = "1000.50000";
var regex = new Regex("^(?<int>-?[\\d,]+)\\.(?<fract>\\d+)$");
var match = regex.Match(input);
var integralPart = match.Groups["int"].Value.Replace(",", "");
var fractionalPart = match.Groups["fract"].Value;
var result = $"{decimal.Parse(integralPart):n0}.{fractionalPart}";

Answer 3

If you don't mind 1000 converted to 1,000.00 then following is the best as it considers user's locale too.

string output = String.Format("{0:n}", input);

If you want 1000 converted to just 1,000, then you can use the following:

string output2 = String.Format("{0:#,##0.##}", input);

Answer 4

        public string DoFormat(string num)
    {
        int pt = num.IndexOf(".");
        if (pt > 0)
        {
            return string.Format("{0:#,##0}", num.Substring(0, pt)) + num.Substring(pt, num.Length - pt);
        }
        else
        {
            return string.Format("{0:#,##0}", num);
        }
    }

This splits the string at the decimal point and returns whatever is after the decimal point as was typed in BUT formats the integer part of the number with commas.