C# Convert string to double/decimal and back to string, keeping trailing zeroes, adding comas for thousands

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);

score 7 · Accepted Answer · 2017-07-26T17:38:33.073

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)

That's it, all test cases passed (I tested myself). Nice and clean solution. — Alisson Reinaldo Silva, Jul 26 '17 at 17:17
Excellent. Thanks a lot. I was thinking I would need to write some Regex, which I really like to avoid. — Razkar, Jul 26 '17 at 17:27

score 0 · Answer 2 · answered Jul 26 '17 at 17:15

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}";

score 0 · Answer 3 · answered Jul 26 '17 at 17:38

        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.

score -1 · Answer 4 · answered Jul 26 '17 at 17:13

-1

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);

answered Jul 26 '17 at 17:13

Sach

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"If you don't mind 1000 converted to 1,000.00" -- If you don't mind ignoring what's in the question, any answer works, but the question specifically covers this. – Jul 26 '17 at 17:14
This is not what OP asked for. They want to keep the same amount of decimal places. – Alisson Reinaldo Silva Jul 26 '17 at 17:14
Agree, bad answer without properly reading the question. My bad. – Sach Jul 26 '17 at 17:22

C# Convert string to double/decimal and back to string, keeping trailing zeroes, adding comas for thousands

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