C# How to format a double to one decimal place without rounding

Question

I need to format a double value to one decimal place without it rounding.

double value = 3.984568438706
string result = \"\";

What I have tried

Answer 1

Just use modulo operator + built in ToString:

result = (value - (value % 0.1)).ToString("N1") + "%";

Answer 2

ToString() doesn't do it. You have to add extra code. The other answers show math approaches, my approach below is kind of outside-the-box.

string result = value.ToString();
Console.WriteLine("{0}", result.Substring(0, result.LastIndexOf('.') + 2));

This is a fairly simple brute force approach, but it does the trick when the decimal is a '.'. Here's an extension method to ease the pain (and deals with the decimal point).

public static class Extensions
{
    public static string ToStringNoTruncate(this double me, int decimalplaces = 1)
    {
        string result = me.ToString();
        char dec = System.Globalization.CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator[0];
        return result.Substring(0, result.LastIndexOf(dec) + decimalplaces + 1);
    }
}

Answer 3

I know this is a old thread but I've just had to do this. While the approaches here work I want a easy way to be able to affect a lot of calls so using the Math.Truncate on all the calls to string.format wasn't really a good option.

Thus, I made a custom format provider which would allow me to add truncation to the formatting string, eg

string.format(new FormatProvider(), "{0:T}", 1.1299); // 1.12
string.format(new FormatProvider(), "{0:T(3)", 1.12399); // 1.123
string.format(new FormatProvider(), "{0:T(1)0,000.0", 1000.9999); // 1,000.9

The implementation is pretty simple and is easily extendible to other requirements.

public class FormatProvider : IFormatProvider, ICustomFormatter
{
    public object GetFormat(Type formatType)
    {
        if (formatType == typeof (ICustomFormatter))
        {
            return this;
        }
        return null;
    }

    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        if (arg.GetType() != typeof (double))
        {
            try
            {
                return HandleOtherFormats(format, arg);
            }
            catch (FormatException e)
            {
                throw new FormatException(string.Format("The format of '{0}' is invalid.", format));
            }
        }

        if (format.StartsWith("T"))
        {
            int dp = 2;
            int idx = 1;
            if (format.Length > 1)
            {
                if (format[1] == '(')
                {
                    int closeIdx = format.IndexOf(')');
                    if (closeIdx > 0)
                    {
                        if (int.TryParse(format.Substring(2, closeIdx - 2), out dp))
                        {
                            idx = closeIdx + 1;
                        }
                    }
                    else
                    {
                        throw new FormatException(string.Format("The format of '{0}' is invalid.", format));
                    }
                }
            }
            double mult = Math.Pow(10, dp);
            arg = Math.Truncate((double)arg * mult) / mult;
            format = format.Substring(idx);
        }

        try
        {
            return HandleOtherFormats(format, arg);
        }
        catch (FormatException e)
        {
            throw new FormatException(string.Format("The format of '{0}' is invalid.", format));
        }
    }

    private string HandleOtherFormats(string format, object arg)
    {
        if (arg is IFormattable)
        {
            return ((IFormattable) arg).ToString(format, CultureInfo.CurrentCulture);
        }
        return arg != null ? arg.ToString() : String.Empty;
    }
}

Answer 4

result=string.Format("{0:0.0}",Math.Truncate(value*10)/10);

Answer 5

I would make a utility method to handle this:

static double Truncate(double value, int digits)
{
    double mult = System.Math.Pow(10.0, digits);
    return System.Math.Truncate(value * mult) / mult;
}

You could then do:

result = Truncate(value, 1).ToString("##.#", System.Globalization.CultureInfo.InvariantCulture) + "%"; 

Note that you may also want Math.Floor instead of truncate - but it depends on how you want negative values handled.

Answer 6

( Math.Truncate( ( value * 10 ) ) / 1000 ).ToString( "#.#%" )