How can I String.Format a TimeSpan object with a custom format in .NET?

Question

What is the recommended way of formatting TimeSpan objects into a string with a custom format?

Answer 1

Here is my version. It shows only as much as necessary, handles pluralization, negatives, and I tried to make it lightweight.

Output Examples

0 seconds
1.404 seconds
1 hour, 14.4 seconds
14 hours, 57 minutes, 22.473 seconds
1 day, 14 hours, 57 minutes, 22.475 seconds

Code

public static class TimeSpanExtensions
{
    public static string ToReadableString(this TimeSpan timeSpan)
    {
        int days = (int)(timeSpan.Ticks / TimeSpan.TicksPerDay);
        long subDayTicks = timeSpan.Ticks % TimeSpan.TicksPerDay;

        bool isNegative = false;
        if (timeSpan.Ticks < 0L)
        {
            isNegative = true;
            days = -days;
            subDayTicks = -subDayTicks;
        }

        int hours = (int)((subDayTicks / TimeSpan.TicksPerHour) % 24L);
        int minutes = (int)((subDayTicks / TimeSpan.TicksPerMinute) % 60L);
        int seconds = (int)((subDayTicks / TimeSpan.TicksPerSecond) % 60L);
        int subSecondTicks = (int)(subDayTicks % TimeSpan.TicksPerSecond);
        double fractionalSeconds = (double)subSecondTicks / TimeSpan.TicksPerSecond;

        var parts = new List(4);

        if (days > 0)
            parts.Add(string.Format("{0} day{1}", days, days == 1 ? null : "s"));
        if (hours > 0)
            parts.Add(string.Format("{0} hour{1}", hours, hours == 1 ? null : "s"));
        if (minutes > 0)
            parts.Add(string.Format("{0} minute{1}", minutes, minutes == 1 ? null : "s"));
        if (fractionalSeconds.Equals(0D))
        {
            switch (seconds)
            {
                case 0:
                    // Only write "0 seconds" if we haven't written anything at all.
                    if (parts.Count == 0)
                        parts.Add("0 seconds");
                    break;

                case 1:
                    parts.Add("1 second");
                    break;

                default:
                    parts.Add(seconds + " seconds");
                    break;
            }
        }
        else
        {
            parts.Add(string.Format("{0}{1:.###} seconds", seconds, fractionalSeconds));
        }

        string resultString = string.Join(", ", parts);
        return isNegative ? "(negative) " + resultString : resultString;
    }
}