Why does TimeSpan.FromSeconds(double) round to milliseconds?

Question

TimeSpan.FromSeconds takes a double, and can represent values down to 100 nanoseconds, however this method inexplicably rounds the time to whole milliseconds.

Answer 1

FromSeconds uses private method Interval

public static TimeSpan FromSeconds(double value)
{
    return Interval(value, 0x3e8);
}

0x3e8 == 1000

Interval method multiplay value on that const and then cast to long (see last row):

private static TimeSpan Interval(double value, int scale)
{
    if (double.IsNaN(value))
    {
        throw new ArgumentException(Environment.GetResourceString("Arg_CannotBeNaN"));
    }
    double num = value * scale; // Multiply!!!!
    double num2 = num + ((value >= 0.0) ? 0.5 : -0.5);
    if ((num2 > 922337203685477) || (num2 < -922337203685477))
    {
        throw new OverflowException(Environment.GetResourceString("Overflow_TimeSpanTooLong"));
    }
    return new TimeSpan(((long) num2) * 0x2710L); // Cast to long!!!
}

As result we have precision with 3 (x1000) signs. Use reflector to investigate