你觉得这个月的最后一天怎么样？

How about using DaysInMonth:

DateTime createDate = new DateTime (year, month,
DateTime.DaysInMonth(year, month));

(Note to self - must make this easy in Noda Time...)

You can use the method DateTime.DaysInMonth(year,month) to get the number of days in any given month.

if you're interested in a custom code version:

var anyDt = DateTime.Now;
var lastDayOfMonth = anyDt.AddMonths(1).AddDays(anyDt.AddMonths(1).Day).Date;

or:

var anyDt = DateTime.Now;
var lastDayOfMonth = anyDt.AddDays(1-anyDt.Day).AddMonths(1).AddDays(-1).Date;

or as a method:

DateTime LastDayInMonth(DateTime anyDt)
{ return anyDt.AddMonths(1).AddDays(anyDt.AddMonths(1).Day).Date; }

or as an extension method:

DateTime LastDayInMonth(DateTime this anyDt)
{ return anyDt.AddMonths(1).AddDays(anyDt.AddMonths(1).Day).Date; }

Here's an elegant approach I found in a useful DateTime extension library on CodePlex:

http://datetimeextensions.codeplex.com/

Here's some sample code:

    public static DateTime First(this DateTime current)
{
DateTime first = current.AddDays(1 - current.Day);
return first;
}


public static DateTime First(this DateTime current, DayOfWeek dayOfWeek)
{
DateTime first = current.First();


if (first.DayOfWeek != dayOfWeek)
{
first = first.Next(dayOfWeek);
}


return first;
}


public static DateTime Last(this DateTime current)
{
int daysInMonth = DateTime.DaysInMonth(current.Year, current.Month);


DateTime last = current.First().AddDays(daysInMonth - 1);
return last;
}

It has a few other useful extensions as well that may be helpful to you.