今天在尝试解决同样的问题时偶然发现了这个帖子。
从我的测试来看，以下似乎运行良好，循环只需要两次（我认为？也许 3 max(?)）就可以得到答案：
public static DateTime GetNearestSpecificDay(DateTime start, int dayNum)
{
if (dayNum >= 1 && dayNum <= 31)
{
DateTime result = start;
while (result.Day != dayNum)
result = dayNum > result.Day ? result.AddDays(dayNum - result.Day) : new DateTime(result.Month == 12 ? result.Year + 1 : result.Year, (result.Month % 12) + 1, 1);
return result;
}
else
return DateTime.Today;
}
编辑：根据要求，这里有一个不太紧凑的版本，逐步介绍了逻辑。我还更新了原始代码，以考虑到 12 月所需的年份变化。
public static DateTime GetNearestSpecificDay(DateTime start, int dayNum)
{
// Check if target day is valid in the Gregorian calendar
if (dayNum >= 1 && dayNum <= 31)
{
// Declare a variable which will hold our result & temporary results
DateTime result = start;
// While the current result's day is not the desired day
while (result.Day != dayNum)
{
// If the desired day is greater than the current day
if (dayNum > result.Day)
{
// Add the difference to try and skip to the target day (if we can, if the current month does not have enough days, we will be pushed into the next month and repeat)
result = result.AddDays(dayNum - result.Day);
}
// Else, get the first day of the next month, then try the first step again (which should get us where we want to be)
else
{
// If the desired day is less than the current result day, it means our result day must be in the next month (it obviously can't be in the current)
// Get the next month by adding 1 to the current month mod 12 (so when we hit december, we go to january instead of trying to use a not real 13th month)
// If result.Month is November, 11%12 = 11; 11 + 1 = 12, which rolls us into December
// If result.Month is December, 12%12 = 0; 0 + 1 = 1, which rolls us into January
var month = (result.Month % 12) + 1;
// Get current/next year.
// Since we are adding 1 to the current month, we can assume if the previous month was 12 that we must be entering into January of next year
// Another way to do this would be to check if the new month is 1. It accomplishes the same thing but I chose 12 since it doesn't require an extra variable in the original code.
// Below can be translated as "If last result month is 12, use current year + 1, else, use current year"
var year = result.Month == 12 ? result.Year + 1 : result.Year;
// Set result to the start of the next month in the current/next year
result = new DateTime(year, month, 1);
}
}
// Return result
return result;
}
else
// If our desired day is invalid, just return Today. This can be an exception or something as well, just using Today fit my use case better.
return DateTime.Today;
}