如何根据同月的日期查找一个月中特定日期的第五个或结束日期

我一直在尝试根据同月的日期查找一个月中某一天的第五周日期，例如第五周星期一日期、第五周星期二日期、星期三......等等。该日期可以属于同月的任何一周。 我尝试过像

DateTime MonthEventDate=05/01/2016; //Date format in dd/MM/yyyy format

DayOfWeek EventDay="Sunday"; //For Example i want to find 5th Sunday in January Month, but days too will change everytime based on user selection

string SelectedWeek="5"; //Here i'm getting the week in which i need to find the given date i.e, 5th Monday or Tuesday & so on  
if (SelectedWeek == "5")
{
    //Here i tried to add number of days to my initial day to find 5th day date, but every time its returning next month value 
    MonthEventDate = MonthEventDate.AddDays((EventDay < MonthEventDate.DayOfWeek ? 31 : 28) + EventDay - MonthEventDate.DayOfWeek);
}

我知道逻辑是错误的，但我想获取一周中第 5 天的日期，如果该天不存在，则返回 0。寻找一些指导 注意：这里的月份将根据用户输入而变化，因此如何返回第五天的日期（如果给定月份中存在）

这应该给你第五天（如果有的话）......

 DateTime dayInMonth = DateTime.Now;
 DayOfWeek dayToFind = DayOfWeek.Friday;

 var fifth= Enumerable.Range(1, DateTime.DaysInMonth(dayInMonth.Year, dayInMonth.Month))
            .Select(day => new DateTime(dayInMonth.Year, dayInMonth.Month, day))
            .Where(day => day.DayOfWeek == dayToFind)
            .Skip(4)
            .FirstOrDefault();