c# - Linq语句中序列不包含匹配元素错误

Question

我正在尝试调试我的方法，但我不知道它有什么问题。

有时它会抛出错误，有时又没问题。我不知道出了什么问题。

这是我的方法:

private void GetWorkingWeek(int month, int year)
    {
        var cal = System.Globalization.CultureInfo.CurrentCulture.Calendar;


        var daysInMonth = Enumerable.Range(1, cal.GetDaysInMonth(year, month));

        var listOfWorkWeeks = daysInMonth
            .Select(day => new DateTime(year, month, day))
            .GroupBy(d => cal.GetWeekOfYear(d, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday))
            .Select(g => Tuple.Create(g.Key, g.First(), g.Last(d => d.DayOfWeek != DayOfWeek.Saturday && d.DayOfWeek != DayOfWeek.Sunday)))
            .ToList();
         foreach (var weekGroup in listOfWorkWeeks)
                {
                    Console.WriteLine("Week{0} = {1} {2} to {1} {3}"
                        , weekNum++
                        , System.Globalization.CultureInfo.CurrentCulture.DateTimeFormat.GetMonthName(month)
                        , weekGroup.Item2.Day
                        , weekGroup.Item3.Day);
                }
    }

这是出现错误的行:

var listOfWorkWeeks = daysInMonth
            .Select(day => new DateTime(year, month, day))
            .GroupBy(d => cal.GetWeekOfYear(d, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday))
            .Select(g => Tuple.Create(g.Key, g.First(), g.Last(d => d.DayOfWeek != DayOfWeek.Saturday && d.DayOfWeek != DayOfWeek.Sunday)))
            .ToList();

这是错误:

 InvalidOperationException : Sequence contains no matching element

Answer 1

var listOfWorkWeeks = daysInMonth
        .Select(day => new DateTime(year, month, day))
        .GroupBy(d => cal.GetWeekOfYear(d, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday))
        .Select(g => Tuple.Create(g.Key, g.FirstOrDefault(), g.LastOrDefault(d => d.DayOfWeek != DayOfWeek.Saturday && d.DayOfWeek != DayOfWeek.Sunday)))
        .ToList();

尝试使用FirstOrDefault和LastOrDefault代替First和Last，这些方法将返回默认值如果没有元素与您作为参数提供的 lambda 表达式匹配，则调用它们的类型。

在 g.FirstOrDefault() 的情况下，如果 g 为空并且在 g.LastOrDefault(d => d.DayOfWeek != DayOfWeek. Saturday && d.DayOfWeek != DayOfWeek.Sunday) 如果所有日期都是星期六或星期日，则返回默认值。