序列在Linq语句中没有匹配的元素错误

Question

我想调试我的方法，但我不知道它有什么问题。

有时候会抛出错误，有时候也没关系。我不知道什么是错的。

这里是我的方法：

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); 
       } 
    } 

这是错误出现的行：

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而不是First和Last，如果没有元素匹配您提供的lambda表达式作为参数，则这些方法将返回它们被调用的类型的默认值。

如果是g.FirstOrDefault()，如果g为空，则返回默认值，如果为g.LastOrDefault(d => d.DayOfWeek != DayOfWeek.Saturday && d.DayOfWeek != DayOfWeek.Sunday)，则在所有日期为星期六或星期日时返回默认值。