序列在Linq語句中沒有匹配的元素錯誤

Question

我想調試我的方法，但我不知道它有什麼問題。

有時候會拋出錯誤，有時候也沒關係。我不知道什麼是錯的。

這裏是我的方法：

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); 
       } 
    } 

這是錯誤出現的行：

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(); 

這是錯誤：

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(); 

嘗試使用FirstOrDefault和LastOrDefault而不是First和Last，如果沒有元素匹配您提供的lambda表達式作爲參數，則這些方法將返回它們被調用的類型的默認值。

如果是g.FirstOrDefault()，如果g爲空，則返回默認值，如果爲g.LastOrDefault(d => d.DayOfWeek != DayOfWeek.Saturday && d.DayOfWeek != DayOfWeek.Sunday)，則在所有日期爲星期六或星期日時返回默認值。