为什么将DateTime值与被视为无效转换的DateTime值进行比较？

Question

我有这个代码来比较两个DateTime值:

DateTime currentWeek = Convert.ToDateTime(comboBoxWeekToSchedule.SelectedValue);
List<Student> thisWeeksStudents = (List<Student>)studentsList.Where(i => i.WeekOfLastAssignment.Equals(currentWeek));

学生班与本次讨论最相关的成员是:

public DateTime WeekOfLastAssignment { get; set; }

在此之后:

DateTime currentWeek = Convert.ToDateTime(comboBoxWeekToSchedule.SelectedValue);

...执行,"currentWeek"的值是"2/22/2016 12:00:00 AM "

我试图过滤"WeekOfLastAssignment"元素等于"currentWeek"值的元素的通用列表; 这些(如调试器中所示)表示为" {8/14/2015 10:52:55 PM} "

IOW,它们似乎具有相同的格式(除了包装"{"和"}",我怀疑是问题).

我可以看到可能没有确切的匹配,因为组合框值总是有午夜作为他们的时间元素.所以也许我将不得不执行一个"LIKE%"类型的LINQ操作("包含"可能？),但第一个问题是越过这个无效的强制转换.

这是我尝试分配到名为"thisWeeksStudents"的通用列表后得到的错误的准确文本:

System.InvalidCastException was unhandled
  HResult=-2147467262
  Message=Unable to cast object of type 'WhereListIterator`1[AYttFMScheduler.Student]' to type 'System.Collections.Generic.List`1[AYttFMScheduler.Student]'.

我需要做些什么来纠正这种情况？

如果它可能是相关的,comboBoxWeekToSchedule填充了如下值:

private void PopulateComboBoxWithSchedulableWeeks()
{
    int WEEKS_TO_OFFER_COUNT = 13;
    List<String> schedulableWeeks = AYttFMConstsAndUtils.GetWeekBeginnings(WEEKS_TO_OFFER_COUNT).ToList();
    BindingSource bs = new BindingSource();
    bs.DataSource = schedulableWeeks;
    comboBoxWeekToSchedule.DataSource = bs;
}

public static List<String> GetWeekBeginnings(int countOfWeeks)
{
    // from http://stackoverflow.com/questions/6346119/datetime-get-next-tuesday
    DateTime today = DateTime.Today;
    // The (... + 7) % 7 ensures we end up with a value in the range [0, 6]
    int daysUntilMonday = ((int)DayOfWeek.Monday - (int)today.DayOfWeek + 7) % 7;
    DateTime nextMonday = today.AddDays(daysUntilMonday);

    List<String> mondays = new List<string>();
    // Need all Mondays, even though 1st Monday is BR only
    if (!IsAssemblyOrConventionWeek(nextMonday))
    {
        mondays.Add(nextMonday.ToLongDateString());
    }

    for (int i = 0; i < countOfWeeks; i++)
    {
        nextMonday = nextMonday.AddDays(7);
        if (!IsAssemblyOrConventionWeek(nextMonday))
        {
            mondays.Add(nextMonday.ToLongDateString());
        }
    }
    return mondays;
}

Answer 1

如果你得到的错误是......

无法转换'WhereListIterator 1[AYttFMScheduler.Student]' to type 'System.Collections.Generic.List1 [AYttFMScheduler.Student]' 类型的对象.

...问题不在于它不能投射DateTime到一个DateTime,而是它无法投射WhereListIterator<T>到一个List<T>.我认为您需要做的就是使用ToList()而不是强制转换,如下所示:

List<Student> thisWeeksStudents = studentsList
    .Where(i => i.WeekOfLastAssignment.Equals(currentWeek))
    .ToList();

对于你的Equals语句永远不是真的问题(由于比较一个总是午夜的日期和一个具有可变时间的日期),尝试获取Date组件,如下所示:

.Where(i => i.WeekOfLastAssignment.Date == currentWeek)