Question

我有以下foreach循环：

List<WorkingJournal> workingJournals = new List<WorkingJournal>();
foreach (WorkRoster workRoster in workRosters)
{
    bool exists = workingJournals.Any(workingJournal => workingJournal.ServicePlan.Id == workRoster.ServicePlan.Id
                                      && workingJournal.Nurse.Id == workRoster.Nurse.Id
                                      && workingJournal.Month == workRoster.Start.Month
                                      && workingJournal.Year == workRoster.Start.Year);

    if (exists == false)
    {
                WorkingJournal workingJournal = new WorkingJournal
                {
                    ServicePlan = workRoster.ServicePlan,
                    Nurse = workRoster.Nurse,
                    Month = workRoster.Start.Month,
                    Year = workRoster.Start.Year
                };

                workingJournals.Add(workingJournal);
    }
}

我开始写作：

from workRoster in workRosters
select new WorkingJournal
                {
                    ServicePlan = workRoster.ServicePlan,
                    Nurse = workRoster.Nurse,
                    Month = workRoster.Start.Month,
                    Year = workRoster.Start.Year
                };

但是现在我对产生不同工作日期的比较感到困惑我觉得group by子句应该在这里，但我不确定应该怎么做。

Answer 1

假设LINQ为对象：

(from workRoster in workRosters
 select new WorkingJournal
            {
                ServicePlan = workRoster.ServicePlan,
                Nurse = workRoster.Nurse,
                Month = workRoster.Start.Month,
                Year = workRoster.Start.Year
            }).Distinct();

请注意，为了实现此功能，您需要为Equals对象实施GetHashCode和WorkingJournal。如果没有，请参阅Anthony的回答：How to perform a this kind of Distinct operation with LINQ?

如果它是LINQ to SQL，您可以按新表达式分组，然后选择组密钥：

from workRoster in workRosters
group workRoster by new WorkingJournal
            {
                ServicePlan = workRoster.ServicePlan,
                Nurse = workRoster.Nurse,
                Month = workRoster.Start.Month,
                Year = workRoster.Start.Year
            } into workRosterGroup
select workRosterGroup.Key;

Answer 2

如果您的课程中有适当的Equals和GetHashCode实施，您只需调用Distinct()。

var result = workRosters.Select(...).Distinct();

如果没有这样的实现，您可以定义IEqualityComparer<WorkingJournal>实现。这将使您为Equals定义GetHashCode和T方法，然后可以由字典或哈希集使用，也可以在Linq中Distinct()的重载中使用。

class JournalComparer : IEqualityComparer<WorkingJournal>
{
    public bool Equals(WorkingJournal left, WorkingJournal right)
    {
         // perform your equality semantics here
    }

    public int GetHashCode(WorkingJournal obj)
    {
         // return some hash code here.
         return obj.ServicePlan.GetHashCode();
    }
}

var comparer = new JournalComparer(); // implements the interface 
var result = workRosters.Select(r => new WorkingJournal { ... }).Distinct(comparer);

如何使用LINQ执行这种不同的操作？

2 个答案: