Linq GroupBy和Lookup <TKey, TElement> 。项目

Question

I'm using LINQ in C#

This code result in dates of type Lookup<TKey, TElement>

   var dates = timetableEvents.GroupBy(x => x.DateTimeStart);

I need instead just a list of DateTimeStart (of type DateTime).

How should I change my LINQ expressions?

Answer 1

Simply use Select instead:

var dates = timetableEvents.Select(x => x.DateTimeStart);

Or if you just want one from each group:

var dates = timetableEvents.GroupBy(x => x.DateTimeStart).Select(x => x.Key);

And if you want it to run a little bit faster, then you can use Distinct (as in Rawling's answer), but you probably won't notice much difference, unless you have a lot objects (though, the intention is much cleaner):

var dates = timetableEvents.Select(x => x.DateTimeStart).Distinct();

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Answer 2

If you only need the start date/times,

var dates = timetableEvents.Select(x => x.DateTimeStart).Distinct();

may be more efficient than

var dates = timetableEvents.GroupBy(x => x.DateTimeStart).Select(x => x.Key);

as it knows it doesn't have to bother grouping the events together.