如何比较linq / lambda查询中可空日期时间的日期部分？

Question

I have something like the following:

new ObservableCollection<SomeData>(SomeDataCollection.Where(a => a.AGD_Category.Equals((int)AGD_CategoryEnum.Med) && ((a.AGG_DateStart.Date <= SelectedUntill) && (a.AGG_DateEnd.Value.Date >= SelectedFrom))));

now my a.AGG_DateEnd is a nullable datetime and whenever there is a null value in the list I get an exception because of the "a.AGG_DateEnd.Value.Date" part.

But the thing is, the a.AGG_DateEnd also contains hours and minutes, but I do not want this causes my comparison to give unreliable results.

What can I do to improve?

Thank you,

Answer 1

A quick hack is:

a.AGG_DateEnd.GetValueOrDefault().Date >= SelectedFrom

This is OK because the default(DateTime) (which equals its own .Date ) is not greater than any (other) DateTime value.

Otherwise the usual thing to do is:

a.AGG_DateEnd.HasValue && a.AGG_DateEnd.Value.Date >= SelectedFrom

where the double & ensures short-cut evaluation.

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Addition after comments:

If you want the opposite answer in the case where it is null , it is:

(a.AGG_DateEnd ?? DateTime.MaxValue).Date >= SelectedFrom

respectively:

!a.AGG_DateEnd.HasValue || a.AGG_DateEnd.Value.Date >= SelectedFrom

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Later edit: Since C# 6.0 (July 2015), you can use the ?. operator instead. So use the shorter code:

a.AGG_DateEnd?.Date >= SelectedFrom

which will (formally, at least) compare two nullable values of type DateTime? . When either operand is "null" (ie .HasValue gives false ), such a comparison gives false .

If you want the opposite bool result in case of null, that is:

!(a.AGG_DateEnd?.Date < SelectedFrom)

Observe how x >= y is different from !(x < y) when we use these lifted operators that operate on Nullable<> values. Any lifted operator (like >= or < ) will return false immediately if one of x and y is null; only if both x and y are non-null, will they be unwrapped and the contents compared with the non-lifted operator whose return value will be used.