Linq具有if条件的多重where子句

Question

I am trying to write Linq with multiple where clause and where clause further contains inline if conditions.

List<MeetingVM> students = (
from s in db.Meetings 
where MeetingIsActive == null || s.IsActive == MeetingIsActive 
where MeetingStat == null || MeetingStat == 5 ? (DateTime.UtcNow >= s.MeetingStartTime && DateTime.UtcNow <= s.MeetingStopTime) : s.Status== MeetingStat
where StartDate == null || (s.MeetingStartTime >= StartDate && s.MeetingStartTime <= EndDate) 
where s.Status!=4 
orderby s.MeetingStartTime ascending 
select new MeetingVM
{
    MeetingStartTime = s.MeetingStartTime,
    MeetingStopTime = s.MeetingStopTime,
    Alias = s.Alias,
    MeetingSubject = s.MeetingSubject,
    UserId = s.UserId,
    Status=s.Status

}).ToList();

This if condition executes (DateTime.UtcNow >= s.MeetingStartTime && DateTime.UtcNow <= s.MeetingStopTime) even if MeetingStat==null.

where MeetingStat == null || MeetingStat==5 ? (DateTime.UtcNow >= s.MeetingStartTime && DateTime.UtcNow <= s.MeetingStopTime) : s.Status== MeetingStat

If i remove this..

MeetingStat==5 ? (DateTime.UtcNow >= s.MeetingStartTime && DateTime.UtcNow <= s.MeetingStopTime) :

It works. But i wanted to write this to check

if (MeetingStat==5)
  (DateTime.UtcNow >= s.MeetingStartTime && DateTime.UtcNow <= s.MeetingStopTime) 
else 
  s.Status== MeetingStat.

What wrong am i doing?

Answer 1

I think the problem is with DateTime.UtcNow Better you store its value in a variable nad then use the same in LINQ as like this:

DateTime UtcTime = DateTime.UtcNow;
// and change the condition like this;
.Where MeetingStat != null && MeetingStat==5 ? (UtcTime >= s.MeetingStartTime && UtcTime <= s.MeetingStopTime) : s.Status== MeetingStat

It is happening so because your predicates inside the Where clause was not getting translated to SQL, If you are using like this means your code will works as expected since UtcTime is now convertible

Answer 2

First, let's get the code formatted in a way that's not stupidly difficult to read:

List<MeetingVM> students = (
    from s in db.Meetings 
    where MeetingIsActive == null || s.IsActive == MeetingIsActive 
    where MeetingStat == null || MeetingStat == 5 ? (DateTime.UtcNow >= s.MeetingStartTime && DateTime.UtcNow <= s.MeetingStopTime) : s.Status== MeetingStat
    where StartDate == null || (s.MeetingStartTime >= StartDate && s.MeetingStartTime <= EndDate) 
    where s.Status!=4 
    orderby s.MeetingStartTime ascending 
    select new MeetingVM
    {
        MeetingStartTime = s.MeetingStartTime,
        MeetingStopTime = s.MeetingStopTime,
        Alias = s.Alias,
        MeetingSubject = s.MeetingSubject,
        UserId = s.UserId,
        Status=s.Status

    }).ToList();

That's a little better.

Now, I'm not 100% sure on this, so someone who knows better can feel free to correct me, but according to C# Operator Precedence , the OR ("||") operator has a higher precedence than the conditional ("?:") operator, so this line:

where MeetingStat == null || MeetingStat == 5 ? ...

is essentially getting evaluated like this:

where (MeetingStat == null || MeetingStat == 5) ? ...

If MeetingStat is equal to true, this would make the expression evaluate the first branch of the ternary operation. Judging from your description in your question, this is not what you want, so I recommend surrounding the ternary operation in parentheses to make your intention clear:

where MeetingStat == null || (MeetingStat == 5 ? ... )