Error cannot apply operator '<' to operands of type 'models.semester' and 'system.datetime', candidates are: bool <(system.datetime, system.datetime) (in struct datetime)

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I am having a Semester1 including StartDate1 and EndDate1. I want to create a new Semester2 and it must ensure EndDate1 < StartDate2. How can I compare them?

I am using .Net Core 5 and Repository pattern

My code:

_dbContext.Semesters.Find(semesterViewModel.Semester.SemesterEndDate) < semesterViewModel.Semester.SemesterStartDate

What I have tried:

I have tried Get EndDate for assignment however it also failed with DateTime

EDIT: Code added from "solution" 2:

C#

public SemesterViewModel CreateSemester(SemesterViewModel semesterViewModel)
{
    if (CheckExistSemester(semesterViewModel.Semester.SemesterName))
    {
        var model = new SemesterViewModel()
        {
            Semester = semesterViewModel.Semester,
            StatusMessage = "Error: Semester already exists!"
        };
        return model;
    }
    
    if (semesterViewModel.Semester.SemesterStartDate < semesterViewModel.Semester.SemesterClosureDate &&
        semesterViewModel.Semester.SemesterClosureDate < semesterViewModel.Semester.SemesterEndDate)
    {
        var newSemester = new Semester
        {
            SemesterName = semesterViewModel.Semester.SemesterName,
            SemesterStartDate = semesterViewModel.Semester.SemesterStartDate,
            SemesterClosureDate = semesterViewModel.Semester.SemesterClosureDate,
            SemesterEndDate = semesterViewModel.Semester.SemesterEndDate
        };
        _dbContext.Semesters.Add(newSemester);
        _dbContext.SaveChanges();
        return null;
    }

Posted 14-Mar-21 20:54pm

huynbt209

Updated 15-Mar-21 3:18am

Richard Deeming

v2

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Richard Deeming 15-Mar-21 9:12am

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OriginalGriff · Answer 1 · 2021-03-14T21:18:00

What you are comparing isn't a pair of DateTime objects - it's (probably, we can;t see your code from here) a DateTime on the right hand side of the comparison, but the left hand side will be a collection of Semester objects.

Try it: split your line into two parts, so you assign the Find result to a variable:

C#

var x = _dbContext.Semesters.Find(semesterViewModel.Semester.SemesterEndDate);

And use x in your comparison.
Now look at exactly what type the system will give x - you had hover the mouse over it in VS for that - and work out from there where the value you want to compare is.

Sorry, but we can't do that for you - we can't see your screen, access your HDD, or read your mind, and we have no idea what you are actually trying to compare!

Richard Deeming · Answer 2 · 2021-03-15T03:18:00

Quote:

C#

_dbContext.Semesters.Find(semesterViewModel.Semester.SemesterEndDate) < semesterViewModel.Semester.SemesterStartDate

The Find method finds the entity with the specified primary key, and returns it. That return value will be a Semester class instance, which could be null if no matching record was found.

You are then trying to test whether that Semester instance is less than a DateTime instance. But since you have not defined any way to compare a Semester instance to a DateTime, the code cannot compile.

It's not entirely clear what you are trying to do here, since the Semester is unlikely to have a DateTime for its primary key. At a complete guess, are you trying to ensure there are no overlapping semesters already in the database?

C#

bool anyOverlappingSemesters = _dbContext.Semesters.Any(s =>
    s.SemesterEndDate >= semesterViewModel.Semester.SemesterStartDate 
    && s.SemesterStartDate <= semesterViewModel.Semester.SemesterEndDate);

Error cannot apply operator '<' to operands of type 'models.semester' and 'system.datetime', candidates are: bool <(system.datetime, system.datetime) (in struct datetime)

2 solutions

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