Question

I have several records in a database that have Start and End Dates

09/15/2011 - 09/30/2011
10/15/2011 - 10/22/2011
11/01/2011 - 11/15/2011

When user stores a record, I need to make sure dates don’t overlap.
My simple code checks date ranges within a specific record (e.g. user enters 9/16/2011 or 10/21/2011, I throw an exception.)

But, on the slim chance a user gets creative (e.g. 10/14/2011 – 10/23/2011 or even 10/14/2011 to 11/16/2011), now they have circumvented my check.

BTW, the user could enter 10/14/2011 to 10/23/2011 if they were editing the record that contained values 10/15/2011 – 10/22/2011.

So, I’m trying to solve this riddle with a linq query. However, what I have isn’t working exactly right.

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UPDATE Nevermind about code not working. While trying to provide an example to expand on Miika’s repsonse, I found my answer. So, giving credit to Miika for pointing me in the right direction and posting my working code below:

Here’s my code:

        Private Sub CheckForOverlap(myMonth As Messages.MyMonth)


                        Dim am As New MyMonth()
                        Dim amCollection As Messages.MyMonthCollection
                        Dim overlappingMyMonthDate As Boolean = False
                        Dim sErrorMsg As String = ""

'...non-applicable code omitted

    Dim query = From s In amCollection _
                                 Let s1 As MyMonth = CType(s, MyMonth) _
                                 Where s1.AttendanceMonthID <> attendanceMonth.AttendanceMonthID And _
                                 (CDate(attendanceMonth.StartDate) < CDate(s1.StartDate) And CDate(attendanceMonth.EndDate) > CDate(s1.EndDate)) _
                                 Select s1

                If query.Count > 0 Then
                    sErrorMsg = "Dates entered surround another entry"
                End If

                        If overlappingMyMonthDate Then
                            Throw New Exception(sErrorMsg)
                        End If

                    End Sub


                End Class

It all came down a LINQ query.

Answer 1

Do you need to do it in code or would SQL be an option? If the data is in a database, you could use the following query to check for overlaps.

SELECT COUNT(*)
FROM Table1
WHERE Table1.StartDate < 'endCheckDate'
    AND Table1.EndDate > 'startCheckDate'

This will return a count of the number of overlaps found. ‘endCheckDate’ and ‘startCheckDate’ are your new query values (in date format). If your data is in a object collection in memory, then you could use LINQ. If you need help with a LINQ statement, let me know.