Null exception being thrown by LINQ to EF - vb.net

Question

This line of code is throwing an exception:
Dim z = (From r In t Where
(DateTime.ParseExact(r.appt_date, "yyyyMMdd", CultureInfo.CurrentCulture).Date.Month _
= myDate.Date.Month) And _
(DateTime.ParseExact(r.appt_date, "yyyyMMdd", CultureInfo.CurrentCulture).Date.Day = _
myDate.Date.Day) And _
(DateTime.ParseExact(r.appt_date, "yyyyMMdd", CultureInfo.CurrentCulture).Date.Year = _
myDate.Date.Year)).ToArray
T is defined as the following:
Dim t = (From p In mydb.C_MYTABLE Select p).ToArray
Since someone is going to ask what I'm trying to do... I have appointment dates stored as strings as yyyyMMdd in a DB, i can not change the DB. I need to be able to filter and sort them by date on the app level. Its my understanding that LINQ will not support conversions inside the query?

Answer 1

(This answer doesn't actually explain your NullReferenceException - but it gives an alternative approach.)
Given that you're checking for an exact date wouldn't it be easiest just to format myDate as yyyyMMdd before the query, and then have:
Dim z = (From r in t Where r.appt_date = formattedDate).ToArray
? That allows you to filter by a specific date - and if you need to filter to a range of dates, I'd expect that you could just use CompareTo. Your dates are in a sortable format, so if you need to find everything between (say) January 10th 2012 and September 20th 2013, you can just say that the string has to be "greater than or equal to" 20120110 and "less than or equal to" 20130920. You can also use this property for sorting - just ordering by the textual representation will be fine.
(That said, if there ever is a database redesign, please urge whoever's responsible to use a more appropriate data type!)

Answer 2

Enity framework its self will not support this. You can howerver select all of the rows out of the database and then sort them at the application level. Like so :
r
.ToList()
.Select(o => new
{
data = o,
appt_date = DateTime.ParseExact(r.appt_date, "yyyyMMdd", CultureInfo.CurrentCulture)
})
.Where(o => o.appt_date < DateTime.Now)
.OrderBy(o => o.appt_date);
This has a very big downside of selecting far to much out of the database.
Alternatively you could format your date into a string and use it to filter within the database and then parse the date:
var formatedDate = DateTime.now.ToString("yyyyMMdd");
r
.Where(o => o.appt_date == formatedDate)
.ToList()
.Select(o => new
{
data = o,
appt_date = DateTime.ParseExact(r.appt_date, "yyyyMMdd", CultureInfo.CurrentCulture)
})
.OrderBy(o => o.appt_date);
Ideally however I ask to improve the quality of the data. If you cant change the 'appt_date' to a Date type can you ask to insert a new column with the correct data type?

Answer 3

I need to be able to filter
You appear to be creating a non-sargable query.
I would suggest that instead of parsing the column you are querying on, create a variable that matches the value you are looking for.
dim myDate = DateTime.Now.ToString("yyyyMMdd");
Dim z = (From r In t Where
(r.appt_date = myDate)).ToArray
and sort them by date
The way the DB is storing the value should look like:
20120101 // Jan 01, 2012
20121231 // Dec 31, 2012
20130101 // Jan 01, 2013
20131231 // Dec 31, 2013
Natural string sorting will sort these in the correct order every time (asc, desc), so .OrderBy() and .OrderByDescending() will work as expected.