Linq DateTime comparison not working - linq

Question

I have the following code:
DateTime timeStamp = Convert.ToDateTime(Request.QueryString["TimeStamp"]);
var result = (from rs in db.VRec
where
rs.TimeStamp == timeStamp &&
rs.Fixure == wFixture
select rs).ToList();
The result shows 0 even though the correct timeStamp is passed.
If I remove the part where I do the TimeStamp comparison:
rs.TimeStamp == timeStamp
The code works fine.
Any idea on why the datetime comparison may not be working?

Answer 1

DateTime has a pretty fine resolution - likely you are comparing timestamps that only differ in milliseconds, which will fail. You probably want something like:
DateTime now = DateTime.Now;
DateTime then = now.Add(TimeSpan.FromMilliseconds(1));
const int EPSILON_MS = 10;
if(now.Subtract(then).TotalMilliseconds < EPSILON_MS)
{
Console.WriteLine("More or less equal!");
}

Answer 2

Linq converts DateTime arguments to DateTime2 in the sql query executed.
That is, when you do the comparison the actual sql executed will compare a DateTime to a DateTime2. This comparison will "cast" the DateTime to a DateTime2 and the millisecond part will be expanded to a greater resolution (in an odd way in my opinion, please enlighten me).
Try to execute the following sql:
declare #d1 datetime = '2016-08-24 06:53:01.383'
declare #d2 datetime2 = '2016-08-24 06:53:01.383'
declare #d3 datetime2 = #d1
select #d1 as 'd1', #d2 'd2', #d3 'converted'
select (case when (#d1 = #d2) then 'True' else 'False' end) as 'Equal',
(case when (#d1 > #d2) then 'True' else 'False' end) as 'd1 greatest'

Answer 3

From the question, I do not know if you want to compare the date with time or only the date part. If you only want to compare date then following would work
var result = (from rs in db.VRec
where
rs.TimeStamp.Date == timeStamp.Date &&
rs.Fixure == wFixture
select rs).ToList();
Since you are using some reference to db, it gives me a feeling that you are fetching your records from database (which ORM you are using is not obvious from the question or tags). Assuming that you are using Entity framework the above query will fail with exception that .Date has no direct translation to sql. If so you can rewrite the query as following to make it work.
var result = (from rs in db.VRec
where
rs.TimeStamp.Day == timeStamp.Day &&
rs.TimeStamp.Month == timeStamp.Month &&
rs.TimeStamp.Year == timeStamp.Year &&
rs.Fixure == wFixture
select rs).ToList();
The benefit of this approach is that you can compare properties to arbitrary deep level i.e you can compare Hours, Minutes,Seconds etc. in your query. The second query is tested in Entity framework 5.