Using linq-to-entities to convert a string to a datetime has a significant problem.

.net c# entity-framework linq-to-entities winforms

Question

How can I convert the string to datetime using linq to entities ....

I have got the below query, where the visit_date column datatype is string...

var memberl = from v in abc.visits
              join m in abc.members on v.member_Id equals m.member_Id
              where Convert.ToDateTime(v.visit_Date) >= startdate && 
                    Convert.ToDateTime(v.visit_Date) <= enddate
              group m by new { m.member_Firstname, 
                               m.member_Lastname, m.member_Id } into g
              orderby g.Count()
              select new
              {
                  numVisits = g.Count(),
                  firstname = g.Key.member_Firstname,
                  lastname = g.Key.member_Lastname
              };

Unfortunately I can't change the schema ...

I have got the error:

linq to entites does not recognise Convert.ToDatetime method

Is there any possible solution for converting string to Datetime?

Updated code:

as per request i Have updated my question

 var data = (from v in abc.visits
                  join m in abc.members on v.member_Id equals m.member_Id
                  select new
                  {
                      MemberID = v.member_Id,
                      VisiteDate = v.visit_Date,
                      FirstName = m.member_Firstname,
                      LastName = m.member_Lastname
                  }).ToList();

      var membersdata = from d in data
                        where Convert.ToDateTime(d.VisiteDate) >= startdate && Convert.ToDateTime(d.VisiteDate) <= enddate
                        group m by new { d.FirstName, d.LastName, d.MemberID } into g
                        orderby g.Count()
                        select new
                        {
                            numVisits = g.Count(),
                            firstname = g.Key.FirstName,
                            lastname = g.Key.LastName
                        }; 
1
13
10/12/2011 2:07:28 PM

Accepted Answer

I don't think EF supports a translation for a String to DateTime or vice-versa conversion.

As I see it, you have two options, depending on the format of the date in the string field:

If the format is fairly simple, a string comparison might be enough:

// Convert the boundaries to strings first
// TODO: Set the ToString format option to match the database format
string startDateAsString = startdate.ToString("yyyyMMdd");
string endDateAsString = enddate.ToString("yyyyMMdd");

// Query based on string comparison
var memberl = from v in abc.visits
              join m in abc.members on v.member_Id equals m.member_Id
              where v.visit_Date.CompareTo(startDateAsString) >= 0 && 
                    v.visit_Date.CompareTo(endDateAsString) <= 0
              group m by new { m.member_Firstname, 
                               m.member_Lastname, m.member_Id } into g
              orderby g.Count()
              select new
              {
                  numVisits = g.Count(),
                  firstname = g.Key.member_Firstname,
                  lastname = g.Key.member_Lastname
              };

If the string representation of the date is more complex, and a simple string comparison cannot help, you might consider creating a view on the visits table, which does the conversion for you at database level:

CREATE VIEW VisitsWithDate (MemberId, VisitDate)
AS
SELECT MemberId, Convert(datetime, VisitDate, 112) -- For instance
FROM Visits

Followed by importing this view into your DataModel. You might need to do some magic to make the relationships work.

Hope it helps.

18
10/12/2011 2:17:34 PM

Popular Answer

Convert.ToDatetime is supported by Linq2SQL. The only supported method of Linq to entities are these: http://msdn.microsoft.com/en-us/library/bb738681.aspx

about your problem ... try to convert startdate and enddate in string and compare the string value in linq expression.



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