big issue in converting string to datetime using linq-to-entities

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...

Answer 1

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.

Answer 2

Because DateTime and DateTimeOffset are structs, they are not intrinsically nullable. When you need nullability, there are two ways around this:

1. Use a Nullable type (i.e., DateTime? or DateTimeOffset?).
2. Use the static field DateTime.MinValue or DateTimeOffset.MinValue (the default values for these types)

Answer 3

Try converting the results to a List<> first, and filter the results from the list:

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 memberl = from d in data    
              where Convert.ToDateTime(d.VisitDate) >= startdate && Convert.ToDateTime(d.VisitDate) <= enddate           
              group d 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           
              }; 

Answer 4

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.