我怎么能转换成字符串使用LINQ to实体DATETIME ....

我有以下的查询，其中 visit_date 列的数据类型为字符串...

  VAR memberl =从V IN abc.visits
              加入米abc.members上v.member_Id等于m.member_Id
              其中，Convert.ToDateTime（v.visit_Date）＆GT; =起始日期和放大器;＆安培;
                    Convert.ToDateTime（v.visit_Date）＆LT; =结束日期
              通过新的{m.member_Firstname组男，
                               m.member_Lastname，m.member_Id}为G
              排序依据g.Count（）
              选择新
              {
                  numVisits = g.Count（），
                  名字= g.Key.member_Firstname，
                  姓氏= g.Key.member_Lastname
              };
 

不幸的是我无法改变的模式...

我已经得到了错误：

 的LINQ to entites的不承认Convert.ToDatetime方法
 

是否有将字符串转换为datetime任何可能的解决方案？

更新code：

为每个请求我已经更新了我的问题。

  VAR数据=（从V IN abc.visits
                  加入米abc.members上v.member_Id等于m.member_Id
                  选择新
                  {
                      会员id = v.member_Id，
                      VisiteDate = v.visit_Date，
                      名字= m.member_Firstname，
                      姓氏= m.member_Lastname
                  }）了ToList（）。

      VAR membersdata =从D数据
                        其中，Convert.ToDateTime（d.VisiteDate）＆GT; =起始日期和放大器;＆安培; Convert.ToDateTime（d.VisiteDate）＆LT; =结束日期
                        通过新的{d.FirstName，d.LastName，d.MemberID}为G m组
                        排序依据g.Count（）
                        选择新
                        {
                            numVisits = g.Count（），
                            名字= g.Key.FirstName，
                            姓氏= g.Key.LastName
                        };
 

解决方案

我不认为EF支持转换为字符串日期时间，反之亦然转换。

在我看来，你有两个选择，具体取决于字符串字段的日期格式为：

如果该格式是相当简单的，字符串比较可能就足够了：

  //将界限第一为字符串
// TODO：设置的ToString格式选项数据库格式匹配
字符串startDateAsString = startdate.ToString（年月日）;
字符串endDateAsString = enddate.ToString（年月日）;

//查询基于字符串比较
VAR memberl =从V IN abc.visits
              加入米abc.members上v.member_Id等于m.member_Id
              其中v.visit_Date.CompareTo（startDateAsString）＆GT; = 0＆安培;＆安培;
                    v.visit_Date.CompareTo（endDateAsString）＆LT; = 0
              通过新的{m.member_Firstname组男，
                               m.member_Lastname，m.member_Id}为G
              排序依据g.Count（）
              选择新
              {
                  numVisits = g.Count（），
                  名字= g.Key.member_Firstname，
                  姓氏= g.Key.member_Lastname
              };
 

如果日期的字符串，再presentation比较复杂，简单的字符串比较不能帮忙，你可以考虑创建一个视图在访问次数表，由它来完成转换为你在数据库级别：

  CREATE VIEW VisitsWithDate（会员id，VisitDate）
如
SELECT会员id，转换（日期时间，VisitDate，112） - 例如
从访问
 

随后导入这种观点到您的DataModel。你可能需要做一些魔法使的关系的工作。

希望它帮助。

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
                        }; 

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.