如何使用 linq to entities 将字符串转换为日期时间....

我有下面的查询，其中 visit_date 列数据类型是字符串...

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
              };

很遗憾，我无法更改架构...

我遇到了错误:

linq to entites does not recognise Convert.ToDatetime method

是否有将字符串转换为日期时间的可能解决方案？

更新后的代码:

根据要求我已经更新了我的问题

 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
                        }; 

最佳答案

我认为 EF 不支持将字符串转换为 DateTime，反之亦然。

在我看来，您有两种选择，具体取决于字符串字段中日期的格式:

如果格式相当简单，字符串比较可能就足够了:

// 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
              };

如果日期的字符串表示更复杂，并且简单的字符串比较无济于事，您可以考虑在 visits 表上创建一个view，它执行在数据库级别为您转换:

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

然后将此 View 导入您的 DataModel。您可能需要施展魔法才能让这种关系发挥作用。

希望对您有所帮助。

关于c# - 使用 linq-to-entities 将字符串转换为日期时间的大问题，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7740693/