c# – 使用linq-to-entities将字符串转换为datetime的一个大问题

如何使用 linq将字符串转换为日期时间到实体….

我有以下查询,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
              };

不幸的是我无法改变架构……

我有错误：

linq to entites does not recognise Convert.ToDatetime method

是否有任何可能的解决方案将字符串转换为Datetime？

更新的代码：

根据要求我更新了我的问题

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
                        };

我不认为EF支持将String转换为DateTime或反之亦然.

在我看来,你有两个选择,取决于字符串字段中日期的格式：

如果格式非常简单,那么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形式更复杂,并且简单的字符串比较无法提供帮助,您可以考虑在访问表上创建一个视图,该视图在数据库级别为您进行转换：

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

然后将此视图导入DataModel.您可能需要做一些魔术才能使关系发挥作用.

希望能帮助到你.