使用链接到实体将字符串转换为日期时间的大问题

本文关键字：日期 时间 大问题 转换 字符串 链接 实体 | 更新日期: 2023-09-27 18:10:58

如何使用linq to entities将字符串转换为日期时间....

我得到了下面的查询，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
              };

不幸的是我不能改变schema…

我得到了错误:

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日期时间?

更新代码:

根据请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
                        };

使用链接到实体将字符串转换为日期时间的大问题

我不认为EF支持String到DateTime的转换，反之亦然。

在我看来，根据字符串字段中日期的格式，您有两个选项:

如果格式相当简单，一个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，简单的字符串比较无法提供帮助，您可以考虑在visits表上创建view，它将在数据库级别为您完成转换:

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

然后将这个视图导入到您的DataModel中。你可能需要施展一些魔法来让这些关系发挥作用。

希望能有所帮助。

Convert.ToDatetime是Linq2SQL支持的。Linq唯一支持的实体方法如下:http://msdn.microsoft.com/en-us/library/bb738681.aspx

关于你的问题…尝试将startdate和enddate转换为字符串，并比较linq表达式中的字符串值。

首先尝试将结果转换为List<>，然后从列表中过滤结果:

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           
              };

因为DateTime和DateTimeOffset是结构体，它们本质上不是可空的。当您需要可空性时，有两种方法:

  1. 使用可空类型(即DateTime?还是DateTimeOffset ?)。
  2. 使用静态字段DateTime。MinValue或DateTimeOffset。MinValue(默认值)
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