在 linq 表达式中使用 ToString() 来转换 DateTime 值

Question

我正在尝试将 DateTime 值转换为字符串，但出现此运行时错误：

附加信息：LINQ to Entities 无法识别方法 'System.String ToString()' 方法，并且此方法不能 翻译成商店表达式。

这是我正在使用的查询：

using (var db = new DbContext())
{
    var results = (from u in db.AspNetUserses
        join upi in db.UserPersonalInfoes on u.Id equals upi.UserId into upis
        from upi in upis.DefaultIfEmpty()
        join up in db.UserPreferenceses on u.Id equals up.UserId into ups
        from up in ups.DefaultIfEmpty()
                   join us in db.UserStatses on u.Id equals us.UserId into uss
                   from us in uss.DefaultIfEmpty()
        select new
        {
            Username = u.UserName,
            Telephone = (upi == null ? String.Empty : upi.Telephone),
            ID = u.Id,
            LastLogin = (us == null ? String.Empty : us.LastLoginDate) 
        }).ToList();

    gvUsers.DataSource = results;
    gvUsers.DataBind();

}

知道如何解决这个错误吗？

Answer 1

创建第二个列表并在那里进行转换。

using (var db = new DbContext())
{
    var results = (from u in db.AspNetUserses
        join upi in db.UserPersonalInfoes on u.Id equals upi.UserId into upis
        from upi in upis.DefaultIfEmpty()
        join up in db.UserPreferenceses on u.Id equals up.UserId into ups
        from up in ups.DefaultIfEmpty()
               join us in db.UserStatses on u.Id equals us.UserId into uss
               from us in uss.DefaultIfEmpty()
        select new
        {
            Username = u.UserName,
            Telephone = (upi == null ? String.Empty : upi.Telephone),
            ID = u.Id,
            LastLogin = (us == null ? DateTime.MinValue : us.LastLoginDate) 
        }).ToList();

       var list = (from r in results
                   select new {
                     Username = r.UserName,
                     Telephone = r.Telephone,
                     ID = r.ID
                     LastLogin = r.LastLogin == DateTime.MinValue ? "" : r.LastLogin.ToString()
                    }).ToList();


      gvUsers.DataSource = list;
      gvUsers.DataBind();
}