C# 在 LINQ to Entities 中使用 DateTime
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Using DateTime in LINQ to Entities
提问by max
I have a PostgreSQL database that interacts with the program through Entity Framework Code First.
我有一个 PostgreSQL 数据库，它通过 Entity Framework Code First 与程序交互。
Database contains a table "users" that has column "visit" type of DateTime.
数据库包含一个表“users”，其列“visit”类型为DateTime。
The application is described as;
该应用程序被描述为；
public class Users
{ ...
[Required]
[Column("visit")]
public DateTime VisitDate
...
}
I trying to run this query;
我试图运行这个查询；
var rslt = context.Visitors.Where(v => v.VisitDate.Date == DateTime.Now.Date).ToList()
But getting an exception: NotSupportedException
但得到一个例外： NotSupportedException
What's wrong?
怎么了？
采纳答案by MarcinJuraszek
DateTime.Dateproperty is not supported. You have to use SqlFunctions.DatePartmethod instead. It will end up with DATEPARTTSQLmethod within generated SQL query.
DateTime.Date不支持属性。你必须改用SqlFunctions.DatePart方法。它将以DATEPARTTSQL生成的 SQL 查询中的方法结束。
var rslt = context.Visitors
.Where(v => SqlFunctions.DatePart("year", v.VisitDate) == SqlFunctions.DatePart("year", DateTime.Now))
.Where(v => SqlFunctions.DatePart("dayofyear", v.VisitDate) == SqlFunctions.DatePart("dayofyear", DateTime.Now))
.ToList();
回答by Erik Schierboom
The problem is that the LINQ provider is trying to convert DateTime.Now.Dateto a database method, which it cannot do by default. The trick to doing date comparison is to create a DateTimeinstance that has its time component set to the default value. You can get more information hereand here.
问题在于 LINQ 提供程序正在尝试转换DateTime.Now.Date为数据库方法，默认情况下它无法执行此操作。进行日期比较的技巧是创建一个DateTime实例，将其时间组件设置为默认值。您可以在此处和此处获得更多信息。
回答by Mayur Borad
Use the class EntityFunction for trimming the time portion.
使用 EntityFunction 类来修剪时间部分。
using System.Data.Objects;
var bla = (from log in context.Contacts
where EntityFunctions.TruncateTime(log.ModifiedDate) == EntityFunctions.TruncateTime(today.Date)
select log).FirstOrDefault();
来源：http: //social.msdn.microsoft.com/Forums/en-US/csharpgeneral/thread/84d4e18b-7545-419b-9826-53ff1a0e2a62/
回答by Tim S
Mayur Borad's answer (IMHO more correct than the accepted answer) has become out of date:
Mayur Borad 的答案（恕我直言，比公认的答案更正确）已经过时了：
System.Data.Entity.Core.Objects.EntityFunctionsis obsolete. You should use System.Data.Entity.DbFunctionsinstead.
System.Data.Entity.Core.Objects.EntityFunctions已过时。你应该System.Data.Entity.DbFunctions改用。
var today = DateTime.Today; // (Time already removed)
var bla = context.Contacts
.FirstOrDefault(x => DbFunctions.TruncateTime(x.ModifiedDate) == today);
