Asp.net 如何使用EF4.1筛选可为空的DateTime字段？

我有一个带有可空DateTime字段的表：

CREATE TABLE [dbo].[myTable](
  [ID] [int] IDENTITY(1,1) NOT FOR REPLICATION NOT NULL,
  ...
  [FinishedDate] [datetime] NULL,
  etc...

当我尝试这个：

var activeThings = from foo in _context.myTable
                   where foo.FinishedDate == null
                   select foo;

foreach ( var thing in activeThings ) {
   ... do some stuff ...
}

我得不到任何回报。如何根据空值筛选此项

var activeThings = from foo in _context.myTable
                   where !foo.FinishedDate.HasValue //foo.FinishedDate.HasValue==false
                   select foo;

HasValue返回布尔值

is true=>不为空

为false=>null