首先,我做了一些搜索这个问题的答案...但我找不到任何东西,因为我不知道我用什么功能被称为。LINQ:为什么我不能在返回的对象中声明新对象?
以下代码中的DBEntities由devart为Oracle数据库连接生成。
为什么第一个例子工作,第二个例子只给我一个System.NullReferenceException?
有效代码:
using (DBEntities context = new DBEntities())
{
var infos = (from info in context.Infos
where info.Index == index
orderby info.Name
select new
{
Name = info.Name,
MRN = info.MRN,
UnitNumber = (info.UnitNum == null) ? -1 : (decimal)info.UnitNum,
UnitName = (info.UnitName == null) ? String.Empty : info.UnitName
}).Distinct();
foreach (var info in infos)
{
// *do stuff*
}
}
代码,让异常(一旦达到的foreach例外仅见于):
using (DBEntities context = new DBEntities())
{
var infos = (from info in context.Infos
where info.Index == index
orderby info.Name
select new Member()
{
Name = info.Name,
MRN = info.MRN,
CurrentUnit = new Unit()
{
UnitNumber = (info.UnitNum == null) ? -1 : (decimal)info.UnitNum,
UnitName = (info.UnitName == null) ? String.Empty : info.UnitName
}
}).Distinct();
foreach (Member info in infos)
{
// *do stuff*
}
}
编辑: 我想补充一点,也可以使用:
using (DBEntities context = new DBEntities())
{
var infos = (from info in context.Infos
where info.Index == index
orderby info.Name
select new Member()
{
Name = info.Name,
MRN = info.MRN
}).Distinct();
foreach (Member info in infos) //Exception is thrown here.
{
// *do stuff*
}
}
你有什么异常? – Thebigcheeze 2012-04-06 22:20:07
@Thebigcheeze System.NullReferenceException – bsara 2012-04-06 22:23:10
是否确定你的集合包含数据? – Tigran 2012-04-06 22:25:09