LINQ：将查询表达式转换为使用 lambda
我正在尝试将 LINQ To Entities 查询重写为表达式。我的模型是一所可以有很多人的学校。人员继承给教师、学生等。
以下查询对我有用：
IQueryable<DAL.TEACHER> teacher =
from p in School
select p.PERSON as ESBDAL.TEACHER;
我如何将其写为查询表达式？我的想法是这样的：
IQueryable<DAL.TEACHER> teacher =
School.Select(x=>x.PERSON) as IQueryable<DAL.TEACHER>;
不幸的是这个声明不起作用。我是否误解了 .Select() ？
I'm trying to rewrite a LINQ To Entities query to an expression. My model is a School which can have many Persons. Persons are inherited out to teachers, students, etc.
The following query works for me:
IQueryable<DAL.TEACHER> teacher =
from p in School
select p.PERSON as ESBDAL.TEACHER;
How would I write this as a query expression? I thought something like:
IQueryable<DAL.TEACHER> teacher =
School.Select(x=>x.PERSON) as IQueryable<DAL.TEACHER>;
Unfortunately this statement doesn't work. Am I misunderstanding the .Select()?
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它实际上应该是：
记住，您正在尝试将
PERSON转换为TEACHER，因此它会进入委托内部。School.Select(x => x.PERSON)本身的结果实际上是一个IEnumerable，它不能转换为IEnumerable; 。另一件事 - 如果某些
PERSON实例实际上不是TEACHER（即它们是STUDENT），那么您最终会输出中包含null引用。如果PERSON可以是多种类型，您可能会想这样写出来：这实际上会过滤掉所有 的
PERSON实体tTEACHER，因此您只能返回序列中的TEACHER实例（无空值）。另请注意，
from x in y语法是“查询”（特别是查询理解语法）。第二种形式是 lambda 语法。It should actually be:
Remember, you're trying to cast the
PERSONto aTEACHER, so it goes inside the delegate. The result ofSchool.Select(x => x.PERSON)by itself is actually anIEnumerable<PERSON>, which is not convertible toIEnumerable<TEACHER>.One more thing - if some
PERSONinstances are not actuallyTEACHERs (i.e. they areSTUDENTs instead), you're going to end up withnullreferences in the output. IfPERSONcan be more than one type, you would probably want to write it out like this instead:This will actually filter out all of the
PERSONentities that aren'tTEACHERs, so you only get backTEACHERinstances in the sequence (no nulls).Also note that the
from x in ysyntax is the "query" (specifically query comprehension syntax). The second form is lambda syntax.