新手LINQ问题 [英] Newbie LINQ Question
问题描述
我有以下代码摘录...
--- top ---
列表< PersonPersons =新列表< Person> （）;
Persons.Add（新人（彼得，28，珀斯））;
Persons.Add（新人（马修） ;，31，Bundaberg））;
Persons.Add（new Person（Cathryn，36，Perth））;
//这是有效的
IEnumerable< Personp1 =来自人的操作
其中op.City ==" Perth"
select op;
//但是这不起作用
列表< Personp2 =来自人的操作
其中op.City =="珀斯
选择操作;
---底部---
编译器错误消息是：编译器错误消息：CS0266：不能
隐式转换类型
''System.Collections.Generic.IEnumerable< ASP.linq_l inqgrouping_aspx.Person>''
到
''System.Collections.Generic.List< ASP.l inq_linqgrou ping_aspx.Person>''。
存在显式转换（你是否错过演员表？）
我正试图不进入使用var的习惯，因为我在某处（MSDN）读到它会使你的代码对其他人更难以理解，
我倾向于同意。
有人可以告诉我我做错了吗？
提前致谢，
Damien
Hi, I have the following code extract...
---top---
List<PersonPersons = new List<Person>();
Persons.Add(new Person("Peter", 28,"Perth"));
Persons.Add(new Person("Matthew", 31, "Bundaberg"));
Persons.Add(new Person("Cathryn", 36, "Perth"));
//This works
IEnumerable<Personp1 = from op in Persons
where op.City=="Perth"
select op;
//Tut this doesn''t work
List<Personp2 = from op in Persons
where op.City == "Perth"
select op;
---bottom---
The compiler error message is: Compiler Error Message: CS0266: Cannot
implicitly convert type
''System.Collections.Generic.IEnumerable<ASP.linq_l inqgrouping_aspx.Person>''
to
''System.Collections.Generic.List<ASP.linq_linqgrou ping_aspx.Person>''.
An explicit conversion exists (are you missing a cast?)
I''m trying to not get into the habit of using var because I read
somewhere (MSDN) that it makes your code more unreadable to others,
which I tend to agree with.
Can someone please tell me what I''m doing wrong?
Thanks in advance,
Damien
推荐答案
6月20日，8：51 * am，damiensaw ... @ yahoo.com.au
< damiensaw。 .. @ yahoo.com.auwrote：
< snip>
On Jun 20, 8:51*am, "damiensaw...@yahoo.com.au"
<damiensaw...@yahoo.com.auwrote:
<snip>
我正试图不进入使用var的习惯是因为我在某处（MSDN）读到它会使你的代码对其他人更难以理解，
我倾向于同意。
I''m trying to not get into the habit of using var because I read
somewhere (MSDN) that it makes your code more unreadable to others,
which I tend to agree with.
我个人发现var *增加*可读性而不是降低它的价值，但这是个人的事情。
I personally find that var *increases* readability rather than
reducing it, but it''s a personal thing.
有人可以告诉我，我做错了吗？
Can someone please tell me what I''m doing wrong?
正是编译器所说的 -
RHS上的表达式的结果是IEnumerable< Person>，而不是List< Person>。
如果你想在List中，你应该在
查询表达式的结果上调用ToList。
提醒你，如果你只是*使用在哪里子句代码将
可能更简单如下：
列表< Personp2 = people.Where（person =>
person.City == 珀斯.ToList（）;
（请注意，我已将人员更改为人员以跟随命名
惯例和让阅读更舒服:)
Jon
Exactly what the compiler says - the result of the expression on the
RHS is IEnumerable<Person>, not List<Person>.
If you want it in a List, you should call ToList on the result of the
query expression.
Mind you, if you''re *just* using a "where" clause the code would
probably be simpler as:
List<Personp2 = people.Where(person =>
person.City=="Perth").ToList();
(Note that I''ve changed "Persons" to "people" to follow naming
conventions and make for more comfortable reading :)
Jon
da ********** @ yahoo.com.au 写道：
我有以下代码摘录...
---顶部---
列表< PersonPersons =新列表< ;人>（）;
Persons.Add（新人（彼得，28，珀斯））;
Persons.Add（new Person（ Matthew，31，Bundaberg））;
Persons.Add（new Person（" Cathryn"，36，" Perth））;
//这是有效的
IEnumerable< Person p1 =来自op in Persons
其中op.City ==" Perth"
select op;
// Tut这不起作用
列表< Personp2 =来自op in Persons
其中op.City ==" Perth"
select op ;
---底部---
编译器错误消息是：编译器错误消息：CS0266：无法隐含地
转换类型
''System.Collections.Generic.IEnumerable< ASP.linq_l inqgrouping_aspx.Person>''
to
''系统。 Collections.Generic.List< ASP.linq_linqgrou ping_aspx.Person>''。
存在显式转换（你是否错过演员表？）
我'我试图不养成使用var的习惯，因为我在某个地方（MSDN）读到了它使你的代码对其他人来说更难以理解，
whic我倾向于同意。
有人可以告诉我我做错了什么吗？
Hi, I have the following code extract...
---top---
List<PersonPersons = new List<Person>();
Persons.Add(new Person("Peter", 28,"Perth"));
Persons.Add(new Person("Matthew", 31, "Bundaberg"));
Persons.Add(new Person("Cathryn", 36, "Perth"));
//This works
IEnumerable<Personp1 = from op in Persons
where op.City=="Perth"
select op;
//Tut this doesn''t work
List<Personp2 = from op in Persons
where op.City == "Perth"
select op;
---bottom---
The compiler error message is: Compiler Error Message: CS0266: Cannot
implicitly convert type
''System.Collections.Generic.IEnumerable<ASP.linq_l inqgrouping_aspx.Person>''
to
''System.Collections.Generic.List<ASP.linq_linqgrou ping_aspx.Person>''.
An explicit conversion exists (are you missing a cast?)
I''m trying to not get into the habit of using var because I read
somewhere (MSDN) that it makes your code more unreadable to others,
which I tend to agree with.
Can someone please tell me what I''m doing wrong?
来自op.City ==" Perth" select op;''part isn'n
a List< Person but an IEnumerable。如果你想要一个List< Person>，
，你应该这样做：
List< Personp2 =（来自op in Persons
其中op.City ==" Perth"
select op）.ToList（）;
背景是linq到对象，这是你使用的是什么
''人'是一个IEnumerable，而不是一个包含Linq
提供者的IQueryable，只是做一个对你的人有收益的foreach 。
所以查询等于这段代码：
foreach（人物操作人员）
[
if（op.City！=" Perth"）
{
继续;
}
收益率返回操作;
}
只生成一个IEnumerable< PersonEnumerator，由
创建编译器。
FB
-
------------- -------------------------------------------------- ---------
LLBLGen Pro的首席开发人员，富有成效用于.NET的O / R映射器
LLBLGen Pro网站： http：// www .llblgen.com
我的.NET博客： http ：//weblogs.asp.net/fbouma
Microsoft MVP（C＃）
--------------- -------------------------------------------------- -------
the ''from op in Persons where op.City == "Perth" select op;'' part isn''t
a List<Personbut an IEnumerable. If you want to have a List<Person>,
you should do:
List<Personp2 = (from op in Persons
where op.City == "Perth"
select op).ToList();
The background is that linq to objects, which is what you''re using as
''Persons'' is an IEnumerable and not an IQueryable containing a Linq
provider, simply does a foreach with a yield over your persons.
so the query is equal to this code:
foreach(Person op in Persons)
[
if(op.City!="Perth")
{
continue;
}
yield return op;
}
which simply produces an IEnumerable<PersonEnumerator, created by the
compiler.
FB
--
------------------------------------------------------------------------
Lead developer of LLBLGen Pro, the productive O/R mapper for .NET
LLBLGen Pro website: http://www.llblgen.com
My .NET blog: http://weblogs.asp.net/fbouma
Microsoft MVP (C#)
------------------------------------------------------------------------
Jon Skeet [C＃MVP]写道：
Jon Skeet [C# MVP] wrote:
我个人觉得var增加了可读性而不是降低它的价值，但这是个人的事情。
I personally find that var increases readability rather than
reducing it, but it''s a personal thing.
你在这方面的表现相当不错......
http://www.codinghorror.com/blog/archives/001136.html
问候蒂姆。
-
You are in pretty good company on that score...
http://www.codinghorror.com/blog/archives/001136.html
Regards Tim.
--
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