简单方法*在LINQ查询中转换对象？不是* [英] Easy way* to cast an object in a LINQ query? Methinks not*

问题描述

灵感来自Albahari第8章的优秀C＃3.0 in a Nutshell

（这本书太棒了，你必须得到它，如果你必须买一个C＃

book）以及Jon Skeet的书的附录A，我将通过

一些LINQ查询。但是如何施展？ （


见下面的内容，修改自别人的代码。


问题是查询''停止''（抛出一个演员例外）在3，并且

永远不会到达紫罗兰。


如何防止这种情况？


Pseudocode好的，只需要一个提示就可以了，但更多的当然是

更好。


RL


* PS - 通过简单的方式来投射我的意思是非常简短，可以在运行时工作，而不是像列出所有的CASE / SWITCH语句

可能提前，但是，我会得到所有答案。


//

List< objectwords = new List< object {" green"，" blue"，3，" violet"，

5};


//投放对象列表中键入''string''

//并投射每个字符串的前两个字母。


尝试

{

IEnumerable< strin gquery =

words.AsQueryable（）。Cast< string>（）。选择（str = str.Substring（0,2））;


foreach （查询中的字符串s）

Console.WriteLine（s）;

}


catch（ArgumentException ex）

{

Console.WriteLine（ex）;

}

catch（InvalidCastException ex2）

{

Console.WriteLine（" invalid cast！" + ex2）;

}


/ *此代码*应*生成以下输出，但

它突破了查询到达上面的''3'并抛出一个

InvalidCastException - 如何让它完成查询直到

结束？


gr

bl

vi

* /


RL

Inspired by Chapter 8 of Albahari''s excellent C#3.0 in a Nutshell
(this book is amazing, you must get it if you have to buy but one C#
book) as well as Appendix A of Jon Skeet''s book, I am going through
some LINQ queries. But how to cast? (

See the below, modified from somebody else''s code.

The problem is the query ''stops'' (throws a cast exception) at "3", and
never gets to "violet".

How to prevent this?

Pseudocode OK, just a hint is all I need, but more of course is
better.

RL

*PS--by "easy way to cast" I mean something very short, that can work
at runtime, not something like a CASE/SWITCH statement that lists all
possibilities ahead of time, but, I''ll take all answers.

//
List<objectwords = new List<object{ "green", "blue", 3, "violet",
5 };

// Cast the objects in the list to type ''string''
// and project the first two letters of each string.

try
{
IEnumerable<stringquery =
words.AsQueryable().Cast<string>().Select(str =str.Substring(0, 2));

foreach (string s in query)
Console.WriteLine(s);
}

catch (ArgumentException ex)
{
Console.WriteLine(ex);
}
catch (InvalidCastException ex2)
{
Console.WriteLine("invalid cast!" + ex2);
}

/* This code *should* produce the following output, but
it breaks out of the query when it gets to ''3'' above and throws an
InvalidCastException--how to get it to complete the query until the
end?

gr
bl
vi
*/

RL

推荐答案

我建​​议将你的关键线改为：


IEnumerable< stringquery =

words.AsQueryable（）。Cast< string>（）。Select（str = str.Substring（0，

Math.Min（str.Length，2）））;


" raylopez99" < ra ******** @ yahoo.comwrote in message

news：c6 ********************** ************ @ e39g2000 hsf.googlegroups.com ...

I suggest a change your critical line to:

IEnumerable<stringquery =
words.AsQueryable().Cast<string>().Select(str =str.Substring(0,
Math.Min(str.Length, 2)));

"raylopez99" <ra********@yahoo.comwrote in message
news:c6**********************************@e39g2000 hsf.googlegroups.com...

灵感来自Albahari优秀C＃的第8章坚果壳中的3.0

（这本书太棒了，如果你必须购买一本C＃

书，你必须得到它）以及Jon Skeet的附录A''的书，我正在经历

一些LINQ查询。但是如何施展？ （


见下面的内容，修改自别人的代码。


问题是查询''停止''（抛出一个演员例外）在3，并且

永远不会到达紫罗兰。


如何防止这种情况？


Pseudocode好的，只需要一个提示就可以了，但更多的当然是

更好。


RL


* PS - 通过简单的方式来投射我的意思是非常简短，可以在运行时工作，而不是像列出所有的CASE / SWITCH语句

可能提前，但是，我会得到所有答案。


//


List< ; objectwords = new List< object {" green"，" blue"，3，" violet"，

5};


//在列表中转换对象以键入''string''

//并投射每个字符串的前两个字母。


尝试

{

IEnumerable< stringquery =

words.AsQueryable（）。Cast< string>（）。Select（str = str.Substring（0,2））;


foreach（查询中的字符串s）

Console.WriteLine（s）;

}


catch（ArgumentException ex）

{

Console.WriteLine（ex）;

}

catch （InvalidCastException ex2）

{

Console.WriteLine（" invalid cast！" + ex2）;

}


/ *此代码*应*生成以下输出，但

它突破了查询到达上面的''3'并抛出一个

InvalidCastException - 如何让它完成查询直到

结束？


gr

bl

vi

* /


RL

Inspired by Chapter 8 of Albahari''s excellent C#3.0 in a Nutshell
(this book is amazing, you must get it if you have to buy but one C#
book) as well as Appendix A of Jon Skeet''s book, I am going through
some LINQ queries. But how to cast? (

See the below, modified from somebody else''s code.

The problem is the query ''stops'' (throws a cast exception) at "3", and
never gets to "violet".

How to prevent this?

Pseudocode OK, just a hint is all I need, but more of course is
better.

RL

*PS--by "easy way to cast" I mean something very short, that can work
at runtime, not something like a CASE/SWITCH statement that lists all
possibilities ahead of time, but, I''ll take all answers.

//
List<objectwords = new List<object{ "green", "blue", 3, "violet",
5 };

// Cast the objects in the list to type ''string''
// and project the first two letters of each string.

try
{
IEnumerable<stringquery =
words.AsQueryable().Cast<string>().Select(str =str.Substring(0, 2));

foreach (string s in query)
Console.WriteLine(s);
}

catch (ArgumentException ex)
{
Console.WriteLine(ex);
}
catch (InvalidCastException ex2)
{
Console.WriteLine("invalid cast!" + ex2);
}

/* This code *should* produce the following output, but
it breaks out of the query when it gets to ''3'' above and throws an
InvalidCastException--how to get it to complete the query until the
end?

gr
bl
vi
*/

RL

raylopez99< ra ******** @ yahoo.comwrote：

raylopez99 <ra********@yahoo.comwrote:

灵感来自阿尔巴哈里第8章的优秀C＃3.0 in a Nutshell

（这本书太棒了，你必须得到它，如果你必须买一个C＃

book）以及Jon Skeet的书的附录A，我将通过

一些LINQ查询。但是如何施展？ （


见下面的内容，修改自别人的代码。


问题是查询''停止''（抛出一个演员例外）在3，并且

永远不会到达紫罗兰。


如何防止这种情况？

Inspired by Chapter 8 of Albahari''s excellent C#3.0 in a Nutshell
(this book is amazing, you must get it if you have to buy but one C#
book) as well as Appendix A of Jon Skeet''s book, I am going through
some LINQ queries. But how to cast? (

See the below, modified from somebody else''s code.

The problem is the query ''stops'' (throws a cast exception) at "3", and
never gets to "violet".

How to prevent this?

请参阅OfType方法。


-

Jon Skeet - < sk *** @ pobox.com>

网站： http：// www。 pobox.com/~skeet

博客： http://www.msmvps.com/jon.skeet

C＃深度： http://csharpindepth.com

See the "OfType" method.

--
Jon Skeet - <sk***@pobox.com>
Web site: http://www.pobox.com/~skeet
Blog: http://www.msmvps.com/jon.skeet
C# in Depth: http://csharpindepth.com

顺便说一下，我之所以这样说是因为我没有得到无效的演员，但

而不是

索引超出范围错误...

Family Tree Mike< Fa ****** ****** @ ThisOldHouse.com写信息

新闻：uO ************** @ TK2MSFTNGP04.phx.gbl ...

By the way, the reason I said this is that I don''t get an invalid cast, but
rather
an index out of range error...
"Family Tree Mike" <Fa************@ThisOldHouse.comwrote in message
news:uO**************@TK2MSFTNGP04.phx.gbl...

>我建议将你的关键行改为：


IEnumerable< stringquery =

words.AsQueryable（）。Cast< string> ;（）。选择（str = str.Substring（0，

Math.Min（str.Length，2）））;


" raylopez99" ; < ra ******** @ yahoo.comwrote in message

>I suggest a change your critical line to:

IEnumerable<stringquery =
words.AsQueryable().Cast<string>().Select(str =str.Substring(0,
Math.Min(str.Length, 2)));

"raylopez99" <ra********@yahoo.comwrote in message

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