blocks|key|4014244|text|查询的实际类型将是IEnumerable。LINQ有一个deffered+execution的概念，为了让你的查询真正被执行，你需要调用一个遍历LINQ的方法|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4014245|++++string+currentLabel+=+(from+s2f+in+stream2FieldTypesTable
++++++++++++++++++++++++++where+s2f.s2fID+==+item.s2fID
++++++++++++++++++++++++++&&+(s2f.s2fLabel+!=+item.s2fLabel+%7C%7C+s2f.s2fIsRequired+!=+item.s2fIsRequired)
++++++++++++++++++++++++++select+s2f.s2fLabel)
+++++++++++++++++++++++++.FirstOrDefault();|code-block|syntax|javascript|4014246|为了获得精确的实例，它可以是FirstOrDefault()方法或SignleOrDefault()，或者只是First()或Single()。唯一的区别是，如果枚举不能满足它们的期望，没有"OrDefault()“的方法将抛出异常，而具有"OrDefault()”的方法将只返回null。|4014247|编辑|style|BOLD|4014248|Single和First之间的区别是Single期望集合中的一个确切的元素，而First期望至少一个元素。|4014249|entityMap|0|LINK|mutability|MUTABLE|url|http://blogs.msdn.com/b/charlie/archive/2007/12/09/deferred-execution.aspx^0|S|I|0|0|0|0|0|2|0|Q|9|1A|2|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@$A|Z|B|10|1|11]]|C|$]]|$1|D|3|E|5|F|7|12|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|13|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|14|8|@$A|15|B|16|M|N]]|9|@]|C|$]]|$1|O|3|P|5|6|7|17|8|@$A|18|B|19|M|N]|$A|1A|B|1B|M|N]]|9|@]|C|$]]|$1|Q|3|-4|5|6|7|1C|8|@]|9|@]|C|$]]]|R|$S|$5|T|U|V|C|$W|X]]]]

Actual type of your query will be IEnumerable. LINQ has a concept of <a href="http://blogs.msdn.com/b/charlie/archive/2007/12/09/deferred-execution.aspx" rel="noreferrer">deffered execution</a>, in order to get you query actually exectuted you need to call a method that will iterate over IEnumerable:

<pre><code> string currentLabel = (from s2f in stream2FieldTypesTable
 where s2f.s2fID == item.s2fID
 &amp;&amp; (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired != item.s2fIsRequired)
 select s2f.s2fLabel)
 .FirstOrDefault();
</code></pre>

For getting exact one instanse it can be FirstOrDefault() method or SignleOrDefault() or just First() or Single(). The only difference is that methods without "OrDefault()" will throw an exception if enumeration will not satisfy their expectations, and methods with "OrDefault()" will just return null.

Edit

The dirrerence between Single and First is that Single expects exact one element in the collection and First expects at least one element.

blocks|key|350286|text|尝试使用First()或FirstOrDefault()来代替(或补充)+ToString()。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|350287|First和FirstOrDefault返回单个记录，而不是第一个查询所做的记录集合。|350288|entityMap^0|4|7|C|G|11|A|0|0|5|6|E|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|I|$]]

Try <code>First()</code> or <code>FirstOrDefault()</code> instead of (or in addition to) <code>ToString()</code>. 

<code>First</code> and <code>FirstOrDefault</code> return a single record, rather than a collection of records, which is what your first query does.

blocks|key|4014329|text|试试这段代码|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4014330|var+result+=+(from+s2f+in+stream2FieldTypesTable
++++++++++++++++++++++where+s2f.s2fID+==+item.s2fID
++++++++++++++++++++++&&+(s2f.s2fLabel+!=+item.s2fLabel+%7C%7C+s2f.s2fIsRequired+!=+item.s2fIsRequired)
++++++++++++++++++++++select+s2f.s2fLabel).First();|code-block|syntax|javascript|4014331|它是必需，因为您的结果是|4014332|IEnumerable<String>|4014333|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Try this code

<pre><code>var result = (from s2f in stream2FieldTypesTable
 where s2f.s2fID == item.s2fID
 &amp;&amp; (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired != item.s2fIsRequired)
 select s2f.s2fLabel).First();
</code></pre>

it is need, because your result is 

<pre><code>IEnumerable&lt;String&gt;
</code></pre>

blocks|key|350393|text|所有你需要做的就是在你的LINQ前后加上括号，然后使用.FirstOrDefault()就是你只想要一个值(因为你有字符串currentLabel)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|350394|select字符串currentLabel+=+(from+s2f+in+stream2FieldTypesTable+s2f.s2fID+==+item.s2fID+&&+(s2f.s2fLabel+!=+item.s2fLabel+%7C%7C+s2f.s2fIsRequired+!=+item.s2fIsRequired)+select+s2f.s2fLabel).FirstOrDefault();|350395|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

All u need to do is Add parenthesis before and after uour LINQ then use .FirstOrDefault() is u want only single value(which u r bcoz u have string currentLabel).

<blockquote>
 string currentLabel = (from s2f in
 stream2FieldTypesTable
 where s2f.s2fID == item.s2fID
 &amp;&amp; (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired
 != item.s2fIsRequired)
 select s2f.s2fLabel).FirstOrDefault();
</blockquote>

blocks|key|1524102|text|+int+id+=Convert.ToInt32(db.Table.Single(x+=>+x.Id+==+id_factor).Id);|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1524103|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code> int id =Convert.ToInt32(db.Table.Single(x =&gt; x.Id == id_factor).Id);
</code></pre>

I am using this C# with linq to sql:

<pre><code>string currentLabel = from s2f in stream2FieldTypesTable
 where s2f.s2fID == item.s2fID
 &amp;&amp; (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired != item.s2fIsRequired)
 select s2f.s2fLabel;
</code></pre>

I am getting a compiler error saying i can't assign type <code>System.Linq.IQueryable&lt;string&gt;</code> to <code>string</code>.

I tried this code:

<pre><code>string currentLabel = from s2f in stream2FieldTypesTable
 where s2f.s2fID == item.s2fID
 &amp;&amp; (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired != item.s2fIsRequired)
 select s2f.s2fLabel.ToString();
</code></pre>

And that returns the same error. I'm sure this is a simple thing. what am I missing? I just want the first <code>s2fLabel</code> that matches the where clause.

How to use linq to sql to select one column from row

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我在linq to sql中使用这个C#：string currentLabel = from s2f in stream2FieldTypesTable                      where s2f.s2fID == item.s2fID                      && (s2f.s2...

问如何使用linq to sql从行中选择一列
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问如何使用linq to sql从行中选择一列EN