blocks|key|264533|text|var+q+=+from+PropertyDescriptor+t+in+TypeDescriptor.GetProperties(instance)
++++++++let+u+=+t.ComponentType.GetProperty(t.Name)
++++++++where+(u+!=+null)
++++++++select+u;|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|264534|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>var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
 let u = t.ComponentType.GetProperty(t.Name)
 where (u != null)
 select u;
</code></pre>

blocks|key|47988|text|您需要let|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|47989|var+q+=+from+PropertyDescriptor+t+in+TypeDescriptor.GetProperties(instance)
++++++++let+name+=+t.ComponentType.GetProperty(t.Name)
++++++++where+(name+!=+null)
++++++++select+name;|code-block|syntax|javascript|47990|如果你想用查询语法来做这件事，你可以用一种更高效、更干净的方式(afaik)来做：|47991|var+q+=+TypeDescriptor
++++++++++++.GetProperties(instance)
++++++++++++.Select(t+=>+t.ComponentType.GetProperty(t.Name))
++++++++++++.Where(name+=>+name+!=+null);|47992|entityMap^0|3|3|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|P|$]]

You need <code>let</code>:

<pre><code>var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
 let name = t.ComponentType.GetProperty(t.Name)
 where (name != null)
 select name;
</code></pre>

If you wanted to do it in query syntax, you could do it in a more efficient (afaik) and cleaner way:

<pre><code>var q = TypeDescriptor
 .GetProperties(instance)
 .Select(t =&gt; t.ComponentType.GetProperty(t.Name))
 .Where(name =&gt; name != null);
</code></pre>

blocks|key|264580|text|是的，使用let关键字：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|264581|var+q+=+from+PropertyDescriptor+t+in+TypeDescriptor.GetProperties(instance)
++++let+nameProperty+=+t.ComponentType.GetProperty(t.Name)
++++where+(nameProperty+!=+null)
++++select+nameProperty;|code-block|syntax|javascript|264582|entityMap^0|5|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Yes, using the <code>let</code> keyword:

<pre><code>var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
 let nameProperty = t.ComponentType.GetProperty(t.Name)
 where (nameProperty != null)
 select nameProperty;
</code></pre>

blocks|key|264618|text|还有一个鲜为人知的替代方案(+select+a+into+b)：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|264619|var+q+=+from+PropertyDescriptor+t+in+TypeDescriptor.GetProperties(instance)
++++++++select+t.ComponentType.GetProperty(t.Name)+into+u
++++++++where+u+!=+null
++++++++select+u;|code-block|syntax|javascript|264620|这可以转化为：|264621|var+q+=+TypeDescriptor.GetProperties(instance)
++++++++.Select(t+=>+t.ComponentType.GetProperty(t.Name))
++++++++.Where(prop+=>+prop+!=+null);|264622|而let-based版本转换为：|264623|var+q+=+TypeDescriptor.GetProperties(instance)
++++++++.Select(t+=>+new+{+t,+prop+=+t.ComponentType.GetProperty(t.Name)+})
++++++++.Where(x+=>+x.prop+!=+null)
++++++++.Select(x+=>+x.prop);|264624|每项不必要的分配，因为t仍在作用域中(尚未使用)。C#编译器应该把它优化掉，但它没有(或者语言规范不允许，不确定)。|264625|entityMap^0|F|F|0|0|0|0|1|3|0|0|B|1|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|12|8|@$9|13|A|14|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|V|$]]

There is an alternative that few people know about ( <code>select a into b</code>):

<pre><code>var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
 select t.ComponentType.GetProperty(t.Name) into u
 where u != null
 select u;
</code></pre>

This translates into:

<pre><code>var q = TypeDescriptor.GetProperties(instance)
 .Select(t =&gt; t.ComponentType.GetProperty(t.Name))
 .Where(prop =&gt; prop != null);
</code></pre>

Whereas the <code>let</code>-based version translates to:

<pre><code>var q = TypeDescriptor.GetProperties(instance)
 .Select(t =&gt; new { t, prop = t.ComponentType.GetProperty(t.Name) })
 .Where(x =&gt; x.prop != null)
 .Select(x =&gt; x.prop);
</code></pre>

An unnecessary allocation per item because <code>t</code> is still in scope (yet unused). The C# compiler should just optimize that away, but it doesn't (or the language spec does not allow it, not sure).

Can I declare / use some variable in <a href="http://en.wikipedia.org/wiki/Language_Integrated_Query" rel="noreferrer">LINQ</a>?

For example, can I write following LINQ clearer?

<pre><code>var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
 where (t.ComponentType.GetProperty(t.Name) != null)
 select t.ComponentType.GetProperty(t.Name);
</code></pre>

Are there ways to not write / call <code>t.ComponentType.GetProperty(t.Name)</code> two times here?

Can I declare / use some variable in LINQ? Or can I write following LINQ clearer?

编译器

我可以在中声明/使用一些变量吗例如，我可以写下面的LINQ更清晰吗？var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)        where (t.ComponentType.GetProperty(t.Name) ...

问我可以在LINQ中声明/使用一些变量吗？或者我可以写下面的LINQ更清楚？
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问我可以在LINQ中声明/使用一些变量吗？或者我可以写下面的LINQ更清楚？
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