blocks|key|3762213|text|不，它返回null。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3762214|entityMap^0|5|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

No, it returns <code>null</code>.

blocks|key|1438777|text|从MSDN+documentation+for+NameValueCollection.Item+Property+(String)|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1438778|1438779|+Caution|blockquote|style|BOLD|1438780|1438781|在以下情况下，此属性返回null+:+1)如果找不到指定的键；2)如果找到指定的键并且其关联值为null。此属性不区分这两种情况。|1438782|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/8d0bzeeb.aspx^0|1|1T|0|0|0|1|7|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|Z|8|@$A|10|B|11|H|I]]|9|@]|C|$]]|$1|J|3|-4|5|6|7|12|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|13|8|@]|9|@]|C|$]]|$1|M|3|-4|5|6|7|14|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

From the <a href="http://msdn.microsoft.com/en-us/library/8d0bzeeb.aspx" rel="noreferrer">MSDN documentation for NameValueCollection.Item Property (String)</a>:
<blockquote>
Caution
This property returns null in the following cases: 1) if the specified key is not found; and 2) if the specified key is found and its associated value is null. This property does not distinguish between the two cases.
</blockquote>

blocks|key|3762327|text|不，它返回null。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3762328|AppSettings是一个NameValueCollection+-根据NameValueCollection.Get页面上的caution：|3762329|3762330|在以下情况下，此方法返回空引用(在Visual+Basic中为Nothing)：1)如果找不到指定的键；以及2)如果找到指定的键并且其关联值为空引用(在Visual+Basic中为Nothing)。此方法不区分这两种情况。|blockquote|3762331|3762332|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/system.collections.specialized.namevaluecollection.aspx|1|http://msdn.microsoft.com/en-us/library/s8e7f4s6.aspx|2^0|5|4|0|E|J|0|11|N|1|1S|7|2|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|10|8|@]|D|@$9|11|A|12|1|13]|$9|14|A|15|1|16]|$9|17|A|18|1|19]]|E|$]]|$1|H|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]|$1|I|3|J|5|K|7|1B|8|@]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|1D|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]|W|$5|P|Q|R|E|$S|V]]]]

No, it returns <code>null</code>.

AppSettings is a <a href="http://msdn.microsoft.com/en-us/library/system.collections.specialized.namevaluecollection.aspx" rel="noreferrer">NameValueCollection</a> - as per the <a href="http://msdn.microsoft.com/en-us/library/s8e7f4s6.aspx" rel="noreferrer">caution</a> on the <a href="http://msdn.microsoft.com/en-us/library/s8e7f4s6.aspx" rel="noreferrer">NameValueCollection.Get</a> page:

<blockquote>
 This method returns a null reference
 (Nothing in Visual Basic) in the
 following cases: 1) if the specified
 key is not found; and 2) if the
 specified key is found and its
 associated value is a null reference
 (Nothing in Visual Basic). This method
 does not distinguish between the two
 cases.
</blockquote>

blocks|key|3762275|text|不，它返回null。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3762276|ConfigurationManager.AppSettings是一个来自the+MSDN+documentation的NameValueCollection|3762277|3762278|+Get方法不区分由于未找到指定的键而返回的null和由于与键关联的值为而返回的null。|blockquote|BOLD|3762279|3762280|3762281|(我的重点)|3762282|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/system.collections.specialized.namevaluecollection.aspx^0|5|4|0|0|W|1O|J|11|M|0|0|0|M|E|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@$9|16|A|17|1|18]]|E|$]]|$1|H|3|-4|5|6|7|19|8|@]|D|@]|E|$]]|$1|I|3|J|5|K|7|1A|8|@$9|1B|A|1C|B|L]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|1D|8|@]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|1F|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]]]

No, it returns <code>null</code>.

<code>ConfigurationManager.AppSettings</code> is a <code>NameValueCollection</code> - from <a href="http://msdn.microsoft.com/en-us/library/system.collections.specialized.namevaluecollection.aspx" rel="noreferrer">the MSDN documentation</a>:

<blockquote>
 The Get method does not distinguish
 between null which is returned because
 the specified key is not found and
 null which is returned because the
 value associated with the key is null.
</blockquote>

(my emphasis)

blocks|key|1442169|text|其他答案引用了Item属性的文档。看一下下面的代码片段，可能不会立即明白为什么它们是相关的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1442170|ConfigurationManager.AppSettings["blah"]|code-block|syntax|javascript|1442171|在C#中使用方括号语法来访问索引器。这些是特殊的属性，允许以与数组相同的方式对类进行索引。查看NameValueCollection.Item属性的定义，您会注意到它没有使用正常的属性语法。this关键字和索引器参数用于将此属性定义为索引器。|offset|length|1442172|public+string+this[
++++string+name
]+{+get;+set;+}|1442173|在文档中，索引器隐式命名为Item，参数用方括号括起来。|1442174|​|1442175|📷|atomic|1442176|1442177|我不清楚为什么会有引用Get方法的答案--也许一个调用了另一个？|1442178|不管怎样，要回答这个问题...|1442179|号如果你访问一个不存在的键，将不会抛出异常--将返回null。|style|BOLD|1442180|以下是NameValueCollection.Item属性文档中的相关部分。|1442181|1442182|在以下情况下，此属性返回null+:+1)如果找不到指定的键；2)如果找到指定的键并且其关联值为null。此属性不区分这两种情况。|blockquote|1442183|1442184|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/8d0bzeeb.aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-1|1|IMAGE|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/ab28a707f63d0796a287a19d6060fd00.jpeg|imageAlt|2|http://msdn.microsoft.com/en-us/library/s8e7f4s6(v=vs.110).aspx|3|http://msdn.microsoft.com/en-us/library/8d0bzeeb(v=vs.110).aspx^0|0|0|1B|O|0|0|0|0|0|0|1|1|0|0|B|3|2|0|0|0|V|0|3|O|3|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1R|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1S|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1T|8|@]|9|@$I|1U|J|1V|1|1W]]|A|$]]|$1|K|3|L|5|D|7|1X|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|Q|3|R|5|S|7|20|8|@]|9|@$I|21|J|22|1|23]]|A|$]]|$1|T|3|P|5|6|7|24|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|25|8|@]|9|@$I|26|J|27|1|28]]|A|$]]|$1|W|3|X|5|6|7|29|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|2A|8|@$I|2B|J|2C|10|11]]|9|@]|A|$]]|$1|12|3|13|5|6|7|2D|8|@]|9|@$I|2E|J|2F|1|2G]]|A|$]]|$1|14|3|-4|5|6|7|2H|8|@]|9|@]|A|$]]|$1|15|3|16|5|17|7|2I|8|@]|9|@]|A|$]]|$1|18|3|-4|5|6|7|2J|8|@]|9|@]|A|$]]|$1|19|3|-4|5|6|7|2K|8|@]|9|@]|A|$]]]|1A|$1B|$5|1C|1D|1E|A|$1F|1G]]|1H|$5|1I|1D|1J|A|$1K|1L|1M|-4]]|1N|$5|1C|1D|1E|A|$1F|1O]]|1P|$5|1C|1D|1E|A|$1F|1Q]]]]

Other answers reference the documentation for the Item property. It might not be immediately obvious why they are relevant looking at the following code snippet.

<pre><code>ConfigurationManager.AppSettings["blah"]
</code></pre>

The square bracket syntax is used in C# to access indexers. These are special properties that allow a class to be indexed in the same way that an array can be. Looking at the definition of the <a href="http://msdn.microsoft.com/en-us/library/8d0bzeeb.aspx?cs-save-lang=1&amp;cs-lang=csharp#code-snippet-1" rel="nofollow noreferrer">NameValueCollection.Item</a> property, you will notice that it does not use the normal property syntax. The this keyword and the indexer parameters are used to define this property as an indexer.

<pre><code>public string this[
 string name
] { get; set; }
</code></pre>

In the documentation, indexers are implicitly named Item and parameters are surrounded by square brackets.

<img src="https://i.stack.imgur.com/sWa5P.jpg" alt="Indexers as shown in the MSDN documentation.">

It is not clear to me why there were answers that referenced the <a href="http://msdn.microsoft.com/en-us/library/s8e7f4s6(v=vs.110).aspx" rel="nofollow noreferrer">Get</a> method - maybe one calls the other?

At any rate, to answer the question...

No. An exception will not be thrown if you access a non-existent key - a null will be returned.

Here is the relevant section from the <a href="http://msdn.microsoft.com/en-us/library/8d0bzeeb(v=vs.110).aspx" rel="nofollow noreferrer">NameValueCollection.Item</a> property documentation.

<blockquote>
 This property returns null in the following cases: 1) if the specified
 key is not found; and 2) if the specified key is found and its
 associated value is null. This property does not distinguish between
 the two cases.
</blockquote>

blocks|key|695474|text|正如Tim所说，它将只返回null。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|695475|然而，如果你想让它在找不到的时候抛出异常，你可以这样做：|695476|var+myImportantSetting=+ConfigurationManager.AppSettings["important_setting"]+??+throw+new+SettingsPropertyNotFoundException("AppSetting+missing.");|code-block|syntax|javascript|695477|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

As Tim said, it will just return null.
However, if you want it to throw an exception when not found one could do this:
<pre><code>var myImportantSetting= ConfigurationManager.AppSettings[&quot;important_setting&quot;] ?? throw new SettingsPropertyNotFoundException(&quot;AppSetting missing.&quot;);
</code></pre>

blocks|key|3762204|text|是，http://msdn.microsoft.com/en-us/library/system.configuration.configurationmanager.appsettings.aspx|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3762205|编辑:这显然是错误的。左图是下面有用的评论。|3762206|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/system.configuration.configurationmanager.appsettings.aspx^0|2|2Q|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

Yes <a href="http://msdn.microsoft.com/en-us/library/system.configuration.configurationmanager.appsettings.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/system.configuration.configurationmanager.appsettings.aspx</a>

Edit: this is clearly wrong. Left for the helpful comments below.

Will ConfigurationManager.AppSettings[&quot;blah&quot;] throw an exception if &quot;blah&quot; doesn't exist in the web/app.config?

Will ConfigurationManager.AppSettings["blah"] throw an exception if "blah" doesn't exist?

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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如果web/app.config中不存在"blah“，ConfigurationManager.AppSettings"blah”会抛出异常吗？

问如果"blah“不存在，ConfigurationManager.AppSettings["blah"]会抛出异常吗？
EN

回答 7

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问如果"blah“不存在，ConfigurationManager.AppSettings["blah"]会抛出异常吗？EN

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问如果"blah“不存在，ConfigurationManager.AppSettings["blah"]会抛出异常吗？
EN