blocks|key|169060|text|您可以使用其他LINQ方法的组合来处理不匹配条件：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|169061|var+res+=+dictionary.Where(x+=>+x.Value.ID+==+someID)
++++++++++++++++++++.Select(x+=>+x.Value.DisplayName)
++++++++++++++++++++.DefaultIfEmpty("Unknown")
++++++++++++++++++++.First();|code-block|syntax|javascript|169062|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can use a combination of other LINQ methods to handle not matching condition:

<pre><code>var res = dictionary.Where(x =&gt; x.Value.ID == someID)
 .Select(x =&gt; x.Value.DisplayName)
 .DefaultIfEmpty("Unknown")
 .First();
</code></pre>

blocks|key|4674838|text|这是因为FirstOrDefault可能返回null，导致下面的.Value导致异常。您需要将其更改为类似以下内容：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4674839|var+myThing+=+things.FirstOrDefault(t+=>+t.Id+==+idToFind);

if(myThing+==+null)
++++return;+//+we+failed+to+find+what+we+wanted
var+displayName+=+myThing.DisplayName;|code-block|syntax|javascript|4674840|entityMap^0|4|E|M|4|W|6|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|L|$]]

That is because <code>FirstOrDefault</code>can return <code>null</code> causing your following <code>.Value</code> to cause the exception. You need to change it to something like:

<pre><code>var myThing = things.FirstOrDefault(t =&gt; t.Id == idToFind);

if(myThing == null)
 return; // we failed to find what we wanted
var displayName = myThing.DisplayName;
</code></pre>

blocks|key|169143|text|为了添加到解决方案中，这里有一个LINQ语句，它可能会有所帮助|type|unstyled|depth|inlineStyleRanges|entityRanges|data|169144|Utilities.DIMENSION_MemTbl.Where(a+=>+a.DIMENSION_ID+==+format.ContentBrief.DimensionID).Select(a=>a.DIMENSION1).DefaultIfEmpty("").FirstOrDefault();|code-block|syntax|javascript|169145|如果查询结果为null，则结果为空字符串。|169146|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

To add to the solutions, here is a LINQ statement that might help

<pre><code>Utilities.DIMENSION_MemTbl.Where(a =&gt; a.DIMENSION_ID == format.ContentBrief.DimensionID).Select(a=&gt;a.DIMENSION1).DefaultIfEmpty("").FirstOrDefault();
</code></pre>

The result will be an empty string if the result of the query is a null..

blocks|key|4675041|text|只需使用问号技巧进行空检查：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4675042|string+displayName+=+Dictionary.FirstOrDefault(x+=>+x.Value.ID+==+long.Parse(options.ID))?.Value.DisplayName+??+"DEFINE+A+DEFAULT+DISPLAY+NAME+HERE";|code-block|syntax|javascript|4675043|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Simply use the question mark trick for null checks:
<pre><code>string displayName = Dictionary.FirstOrDefault(x =&gt; x.Value.ID == long.Parse(options.ID))?.Value.DisplayName ?? &quot;DEFINE A DEFAULT DISPLAY NAME HERE&quot;;
</code></pre>

blocks|key|169084|text|我假设你正在使用可以为空的数据类型，你可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|169085|var+t+=+things.Where(x+=>+x!=null+&&+x.Value.ID+==+long.Parse(options.ID)).FirstOrDefault();
var+res+=+t+==+null+?+""+:+t.Value;|code-block|syntax|javascript|169086|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

i assume you are working with nullable datatypes, you can do something like this:

<pre><code>var t = things.Where(x =&gt; x!=null &amp;&amp; x.Value.ID == long.Parse(options.ID)).FirstOrDefault();
var res = t == null ? "" : t.Value;
</code></pre>

blocks|key|4675145|text|这个答案是为我们这些需要可视化写作的人准备的(比如我:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4675146|在下面的代码截图中，将抛出一个NullReferenceException，根本原因是ReferenceIdentification_02属性。|offset|length|style|CODE|4675147|​|4675148|📷|atomic|4675149|4675150|调试时，我们看到我正在查询的orderLine.REF数组没有包含ReferenceIdentificationQualifier_01值为==+"RU"的匹配对象，因此此时FirstOrDefault()返回值为NULL|4675151|4675152|4675153|4675154|为了防止NullReferenceException，我首先在orderLine.REF阵列上执行FirstOrDefault()。如果返回值不是null，则检索该值。|4675155|4675156|4675157|4675158|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/ebbc478a1b74e5183cdd75c90ad7f7cf.png|imageAlt|1|https://ask.qcloudimg.com/http-save/yehe-900000/9d25d0dd9579892262d01be3ff81d17d.png|2|https://ask.qcloudimg.com/http-save/yehe-900000/facb092a801a12972d5fcd562bc194b9.png^0|0|F|M|17|Q|0|0|0|1|0|0|0|E|D|X|Z|1Y|7|2F|G|2Z|4|0|0|0|1|1|0|0|4|M|V|D|1D|G|21|4|0|0|0|1|2|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1B|8|@$D|1C|E|1D|F|G]|$D|1E|E|1F|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|1G|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|1H|8|@]|9|@$D|1I|E|1J|1|1K]]|A|$]]|$1|M|3|I|5|6|7|1L|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|1M|8|@$D|1N|E|1O|F|G]|$D|1P|E|1Q|F|G]|$D|1R|E|1S|F|G]|$D|1T|E|1U|F|G]|$D|1V|E|1W|F|G]]|9|@]|A|$]]|$1|P|3|I|5|6|7|1X|8|@]|9|@]|A|$]]|$1|Q|3|K|5|L|7|1Y|8|@]|9|@$D|1Z|E|20|1|21]]|A|$]]|$1|R|3|I|5|6|7|22|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|23|8|@$D|24|E|25|F|G]|$D|26|E|27|F|G]|$D|28|E|29|F|G]|$D|2A|E|2B|F|G]]|9|@]|A|$]]|$1|U|3|I|5|6|7|2C|8|@]|9|@]|A|$]]|$1|V|3|K|5|L|7|2D|8|@]|9|@$D|2E|E|2F|1|2G]]|A|$]]|$1|W|3|I|5|6|7|2H|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|2I|8|@]|9|@]|A|$]]]|Y|$Z|$5|10|11|12|A|$13|14|15|-4]]|16|$5|10|11|12|A|$13|17|15|-4]]|18|$5|10|11|12|A|$13|19|15|-4]]]]

This answer is for those of us who need a visual write up (like me :)
In the code screenshot below, a <code>NullReferenceException</code> will be thrown, the root cause is the <code>ReferenceIdentification_02</code> property.
<a href="https://i.stack.imgur.com/lpOrw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lpOrw.png" alt="enter image description here" /></a>
When debugging, we see that the <code>orderLine.REF</code> array, I am querying does not include a matching object whose <code>ReferenceIdentificationQualifier_01</code> value <code>== &quot;RU&quot;</code>, so at that point <code>FirstOrDefault()</code> return value is <code>NULL</code>
<a href="https://i.stack.imgur.com/C9o4H.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/C9o4H.png" alt="enter image description here" /></a>
to prevent the <code>NullReferenceException</code>, I do a <code>FirstOrDefault()</code> on the <code>orderLine.REF</code> array first. If the returned value is not <code>null</code> then I retrieve the value.
<a href="https://i.stack.imgur.com/djolD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/djolD.png" alt="enter image description here" /></a>

blocks|key|4675006|text|您可以将with+'Where‘语句与FirstOrDefault()一起使用。就像这样。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4675007|+var+modelItem+=++_dbcontext.ModelName.Where(n+=>+n.NewsTagId+==+newsTag.Id).FirstOrDefault();|code-block|syntax|javascript|4675008|如果不匹配查询，则返回第一项。更好的做法是在查询后检查NULL。|4675009|++if(modelItem+==+null)
++{
+++++++return+"Not+Found."
++}
++else
++{
+++++++//+continue+process
++}|4675010|entityMap^0|J|G|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 can use with 'Where' statement with <code>FirstOrDefault()</code>.
like this.
<pre><code> var modelItem = _dbcontext.ModelName.Where(n =&gt; n.NewsTagId == newsTag.Id).FirstOrDefault();
</code></pre>
It returns first item if does not match query.
It is better practice to check the NULL after query.
<pre><code> if(modelItem == null)
 {
 return &quot;Not Found.&quot;
 }
 else
 {
 // continue process
 }
</code></pre>

Here is my code:

<pre><code>string displayName = Dictionary.FirstOrDefault(x =&gt; x.Value.ID == long.Parse(options.ID)).Value.DisplayName;
</code></pre>

The code works fine if <code>x.Value.ID</code> matches <code>options.ID</code>. However, I get a <code>NullReferenceException</code> if it doesn't.

FirstOrDefault returns NullReferenceException if no match is found

下面是我的代码：string displayName = Dictionary.FirstOrDefault(x => x.Value.ID == long.Parse(options.ID)).Value.DisplayName;如果x.Value.ID与options.ID匹配，代码就能正常工作。但是，如果不是这样，我会得到一个NullReferenceException。

问如果未找到匹配项，则FirstOrDefault返回NullReferenceException
EN

回答 7

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问如果未找到匹配项，则FirstOrDefault返回NullReferenceExceptionEN

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问如果未找到匹配项，则FirstOrDefault返回NullReferenceException
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