blocks|key|4462251|text|问题是集合中没有满足条件的项。SingleOrDefault返回null+(用于引用类型)，因此在第一个版本中，您将收到一个NullReferenceException。如果没有返回的元素，Single会抛出异常，这就是第二种方法失败的原因。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4462252|您需要检查是否有任何结果：|4462253|var+restaurant+=+(from+r+in+objCtx.Restaurants
++++++++++++++++++where+r.id+==+item.restaurantid
++++++++++++++++++select+r).SingleOrDefault<Restaurant>();
string+restaurantName;
if+(restaurant+!=+null)
++++restaurantName+=+restaurant.Name;
else
++++restaurantName+=+string.Empty;|code-block|syntax|javascript|4462254|entityMap^0|F|F|1Q|M|2N|6|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

The problem is that there are not items in the collection that fulfill the condition. <code>SingleOrDefault</code> returns null (for reference types), so that in the frst version you receive a <code>NullReferenceException</code>. <code>Single</code> throws an exception if there are no elements that are returned, that's why the second approach fails.

You need to check whether there are any results: 

<pre><code>var restaurant = (from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r).SingleOrDefault&lt;Restaurant&gt;();
string restaurantName;
if (restaurant != null)
 restaurantName = restaurant.Name;
else
 restaurantName = string.Empty;
</code></pre>

blocks|key|1454550|text|如您所述，如果您的第二个代码示例(使用Single的代码示例)不返回任何元素，那么第一个代码示例(使用SingleOrDefault的代码示例)返回null。.Name属性不能从null中获取。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1454551|我认为你需要验证你的查询。直接查看数据，或者甚至只是循环遍历数据，看看输出是什么：|1454552|var+restaurants+=+from+r+in+objCtx.Restaurants
++++++++++++++++++where+r.id+==+item.restaurantid
++++++++++++++++++select+r;

foreach+(var+restaurant+in+restaurants)
{
++++string+name+=+restaurant.Name;
}|code-block|syntax|javascript|1454553|我猜循环永远不会执行，因为它是空的，您需要找出查询的错误所在。|1454554|如果您需要处理可能不返回任何元素的场景，那么只需要一个If检查：|1454555|var+restaurant+=+(from+r+in+objCtx.Restaurants
++++++++++++++++++where+r.id+==+item.restaurantid
++++++++++++++++++select+r).SingleOrDefault<Restaurant>();

string+restaurantName;
if+(restaurant+==+null)
++++restaurantName+=+string.Empty;
else
++++restaurantName+=+restaurant.Name;|1454556|entityMap^0|J|6|1F|F|22|4|27|5|2H|4|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|15|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|17|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|18|8|@]|D|@]|E|$]]|$1|Q|3|R|5|J|7|19|8|@]|D|@]|E|$K|L]]|$1|S|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]]|T|$]]

If, as you say, your second code sample (the one using <code>Single</code>) return no elements, then the first one (the one using <code>SingleOrDefault</code>) returned <code>null</code>. And you can't get the <code>.Name</code> property off of <code>null</code>.

I think you need to verify your query. Look at the data directly or perhaps even just loop through the data and see what the output is:

<pre><code>var restaurants = from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r;

foreach (var restaurant in restaurants)
{
 string name = restaurant.Name;
}
</code></pre>

I'm guessing that the loop will never execute because its empty and you need to figure out what's wrong with your query.

If you need to handle the scenario where it might return no elements, then you just need an if check:

<pre><code>var restaurant = (from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r).SingleOrDefault&lt;Restaurant&gt;();

string restaurantName;
if (restaurant == null)
 restaurantName = string.Empty;
else
 restaurantName = restaurant.Name;
</code></pre>

blocks|key|4462284|text|问题是，linq查询返回一个空集合：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4462285|from+r+in+objCtx.Restaurants
where+r.id+==+item.restaurantid
select+r|code-block|syntax|javascript|4462286|在第一个示例中调用SingleOrDefault时，它返回null+(即默认值)。您收到一个object+reference+not+set+...，因为您试图在空引用上调用Name属性。|offset|length|style|CODE|4462287|在第二个例子中，您调用了Single。在具有0或%2B1元素的列表上执行此操作时，将引发异常。(正如你所经历的)。|4462288|解决方案是要么确保始终检索单个实例，要么在访问该对象上的任何属性之前进行检查。|4462289|entityMap^0|0|0|9|F|T|4|1A|S|2F|4|0|C|6|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@$I|V|J|W|K|L]|$I|X|J|Y|K|L]|$I|Z|J|10|K|L]|$I|11|J|12|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@$I|14|J|15|K|L]]|9|@]|A|$]]|$1|O|3|P|5|6|7|16|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$]]

The problem is that your linq query returns an empty collection:

<pre><code>from r in objCtx.Restaurants
where r.id == item.restaurantid
select r
</code></pre>

When you call <code>SingleOrDefault</code>in your first example, it returns <code>null</code> (ie the default value). You receive an <code>object reference not set ...</code>because you're trying to invoke the <code>Name</code> property on a null reference.

In your second example you call <code>Single</code>. When you do that on a list with 0 or +1 elements, an exception will be thrown. (as you are experiencing).

The solution is to either make sure that you always retrieve a single instance, or to do a check before you access any properties on that object

blocks|key|1996806|text|如果序列中没有项，则单个将通过异常。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1996807|尝试：|1996808|var+RestaurantName+=+((from+r+in+objCtx.Restaurants
++++++++++++++++++++++++++where+r.id+==+item.restaurantid
++++++++++++++++++++++++++select+r).Any())+?+(from+r+in+objCtx.Restaurants
++++++++++++++++++++++++++where+r.id+==+item.restaurantid
++++++++++++++++++++++++++select+r).SingleOrDefault<Restaurant>().Name+:+string.empty;|code-block|syntax|javascript|1996809|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|$]]

Single will through an exception if there are no items in the sequence.

Try:

<pre><code>var RestaurantName = ((from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r).Any()) ? (from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r).SingleOrDefault&lt;Restaurant&gt;().Name : string.empty;
</code></pre>

blocks|key|4462358|text|正如其他人所说，您的结果集是空的。这将导致Single失败(这已记录在案)，并将导致SingleOrDefault返回给定类型的默认值(在类的情况下为null)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4462359|除了在其他答案(使用空守卫)中提供的解决方案之外，下面还有一个使用更多LINQ的解决方案：|4462360|var+RestaurantName+=+(from+r+in+objCtx.Restaurants
++++++++++++++++++++++where+r.id+==+item.restaurantid
++++++++++++++++++++++select+r).DefaultIfEmpty(new+Restaurant()+{+Name="None"+})
+++++++++++++++++++++++++++++++.SingleOrDefault().Name;|code-block|syntax|javascript|4462361|DefaultIfEmpty将SingleOrDefault返回的“默认”值从null更改为提供的值。这样您就可以安全地访问Name属性。根据构建Restaurant类的具体方式，您可能希望以不同的方式创建默认值。您也应该能够提供类似new+{+Name+=+"None"+}之类的默认设置，创建一个匿名对象，但这取决于您。|4462362|entityMap^0|L|6|16|F|23|4|0|0|0|0|E|F|F|13|4|1Q|4|21|A|39|L|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]|$9|14|A|15|B|C]|$9|16|A|17|B|C]|$9|18|A|19|B|C]|$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|P|$]]

As others have said, your result set is empty. This will cause <code>Single</code> to fail (this is documented), and it will cause <code>SingleOrDefault</code> to return the default value for the given type (<code>null</code> in the case of a class).

In addition to the solutions presented in other answers (using null guards), here's another using more LINQ:

<pre><code>var RestaurantName = (from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r).DefaultIfEmpty(new Restaurant() { Name="None" })
 .SingleOrDefault().Name;
</code></pre>

<code>DefaultIfEmpty</code> will change the "default" value returned by <code>SingleOrDefault</code> from <code>null</code> to the supplied value. This way you can safely access the <code>Name</code> property. Depending on exactly how your <code>Restaurant</code> class is built, you may want to create the default value differently. You should be able to supply a default of something like <code>new { Name = "None" }</code>, as well, creating an anonymous object, but that's up to you.

I am having issues with a Linq query when it runs I receive the error <code>Object reference not set to an instance of an object.</code>

<pre><code>var RestaurantName = (from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r).SingleOrDefault&lt;Restaurant&gt;().Name;
</code></pre>

I then changed the query to 

<pre><code>var RestaurantName = (from r in objCtx.Restaurants
 where r.id == item.restaurantid
 select r).Single&lt;Restaurant&gt;().Name;
</code></pre>

But I received the error <code>Sequence contains no elements</code>. I checked the variable and it was returning a restaurant name. But I don't understand why I am receiving these errors.

Linq Object Reference Not An Instance Of An Object

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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当Linq查询运行时，我遇到了问题，我收到了错误Object reference not set to an instance of an object.var RestaurantName = (from r in objCtx.Restaurants                      where r.id ...

问Linq对象引用而不是对象的实例
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