blocks|key|2280169|text|若要允许Amount字段为空，只需使用空合并运算符将空值转换为0。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2280170|var+creditsSum+=+(from+u+in+context.User
++++++++++++++join+ch+in+context.CreditHistory+on+u.ID+equals+ch.UserID++++++++++++++++++++++++++++++++++++++++
++++++++++++++where+u.ID+==+userID
++++++++++++++select+ch.Amount+??+0).Sum();|code-block|syntax|javascript|2280171|entityMap^0|4|6|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|$]]

To allow a nullable <code>Amount</code> field, just use the null coalescing operator to convert nulls to 0.

<pre><code>var creditsSum = (from u in context.User
 join ch in context.CreditHistory on u.ID equals ch.UserID 
 where u.ID == userID
 select ch.Amount ?? 0).Sum();
</code></pre>

blocks|key|2260800|text|linq到SQL查询不是以代码的形式执行，而是转换为SQL。有时，这是一种“漏出的抽象”，会产生意想不到的行为。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2260801|其中一种情况是null处理，其中可能在不同的地方出现意外的空值。在这种情况下，...DefaultIfEmpty(0).Sum(0)可以提供帮助(非常简单)，在这种情况下，可能没有元素，而sql的SUM返回null，而c#预期为0。|offset|length|style|CODE|2260802|一种更普遍的方法是使用??，当生成的COALESCE有返回意外null的风险时，它将被转换为COALESCE：|2260803|var+creditsSum+=+(from+u+in+context.User
++++++++++++++join+ch+in+context.CreditHistory+on+u.ID+equals+ch.UserID++++++++++++++++++++++++++++++++++++++++
++++++++++++++where+u.ID+==+userID
++++++++++++++select+(int?)ch.Amount).Sum()+??+0;|code-block|syntax|javascript|2260804|这第一个转换为int?，它告诉C#编译器这个表达式确实可以返回null，即使Sum()返回一个int。然后我们使用普通的??算子来处理null的情况。|2260805|基于这个答案，我编写了一个，其中包含LINQ和LINQ实体的详细信息。|2260806|entityMap^0|0|13|R|2Q|3|2V|4|0|B|2|I|8|1A|8|0|0|7|4|V|4|12|5|1B|3|1O|2|1V|4|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]|$D|Y|E|Z|F|G]|$D|10|E|11|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|12|8|@$D|13|E|14|F|G]|$D|15|E|16|F|G]|$D|17|E|18|F|G]]|9|@]|A|$]]|$1|J|3|K|5|L|7|19|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1A|8|@$D|1B|E|1C|F|G]|$D|1D|E|1E|F|G]|$D|1F|E|1G|F|G]|$D|1H|E|1I|F|G]|$D|1J|E|1K|F|G]|$D|1L|E|1M|F|G]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1N|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1O|8|@]|9|@]|A|$]]]|T|$]]

A linq-to-sql query isn't executed as code, but rather translated into SQL. Sometimes this is a "leaky abstraction" that yields unexpected behaviour. 

One such case is null handling, where there can be unexpected nulls in different places. <code>...DefaultIfEmpty(0).Sum(0)</code> can help in this (quite simple) case, where there might be no elements and sql's <code>SUM</code> returns <code>null</code> whereas c# expect 0.

A more general approach is to use <code>??</code> which will be translated to <code>COALESCE</code> whenever there is a risk that the generated SQL returns an unexpected null:

<pre><code>var creditsSum = (from u in context.User
 join ch in context.CreditHistory on u.ID equals ch.UserID 
 where u.ID == userID
 select (int?)ch.Amount).Sum() ?? 0;
</code></pre>

This first casts to <code>int?</code> to tell the C# compiler that this expression can indeed return <code>null</code>, even though <code>Sum()</code> returns an <code>int</code>. Then we use the normal <code>??</code> operator to handle the <code>null</code> case.

Based on this answer, I wrote a <a href="http://coding.abel.nu/2012/08/null-sematics-in-linqs-sum/">blog post</a> with details for both LINQ to SQL and LINQ to Entities.

blocks|key|2260839|text|您使用的是aggregate函数，它没有让项执行操作，您必须验证linq查询是否提供了如下结果：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2260840|var+maxOrderLevel+=sdv.Any()?+sdv.Max(s+=>+s.nOrderLevel):0|code-block|syntax|javascript|2260841|entityMap^0|5|9|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|$]]

You are using <code>aggregate</code> function which not getting the items to perform action , you must verify linq query is giving some result as below:

<pre><code>var maxOrderLevel =sdv.Any()? sdv.Max(s =&gt; s.nOrderLevel):0
</code></pre>

blocks|key|2260849|text|我使用了这段代码，它的响应是正确的，只有输出值是空的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2260850|var+packesCount+=+await+botContext.Sales.Where(s+=>+s.CustomerId+==+cust.CustomerId+&&+s.Validated)
++++++++++++++++++++++++++++++++.SumAsync(s+=>+(int?)s.PackesCount);
++++++++++++++++++++++++++++if(packesCount+!=+null)
++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++//+your+code
++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++else
++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++//+your+code
++++++++++++++++++++++++++++}|code-block|syntax|javascript|2260851|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 have used this code and it responds correctly, only the output value is nullable.

<pre><code>var packesCount = await botContext.Sales.Where(s =&gt; s.CustomerId == cust.CustomerId &amp;&amp; s.Validated)
 .SumAsync(s =&gt; (int?)s.PackesCount);
 if(packesCount != null)
 {
 // your code
 }
 else
 {
 // your code
 }
</code></pre>

blocks|key|625037|text|我看这个问题已经回答了。但是，如果您希望将其分成两个语句，则可以考虑使用下面的语句。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|625038|var+credits+=+from+u+in+context.User
++++++++++++++join+ch+in+context.CreditHistory+
++++++++++++++++++on+u.ID+equals+ch.UserID++++++++++++++++++++++++++++++++++++++++
++++++++++++++where+u.ID+==+userID
++++++++++++++select+ch;

var+creditSum=+credits.Sum(x+=>+(int?)x.Amount)+??+0;|code-block|syntax|javascript|625039|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 see that this question is already answered. But if you want it to be split into two statements, following may be considered.

<pre><code>var credits = from u in context.User
 join ch in context.CreditHistory 
 on u.ID equals ch.UserID 
 where u.ID == userID
 select ch;

var creditSum= credits.Sum(x =&gt; (int?)x.Amount) ?? 0;
</code></pre>

blocks|key|2280313|text|当我试图从视图中进行选择时，出现了此错误消息。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2280314|问题是视图最近获得了一些新的空行(在SubscriberId列中)，而且它还没有在EDMX+(EF数据库先更新)中更新。|2280315|该列必须为Nullable类型才能工作。|2280316|Context.Dealers.Where(x+=>+x.dealerCode+==+dealerCode).FirstOrDefault()；|2280317|视图刷新前：|2280318|public+int+SubscriberId+{+get;+set;+}|code-block|syntax|javascript|2280319|视图刷新后：|2280320|public+Nullable<int>+SubscriberId+{+get;+set;+}|2280321|在EDMX中删除和添加视图是有效的。|2280322|希望它能帮到别人。|2280323|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|11|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|12|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|13|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|14|8|@]|9|@]|A|$]]|$1|Q|3|R|5|L|7|15|8|@]|9|@]|A|$M|N]]|$1|S|3|T|5|6|7|16|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|17|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|X|$]]

Had this error message when I was trying to select from a view.

The problem was the view recently had gained some new null rows (in SubscriberId column), and it had not been updated in EDMX (EF database first).

The column had to be Nullable type for it to work.

var dealer = Context.Dealers.Where(x => x.dealerCode == dealerCode).FirstOrDefault();

Before view refresh:

<pre><code>public int SubscriberId { get; set; }
</code></pre>

After view refresh:

<pre><code>public Nullable&lt;int&gt; SubscriberId { get; set; }
</code></pre>

Deleting and adding the view back in EDMX worked.

Hope it helps someone.

blocks|key|2260924|text|在运行时使用以下代码在实体框架6中获得了此错误：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2260925|var+fileEventsSum+=+db.ImportInformations.Sum(x+=>+x.FileEvents)|code-block|syntax|javascript|2260926|从LeandroSoares:更新|offset|length|style|BOLD|2260927|在单次执行时使用此方法：|2260928|var+fileEventsSum+=+db.ImportInformations.Sum(x+=>+(int?)x.FileEvents)+??+0|2260929|原版：|2260930|改变为这个，然后它就起作用了：|2260931|var+fileEventsSum+=+db.ImportInformations.Any()+?+db.ImportInformations.Sum(x+=>+x.FileEvents)+:+0;|2260932|entityMap^0|0|0|0|F|0|0|0|0|3|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|14|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|15|8|@$I|16|J|17|K|L]]|9|@]|A|$]]|$1|S|3|T|5|6|7|18|8|@]|9|@]|A|$]]|$1|U|3|V|5|D|7|19|8|@]|9|@]|A|$E|F]]|$1|W|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|X|$]]

Got this error in Entity Framework 6 with this code at runtime:

<pre><code>var fileEventsSum = db.ImportInformations.Sum(x =&gt; x.FileEvents)
</code></pre>

Update from LeandroSoares:

Use this for single execution:

<pre><code>var fileEventsSum = db.ImportInformations.Sum(x =&gt; (int?)x.FileEvents) ?? 0
</code></pre>

Original:

Changed to this and then it worked:

<pre><code>var fileEventsSum = db.ImportInformations.Any() ? db.ImportInformations.Sum(x =&gt; x.FileEvents) : 0;
</code></pre>

blocks|key|625078|text|我也面临着同样的问题，并通过使用"?“来解决列为可空的问题。操作符。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|625079|Sequnce+=+db.mstquestionbanks.Where(x+=>+x.IsDeleted+==+false+&&+x.OrignalFormID+==+OriginalFormIDint).Select(x=>(int?)x.Sequence).Max().ToString();|code-block|syntax|javascript|625080|有时返回null。|625081|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|$]]

I was also facing the same problem and solved through making column as nullable using "?" operator.

<pre><code>Sequnce = db.mstquestionbanks.Where(x =&gt; x.IsDeleted == false &amp;&amp; x.OrignalFormID == OriginalFormIDint).Select(x=&gt;&lt;b&gt;(int?)x.Sequence&lt;/b&gt;).Max().ToString();
</code></pre>

Sometimes null is returned.

I have the following code. I'm getting error: 

<blockquote>
 "The cast to value type 'Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type." 
</blockquote>

when CreditHistory table has no records.

<pre><code>var creditsSum = (from u in context.User
 join ch in context.CreditHistory on u.ID equals ch.UserID 
 where u.ID == userID
 select ch.Amount).Sum();
</code></pre>

How can I modify the query to accept null values?

The cast to value type 'Int32' failed because the materialized value is null

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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我有下面的代码。我搞错了：“转换为值类型'Int32‘失败，因为物化值为空。结果类型的泛型参数或查询必须使用可空类型。”当CreditHistory表没有记录时。var creditsSum = (from u in context.User                  join ch in context.Cr...

问转换为值类型'Int32‘失败，因为物化值为null
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问转换为值类型'Int32‘失败，因为物化值为nullEN