blocks|key|1259207|text|你会想把它分成两个陈述。返回查询的结果(这将击中数据库)，然后在单独的步骤中第二次枚举结果，将转换转换为新的对象列表。第二个“查询”不会击中数据库，因此您将能够在其中使用someMethod()。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1259208|Linq到实体是一件有点奇怪的事情，因为它使从C#查询数据库的转换非常无缝:但是您必须始终提醒自己，“这个C#将被转换成一些SQL。”因此，您必须扪心自问，“所有这些C#实际上能作为SQL执行吗？”如果它不能--如果你在里面调用someMethod()+--你的查询就会有问题。通常的解决办法是把它分开。|1259209|(使用.AsEnumerable()的@BrokenGlass的另一个回答基本上是另一种方法。)|1259210|entityMap^0|2D|C|0|36|C|0|3|F|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|K|$]]

You'll want to split it up into two statements. Return the results from the query (which is what will hit the database), and then enumerate the results a second time in a separate step to transform the translation into the new object list. This second "query" won't hit the database, so you'll be able to use the <code>someMethod()</code> inside it.

Linq-to-Entities is a bit of a strange thing, because it makes the transition to querying the database from C# extremely seamless: but you always have to remind yourself, "This C# is going to get translated into some SQL." And as a result, you have to ask yourself, "Can all this C# actually get executed as SQL?" If it can't - if you're calling <code>someMethod()</code> inside it - your query is going to have problems. And the usual solution is to split it up.

(The other answer from @BrokenGlass, using <code>.AsEnumerable()</code>, is basically another way to do just that.)

blocks|key|2110700|text|您必须在Linq-to-Objects上下文中执行您的方法调用，因为在数据库端，方法调用是没有意义的--您可以使用AsEnumerable()来完成--基本上，查询的其余部分将被评估为一个使用Linq-to-Objects的内存集合，并且您可以按照预期使用方法调用：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2110701|var+q+=+(from+a+in+v.A+join+b+in+v.B
++++++++on+a.i+equals+b.j
++++++++where+a.k+==+"aaa"+&&+a.h+==+0
++++++++select+new+{T+=+a.i,+Z+=+a.z+})
++++++++.AsEnumerable()
++++++++.Select(x+=>+new+{+T+=+x.T,+S+=+someMethod(x.Z).ToString()+})|code-block|syntax|javascript|2110702|entityMap^0|4|F|1L|E|2O|F|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|$]]

You have to execute your method call in <code>Linq-to-Objects</code> context, because on the database side that method call will not make sense - you can do this using <code>AsEnumerable()</code> - basically the rest of the query will then be evaluated as an in memory collection using <code>Linq-to-Objects</code> and you can use method calls as expected:

<pre><code>var q = (from a in v.A join b in v.B
 on a.i equals b.j
 where a.k == "aaa" &amp;&amp; a.h == 0
 select new {T = a.i, Z = a.z })
 .AsEnumerable()
 .Select(x =&gt; new { T = x.T, S = someMethod(x.Z).ToString() })
</code></pre>

blocks|key|1228238|text|这是一个老问题，但我看到没有人提到一个“黑客”，它允许在select期间调用方法而不重复。想法是使用构造函数，在构造函数中，您可以任意调用(至少在LINQ和NHibernate中可以很好地工作，不确定LINQ2SQL或EF，但我想应该是一样的)。下面是基准程序的源代码，在我的例子中，重申方法比构造函数方法慢了一倍，我想这并不奇怪--我的业务逻辑是最小的，所以迭代和内存分配很重要。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1228239|我也希望有更好的方法，这个或那个不应该尝试在数据库上执行，|1228240|//+Here+are+the+results+of+selecting+sum+of+1+million+ints+on+my+machine:
//+Name++++Iterations++++++Percent++++
//+reiterate+++++++294+++++53.3575317604356%25++++++
//+constructor+++++551+++++100%25

public+class+A
{
++++public+A()
++++{++++++++++++
++++}

++++public+A(int+b,+int+c)
++++{
++++++++Result+=+Sum(b,+c);
++++}

++++public+int+Result+{+get;+set;+}

++++public+static+int+Sum(int+source1,+int+source2)
++++{
++++++++return+source1+%2B+source2;
++++}
}

class+Program
{
++++static+void+Main(string[]+args)
++++{
++++++++var+range+=+Enumerable.Range(1,+1000000).ToList();

++++++++BenchmarkIt.Benchmark.This("reiterate",+()+=>
++++++++++++{
++++++++++++++++var+tst+=+range
++++++++++++++++++++.Select(x+=>+new+{+b+=+x,+c+=+x+})
++++++++++++++++++++.AsEnumerable()
++++++++++++++++++++.Select(x+=>+new+A
++++++++++++++++++++{
++++++++++++++++++++++++Result+=+A.Sum(x.b,+x.c)
++++++++++++++++++++})
++++++++++++++++++++.ToList();
++++++++++++})
++++++++++++.Against.This("constructor",+()+=>
++++++++++++{
++++++++++++++++var+tst+=+range
++++++++++++++++++++.Select(x+=>+new+A(x,+x))
++++++++++++++++++++.ToList();
++++++++++++})
++++++++++++.For(60)
++++++++++++.Seconds()
++++++++++++.PrintComparison();

++++++++Console.ReadKey();
++++}
}|code-block|syntax|javascript|1228241|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|$]]

That is an old question, but I see nobody mention one "hack", that allows to call methods during select without reiterating. Idea is to use constructor and in constructor you can call whatever you wish (at least it works fine in LINQ with NHibernate, not sure about LINQ2SQL or EF, but I guess it should be the same).
Below I have source code for benchmark program, it looks like reiterating approach in my case is about twice slower than constructor approach and I guess there's no wonder - my business logic was minimal, so things like iteration and memory allocation matters.

Also I wished there was better way to say, that this or that should not be tried to execute on database,

<pre><code>// Here are the results of selecting sum of 1 million ints on my machine:
// Name Iterations Percent 
// reiterate 294 53.3575317604356% 
// constructor 551 100%

public class A
{
 public A()
 { 
 }

 public A(int b, int c)
 {
 Result = Sum(b, c);
 }

 public int Result { get; set; }

 public static int Sum(int source1, int source2)
 {
 return source1 + source2;
 }
}

class Program
{
 static void Main(string[] args)
 {
 var range = Enumerable.Range(1, 1000000).ToList();

 BenchmarkIt.Benchmark.This("reiterate", () =&gt;
 {
 var tst = range
 .Select(x =&gt; new { b = x, c = x })
 .AsEnumerable()
 .Select(x =&gt; new A
 {
 Result = A.Sum(x.b, x.c)
 })
 .ToList();
 })
 .Against.This("constructor", () =&gt;
 {
 var tst = range
 .Select(x =&gt; new A(x, x))
 .ToList();
 })
 .For(60)
 .Seconds()
 .PrintComparison();

 Console.ReadKey();
 }
}
</code></pre>

I want to insert into my table a column named 'S' that will get some string value based on a value it gets from a table column.

For example: <code>for each ID (a.z)</code> I want to gets it's string value stored in another table. The string value is returned from another method that gets it through a Linq query.

<ul>
<li>Is it possible to call a method from Linq?</li>
<li>Should I do everything in the same query?</li>
</ul>

This is the structure of the information I need to get:

<blockquote>
 a.z is the ID in the first square in table #1, from this ID I get another id in table #2, and from that I can get my string value that I need to display under column 'S'. 
 <img src="https://i.stack.imgur.com/q2kvk.png" alt="enter image description here">
</blockquote>

<pre><code>var q = (from a in v.A join b in v.B
 on a.i equals b.j
 where a.k == "aaa" &amp;&amp; a.h == 0
 select new {T = a.i, S = someMethod(a.z).ToString()})
 return q;
</code></pre>

The line <code>S = someMethod(a.z).ToString()</code> causing the following error: 

<blockquote>
 Unable to cast object of type 'System.Data.Linq.SqlClient.SqlColumn'
 to type 'System.Data.Linq.SqlClient.SqlMethodCall'.
</blockquote>

Calling a method inside a Linq query

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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我想在我的表中插入一个名为'S‘的列，它将根据从表列获得的值获得一些字符串值。例如：for each ID (a.z)，我想获取存储在另一个表中的字符串值。字符串值是从另一个通过Linq查询获得它的方法返回的。可以从Linq调用方法吗？应该在同一个查询中执行所有操作吗？这是我需要得到的信息的结构： a.z是表#1中第一...

问在Linq查询中调用方法
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