blocks|key|275680|text|objects是Linq-To-SQL数据库上下文吗？在这种情况下，只能使用=>运算符右侧的简单表达式。原因是，这些表达式不会被执行，但会被转换为SQL，以便对数据库执行。尝尝这个|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|275681|Arr[]+myArray+=+objects.Select(o+=>+new+Obj()+{+
++++Var1+=+o.someVar,
++++Var2+=+o.var2+
}).ToArray();|code-block|syntax|javascript|275682|entityMap^0|0|7|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|$]]

Is <code>objects</code> a Linq-To-SQL database context? In which case, you can only use simple expressions to the right of the => operator. The reason is, these expressions are not executed, but are converted to SQL to be executed against the database.
Try this

<pre><code>Arr[] myArray = objects.Select(o =&gt; new Obj() { 
 Var1 = o.someVar,
 Var2 = o.var2 
}).ToArray();
</code></pre>

blocks|key|279366|text|您可以在IEnumerable集合的lamba表达式中使用语句体。试试这个：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|279367|Obj[]+myArray+=+objects.AsEnumerable().Select(o+=>
{
++++var+someLocalVar+=+o.someVar;

++++return+new+Obj()+
++++{+
++++++++Var1+=+someLocalVar,
++++++++Var2+=+o.var2+
++++};
}).ToArray();|code-block|syntax|javascript|279368|通知：|279369|使用此方法时要仔细考虑，因为这样，所有查询结果都将存储在内存中，这可能会对其余代码产生不必要的副作用。|279370|entityMap^0|4|B|0|0|0|3|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|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|X|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

You can use statement body in lamba expression for IEnumerable collections.
try this one:

<pre><code>Obj[] myArray = objects.AsEnumerable().Select(o =&gt;
{
 var someLocalVar = o.someVar;

 return new Obj() 
 { 
 Var1 = someLocalVar,
 Var2 = o.var2 
 };
}).ToArray();
</code></pre>

Notice: 
Think carefully when using this method, because this way, you will have all query result in memory, that may have unwanted side effects on the rest of your code.

blocks|key|281246|text|这意味着您不能在需要将lambda表达式转换为表达式树的地方使用带有“语句体”的lambda表达式(即使用大括号的lambda表达式)(例如，使用linq2sql时就是这种情况)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|281247|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

It means that you can't use lambda expressions with a "statement body" (i.e. lambda expressions which use curly braces) in places where the lambda expression needs to be converted to an expression tree (which is for example the case when using linq2sql).

blocks|key|277542|text|在不知道你正在做什么(Linq2Objects，Linq2Entities，Linq2Sql?)的情况下，这应该会让它工作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|277543|Arr[]+myArray+=+objects.AsEnumerable().Select(o+=>+{
++++var+someLocalVar+=+o.someVar;

++++return+new+Obj()+{+
++++++++Var1+=+someLocalVar,
++++++++Var2+=+o.var2+
++++};+
}).ToArray();|code-block|syntax|javascript|277544|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|$]]

Without knowing more about what you are doing (Linq2Objects, Linq2Entities, Linq2Sql?), this should make it work:

<pre><code>Arr[] myArray = objects.AsEnumerable().Select(o =&gt; {
 var someLocalVar = o.someVar;

 return new Obj() { 
 Var1 = someLocalVar,
 Var2 = o.var2 
 }; 
}).ToArray();
</code></pre>

blocks|key|277694|text|LINQ+to+SQL返回对象正在实现IQueryable接口。因此，对于Select方法谓词参数，应该只提供一个不带正文的lambda表达式。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|277695|这是因为LINQ+for+SQL代码不是在程序内部执行，而是在SQL+server等远程端执行。这种延迟加载执行类型是通过实现IQueryable实现的，其中它的expect委托被包装在表达式类型类中，如下所示。|277696|Expression<Func<TParam,TResult>>|code-block|syntax|javascript|277697|表达式树不支持带有正文λ表达式，它只支持像var+id+=+cols.Select(+col+=>+col.id+);那样的单行λ表达式|277698|所以，如果你尝试一下，下面的代码将不会工作。|277699|Expression<Func<int,int>>+function+=+x+=>+{
++++return+x+*+2;
}|277700|下面的代码将按照预期工作。|277701|Expression<Func<int,int>>+function+=+x+=>+x+*+2;|277702|entityMap^0|J|A|11|6|0|0|0|L|12|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@$9|16|A|17|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|18|8|@]|D|@]|E|$]]|$1|Q|3|R|5|J|7|19|8|@]|D|@]|E|$K|L]]|$1|S|3|T|5|6|7|1A|8|@]|D|@]|E|$]]|$1|U|3|V|5|J|7|1B|8|@]|D|@]|E|$K|L]]|$1|W|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|X|$]]

The LINQ to SQL return object were implementing <code>IQueryable</code> interface. So for <code>Select</code> method predicate parameter you should only supply single lambda expression without body.

This is because LINQ for SQL code is not execute inside program rather than on remote side like SQL server or others. This lazy loading execution type were achieve by implementing IQueryable where its expect delegate is being wrapped in Expression type class like below.

<pre><code>Expression&lt;Func&lt;TParam,TResult&gt;&gt;
</code></pre>

Expression tree do not support lambda expression with body and its only support single line lambda expression like <code>var id = cols.Select( col =&gt; col.id );</code>

So if you try the following code won't works.

<pre><code>Expression&lt;Func&lt;int,int&gt;&gt; function = x =&gt; {
 return x * 2;
}
</code></pre>

The following will works as per expected.

<pre><code>Expression&lt;Func&lt;int,int&gt;&gt; function = x =&gt; x * 2;
</code></pre>

blocks|key|3198406|text|使用此select重载：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3198407|Obj[]+myArray+=+objects.Select(new+Func<Obj,Obj>(+o+=>
{
++++var+someLocalVar+=+o.someVar;

++++return+new+Obj()+
++++{+
+++++++Var1+=+someLocalVar,
+++++++Var2+=+o.var2+
++++};
})).ToArray();|code-block|syntax|javascript|3198408|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|$]]

Use this overload of select:

<pre><code>Obj[] myArray = objects.Select(new Func&lt;Obj,Obj&gt;( o =&gt;
{
 var someLocalVar = o.someVar;

 return new Obj() 
 { 
 Var1 = someLocalVar,
 Var2 = o.var2 
 };
})).ToArray();
</code></pre>

blocks|key|279454|text|9年的聚会太晚了，但对你的问题采取了不同的方法(没有人提到过吗？)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|279455|语句体可以很好地用于Func<>，但不能用于Expression<Func<>>。IQueryable.Select想要一个Expression<>，因为它们可以被转换为实体框架--+Func<>不能。|offset|length|style|CODE|279456|因此，您要么使用AsEnumerable并开始处理内存中的数据(如果不是非常必要，则不推荐使用)，或者继续使用推荐的IQueryable<>。有一种叫做linq+query的东西可以让一些事情变得更简单：|279457|IQueryable<Obj>+result+=+from+o+in+objects
+++++++++++++++++++++++++let+someLocalVar+=+o.someVar
+++++++++++++++++++++++++select+new+Obj
+++++++++++++++++++++++++{
+++++++++++++++++++++++++++Var1+=+someLocalVar,
+++++++++++++++++++++++++++Var2+=+o.var2
+++++++++++++++++++++++++};|code-block|syntax|javascript|279458|使用let，您可以定义一个变量并在select+(或where，...)中使用它，然后继续使用IQueryable，直到您真正需要执行和获取对象。|279459|之后，您可以Obj[]+myArray+=+result.ToArray()|279460|entityMap^0|0|A|6|M|I|15|H|1Q|C|2K|6|0|8|C|1M|C|24|A|0|0|2|3|H|6|Q|5|1B|A|0|6|W|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]|$D|12|E|13|F|G]|$D|14|E|15|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|16|8|@$D|17|E|18|F|G]|$D|19|E|1A|F|G]|$D|1B|E|1C|F|G]]|9|@]|A|$]]|$1|J|3|K|5|L|7|1D|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1E|8|@$D|1F|E|1G|F|G]|$D|1H|E|1I|F|G]|$D|1J|E|1K|F|G]|$D|1L|E|1M|F|G]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1N|8|@$D|1O|E|1P|F|G]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]]|T|$]]

9 years too late to the party, but a different approach to your problem (that nobody has mentioned?):
The statement-body works fine with <code>Func&lt;&gt;</code> but won't work with <code>Expression&lt;Func&lt;&gt;&gt;</code>. <code>IQueryable.Select</code> wants an <code>Expression&lt;&gt;</code>, because they can be translated for Entity Framework - <code>Func&lt;&gt;</code> can not.
So you either use the <code>AsEnumerable</code> and start working with the data in memory (not recommended, if not really neccessary) or you keep working with the <code>IQueryable&lt;&gt;</code> which is recommended.
There is something called <code>linq query</code> which makes some things easier:
<pre class="lang-cs prettyprint-override"><code>IQueryable&lt;Obj&gt; result = from o in objects
 let someLocalVar = o.someVar
 select new Obj
 {
 Var1 = someLocalVar,
 Var2 = o.var2
 };
</code></pre>
with <code>let</code> you can define a variable and use it in the <code>select</code> (or <code>where</code>,...) - and you keep working with the <code>IQueryable</code> until you really need to execute and get the objects.
Afterwards you can <code>Obj[] myArray = result.ToArray()</code>

blocks|key|277582|text|这意味着包含([parameters])+=>+{+some+code+};的TDelegate类型的Lambda表达式不能转换为Expression<TDelegate>。这是规则。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|277583|简化查询。您提供的代码可以重写为以下代码，并将进行编译：|277584|Arr[]+myArray+=+objects.Select(o+=>+new+Obj()
++++++++++++++++{
+++++++++++++++++++Var1+=+o.someVar,
+++++++++++++++++++Var2+=+o.var2
++++++++++++++++}+).ToArray();|code-block|syntax|javascript|277585|entityMap^0|6|W|13|9|1T|L|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|$]]

It means that a Lambda expression of type <code>TDelegate</code> which contains a <code>([parameters]) =&gt; { some code };</code> cannot be converted to an <code>Expression&lt;TDelegate&gt;</code>. It's the rule.

Simplify your query. The one you provided can be rewritten as the following and will compile:

<pre><code>Arr[] myArray = objects.Select(o =&gt; new Obj()
 {
 Var1 = o.someVar,
 Var2 = o.var2
 } ).ToArray();
</code></pre>

blocks|key|3198233|text|Arr是Obj的基本类型吗？Obj类是否存在？只有当Arr是Obj的基类型时，你的代码才能工作。您可以尝试执行以下操作：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3198234|Obj[]+myArray+=+objects.Select(o+=>
{
++++var+someLocalVar+=+o.someVar;

++++return+new+Obj()+
++++{+
+++++++Var1+=+someLocalVar,
+++++++Var2+=+o.var2+
++++};
}).ToArray();|code-block|syntax|javascript|3198235|entityMap^0|0|3|4|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

Is <code>Arr</code> a base type of <code>Obj</code>? Does the Obj class exist? Your code would work only if Arr is a base type of Obj. You can try this instead:

<pre><code>Obj[] myArray = objects.Select(o =&gt;
{
 var someLocalVar = o.someVar;

 return new Obj() 
 { 
 Var1 = someLocalVar,
 Var2 = o.var2 
 };
}).ToArray();
</code></pre>

blocks|key|281281|text|对于您的具体情况，body是用来创建变量的，切换到IEnumerable将强制所有操作都在客户端处理，我提出以下解决方案。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|281282|Obj[]+myArray+=+objects
.Select(o+=>+new
{
++++SomeLocalVar+=+o.someVar,+//+You+can+even+use+any+LINQ+statement+here
++++Info+=+o,
}).Select(o+=>+new+Obj()
{
++++Var1+=+o.SomeLocalVar,
++++Var2+=+o.Info.var2,
++++Var3+=+o.SomeLocalVar.SubValue1,
++++Var4+=+o.SomeLocalVar.SubValue2,
}).ToArray();|code-block|syntax|javascript|281283|编辑:为C#编码约定重命名|281284|entityMap^0|P|B|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

For your specific case, the body is for creating a variable, and switching to <code>IEnumerable</code> will force all the operations to be processed on client-side, I propose the following solution.

<pre><code>Obj[] myArray = objects
.Select(o =&gt; new
{
 SomeLocalVar = o.someVar, // You can even use any LINQ statement here
 Info = o,
}).Select(o =&gt; new Obj()
{
 Var1 = o.SomeLocalVar,
 Var2 = o.Info.var2,
 Var3 = o.SomeLocalVar.SubValue1,
 Var4 = o.SomeLocalVar.SubValue2,
}).ToArray();
</code></pre>

Edit: Rename for C# Coding Convention

blocks|key|2985241|text|正如在其他回复中所述，您只能在=>运算符的右侧使用简单表达式。我建议这个解决方案，它只需要创建一个方法来完成您想要在lambda中拥有的内容：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2985242|public+void+SomeConfiguration()+{
++++//+...
++++Obj[]+myArray+=+objects.Select(o+=>+Method()).ToArray();
++++//+..
}

public+Obj+Method()+{
++++var+someLocalVar+=+o.someVar;

++++return+new+Obj()+{+
++++Var1+=+someLocalVar,
++++Var2+=+o.var2+};
}|code-block|syntax|javascript|2985243|entityMap^0|F|2|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|$]]

As stated on other replies, you can only use simple expressions to the right of the <code>=&gt;</code> operator. I suggest this solution, which consists of just creating a method that does what you want to have inside of the lambda:
<pre><code>public void SomeConfiguration() {
 // ...
 Obj[] myArray = objects.Select(o =&gt; Method()).ToArray();
 // ..
}

public Obj Method() {
 var someLocalVar = o.someVar;

 return new Obj() { 
 Var1 = someLocalVar,
 Var2 = o.var2 };
}
</code></pre>

In using the EntityFramework, I get the error "<code>A lambda expression with a statement body cannot be converted to an expression tree</code>" when trying to compile the following code:

<pre><code>Obj[] myArray = objects.Select(o =&gt;
{
 var someLocalVar = o.someVar;

 return new Obj() { 
 Var1 = someLocalVar,
 Var2 = o.var2 };
}).ToArray();
</code></pre>

I don't know what the error means and most of all how to fix it. Any help?

"A lambda expression with a statement body cannot be converted to an expression tree"

SQL数据库

数据库

在使用EntityFramework，时，我在尝试编译以下代码时收到错误"A lambda expression with a statement body cannot be converted to an expression tree“：Obj[] myArray = objects.Select(o =>{    var someLocalVar = o.someVar;    retur

问“不能将具有语句体的lambda表达式转换为表达式树”
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