blocks|key|856572|text|是否导入了包含定义命名空间的类的命名空间。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|856573|例如：如果没有|856574|Imports+System.Linq|code-block|syntax|javascript|856575|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|$]]

Have you imported the namespace containing the class that defines the namespace.

Eg. you won't see any of the LINQ to Objects extension methods without a

<pre><code>Imports System.Linq
</code></pre>

blocks|key|660479|text|如果它抛出一个异常，而不是提供编译时错误，那就意味着您已经删除了选项严格+.我不知道在这种情况下扩展方法会发生什么，因为它们通常是在编译时解析的，但是在选项严格关闭的情况下，已经开始了后期绑定。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|660480|我建议你严格选择，一切都会好的.|660481|(您还需要导入名称空间，按照Richard的回答，但我认为您已经这样做了。无论如何，如果您忘记在打开选项后执行该操作，您将看到编译时错误。|660482|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/zcd4xwzs.aspx^0|W|4|0|0|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|U|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]]]

If it's throwing an exception rather than giving a compile-time error, that suggests you've got <a href="http://msdn.microsoft.com/en-us/library/zcd4xwzs.aspx" rel="nofollow">Option Strict</a> off... I don't know what happens to extension methods in that case, as they're normally resolved at compile time, but with Option Strict off you've got late binding going on.

I suggest you turn Option Strict on, and all should be well...

(You'll also need to import the namespace, as per Richard's answer, but I'd assumed you'd already done that. You'll see a compile-time error if you forget to do that after turning option strict on, anyway.)

blocks|key|658460|text|感谢乔恩为我指明了正确的方向，但这里已经足够了，需要一个完整的答案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|658461|扩展方法是一个编译器技巧，所以(在C#中)：|658462|var+something+=+this.SomethingManager.GetSomething(key);
var+result+=+something.ExtensionMethod("extra");|code-block|syntax|javascript|658463|在编译时转换为：|658464|ISomething+something+=+this.SomethingManager.GetSomething(key);
int+result+=+SomethingExtensions.ExtensionMethod(something,+"extra");|658465|作为类的一种方法出现的静态扩展方法只是编译器的聪明。|658466|问题是VB中的Dim与C#中的var不一样。感谢VB的后期绑定，它更接近于dynamic。|offset|length|style|CODE|658467|因此，使用VB中的原始示例：|658468|Dim+something+=+Me.SomethingManager.GetSomething(key)
Dim+result+=+something.ExtensionMethod("extra")|658469|与C#不同的是，在VB中，something的类型一直保留到运行时，使扩展方法正常工作的编译器聪明无法实现。如果显式声明了该类型，则可以解析扩展方法，但是如果后期绑定，则扩展方法永远无法工作。|658470|如果他们使用Option+Strict+On+(就像Jon建议的那样)，那么他们就不得不总是声明类型，早期绑定就会发生，编译器的聪明使得扩展方法能够工作。这是最好的实践，但由于他们还没有这么做，这对他们来说将是一个痛苦的改变。|658471|这个故事的寓意是:不要在API中使用扩展方法，如果你期望VB中的任何人靠近它:-S|658472|entityMap^0|0|0|0|0|0|0|7|3|F|3|11|7|0|0|0|D|9|0|6|G|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|17|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|18|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|19|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|1A|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|1B|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1C|8|@$Q|1D|R|1E|S|T]|$Q|1F|R|1G|S|T]|$Q|1H|R|1I|S|T]]|9|@]|A|$]]|$1|U|3|V|5|6|7|1J|8|@]|9|@]|A|$]]|$1|W|3|X|5|F|7|1K|8|@]|9|@]|A|$G|H]]|$1|Y|3|Z|5|6|7|1L|8|@$Q|1M|R|1N|S|T]]|9|@]|A|$]]|$1|10|3|11|5|6|7|1O|8|@$Q|1P|R|1Q|S|T]]|9|@]|A|$]]|$1|12|3|13|5|6|7|1R|8|@]|9|@]|A|$]]|$1|14|3|-4|5|6|7|1S|8|@]|9|@]|A|$]]]|15|$]]

Cheers to Jon for pointing me in the right direction, but there's enough here to need a whole answer.

Extension methods are a compiler trick, so (in C#):

<pre><code>var something = this.SomethingManager.GetSomething(key);
var result = something.ExtensionMethod("extra");
</code></pre>

Is converted at compile time to be:

<pre><code>ISomething something = this.SomethingManager.GetSomething(key);
int result = SomethingExtensions.ExtensionMethod(something, "extra");
</code></pre>

The static extension method appearing as a method of the class is just compiler cleverness.

The problem is that <code>Dim</code> in VB is not the same as <code>var</code> in C#. Thank's to VB's late binding it's closer to <code>dynamic</code>. 

So with the original example in VB:

<pre><code>Dim something = Me.SomethingManager.GetSomething(key)
Dim result = something.ExtensionMethod("extra")
</code></pre>

Unlike C#, in VB figuring out the type of <code>something</code> is left until run time, and the compiler cleverness that makes the extension methods work doesn't happen. If the type is explicitly declared the extension method can be resolved, but if late-bound then extension methods just can't ever work.

If they use <code>Option Strict On</code> (like Jon suggests) then they're forced to always declare the type, early-binding happens and the compiler cleverness makes the extension methods work. It's best practice anyway, but as they haven't been doing so it would be a painful change for them to make.

The moral of this story: don't use extension methods in your API if you expect anyone from VB land near it :-S

blocks|key|658488|text|根据选项推断，这段代码..。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|658489|Dim+something+=+Me.SomethingManager.GetSomething(key)
Dim+result+=+something.ExtensionMethod("extra")|code-block|syntax|javascript|658490|...is和.|658491|Dim+something+As+Object+=+Me.SomethingManager.GetSomething(key)
Dim+result+As+Object+=+something.ExtensionMethod("extra")|658492|因为something是Object类型的，所以它找不到扩展方法，因为它不是在Object类型上定义的。|offset|length|style|CODE|658493|现在，如果设置Option+Infer+On，您将获得与C#的var关键字相同的结果。类型将自动推断。注意，这也可能破坏现有代码，但可以为特定文件(如Option+Strict+)启用它。|658494|最佳实践是将Option+Strict和Option+Infer设置为On。|658495|entityMap^0|0|0|0|0|2|9|C|6|13|6|0|7|F|V|3|23|D|0|6|D|K|C|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@$M|11|N|12|O|P]|$M|13|N|14|O|P]|$M|15|N|16|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|17|8|@$M|18|N|19|O|P]|$M|1A|N|1B|O|P]|$M|1C|N|1D|O|P]]|9|@]|A|$]]|$1|S|3|T|5|6|7|1E|8|@$M|1F|N|1G|O|P]|$M|1H|N|1I|O|P]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]]|V|$]]

With Option Infer Off, this code...

<pre><code>Dim something = Me.SomethingManager.GetSomething(key)
Dim result = something.ExtensionMethod("extra")
</code></pre>

...is the same as...

<pre><code>Dim something As Object = Me.SomethingManager.GetSomething(key)
Dim result As Object = something.ExtensionMethod("extra")
</code></pre>

Since <code>something</code> is of type <code>Object</code>, it can't find the extension method, since it isn't defined on type <code>Object</code>.

Now, if you set <code>Option Infer On</code>, you will get the same results as with C#'s <code>var</code> keyword. Types will be automatically inferred. Note that this could also break existing code, but it can be enabled for a specific file, like <code>Option Strict</code>.

The best practice would be to set both <code>Option Strict</code> and <code>Option Infer</code> to On.

I have a C# library that has an extension method, something like:

<pre><code>public interface ISomething { ... }
public class SomethingA : ISomething { ... }
public class SomethingB : ISomething { ... }

public static class SomethingExtensions 
{
 public static int ExtensionMethod(this ISomething input, string extra) 
 {
 }
}
</code></pre>

The extension works fine if called from C#, but has an issue if called from an external VB.Net application:

<pre><code>Dim something = Me.SomethingManager.GetSomething(key)
Dim result = something.ExtensionMethod("extra")
</code></pre>

This compiles fine but throws an exception at run time:

<blockquote>
 Public member 'ExtensionMethod' on type 'SomethingB' not found.
</blockquote>

If the VB.Net is changed to explicitly make the type the interface it works:

<pre><code>Dim something as ISomething = Me.SomethingManager.GetSomething(key)
Dim result = something.ExtensionMethod("extra")
</code></pre>

Why? Why does the extension method work on the interface but not the class that implements it? Would I have the same issue if I used a subclass? Is VB.Net's implementation of extension methods incomplete?

Is there anything I can do in the C# library to make VB.Net work without the explicit interface?

Why can't VB.Net find an extension method on an interface?

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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我有一个C#库，它有一个扩展方法，类似于：public interface ISomething { ... }public class SomethingA : ISomething { ... }public class SomethingB : ISomething { ... }public static cl...

问为什么VB.Net不能在接口上找到扩展方法？
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问为什么VB.Net不能在接口上找到扩展方法？EN