为什么不能VB.Net找到一个接口扩展方法？ [英] Why can't VB.Net find an extension method on an interface?

问题描述

我有一个C＃库，有一个扩展方法，是这样的：

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

public interface ISomething { ... }
public class SomethingA : ISomething { ... }
public class SomethingB : ISomething { ... }

public static class SomethingExtensions 
{
    public static int ExtensionMethod(this ISomething input, string extra) 
    {
    }
}

扩展工作正常，如果所谓的从C＃，但如果所谓的从外部VB.Net应用程序的问题：

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

Dim something = Me.SomethingManager.GetSomething(key)
Dim result = something.ExtensionMethod("extra")

这编译正常，但在运行时会抛出一个异常：

This compiles fine but throws an exception at run time:

公共件上键入ExtensionMethod'SomethingB'未找到。

Public member 'ExtensionMethod' on type 'SomethingB' not found.

如果该VB.Net改变，显式地类型它的工作界面：

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

Dim something as ISomething = Me.SomethingManager.GetSomething(key)
Dim result = something.ExtensionMethod("extra")

为什么呢？为什么界面上的扩展方法的工作，但没有实现它的类？我也有同样的问题，如果我用一个子类？是VB.Net的实施扩展方法不完全？

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?

有什么我可以在C＃库做，使VB.Net在工作中没有显式接口？

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

推荐答案

通过选项推断关，这code ...

With Option Infer Off, this code...

Dim something = Me.SomethingManager.GetSomething(key)
Dim result = something.ExtensionMethod("extra")

...是相同的...

...is the same as...

Dim something As Object = Me.SomethingManager.GetSomething(key)
Dim result As Object = something.ExtensionMethod("extra")

由于的东西的类型是对象，它找不到扩展方法，因为它不是在类型定义的对象。

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

现在，如果你设置选项推断在，你会得到相同的结果与C＃的 VAR 关键字。类型将自动推断。需要注意的是，这也可能会破坏现有code，但它可以为特定的文件中启用，如选项严格。

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

最好的做法是同时设置选项严格和选项推断来开。

The best practice would be to set both Option Strict and Option Infer to On.

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