为什么不能直接调用扩展方法？
有人可以向我解释为什么下面第三次调用 DoSomething 无效吗？ （错误消息是“名称‘DoSomething’在当前上下文中不存在”）
public class A { }
public class B : A
{
public void WhyNotDirect()
{
var a = new A();
a.DoSomething(); // OK
this.DoSomething(); // OK
DoSomething(); // ?? Why Not
}
}
public static class A_Ext
{
public static void DoSomething(this A a)
{
Console.WriteLine("OK");
}
}
Can someone explain to me why in the following the 3rd invocation of DoSomething is invalid?
( Error message is "The name 'DoSomething' does not exist in the current context" )
public class A { }
public class B : A
{
public void WhyNotDirect()
{
var a = new A();
a.DoSomething(); // OK
this.DoSomething(); // OK
DoSomething(); // ?? Why Not
}
}
public static class A_Ext
{
public static void DoSomething(this A a)
{
Console.WriteLine("OK");
}
}
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扩展方法可以像其他静态方法一样被调用。
将其更改为
A_Ext.DoSomething(this)。如果您问为什么它不在
this上隐式调用，答案是规范就是这样编写的。我认为原因是在没有限定符的情况下调用它会太误导人。Extension methods can be invoked like other static methods.
Change it to
A_Ext.DoSomething(this).If you're asking why it isn't implicitly invoked on
this, the answer is that that's the way the spec was written. I would assume that the reason is that calling it without a qualifier would be too misleading.因为
DoSomething需要一个参数。DoSomething(a)是合法的。编辑
我在这里读到的问题有点错误。
由于您将其称为普通静态方法，而不是扩展方法，因此您需要在类名前加上前缀。
所以
A_Ext.DoSomething(a);会起作用。如果您像普通静态方法一样调用它，则所有相同的规则都适用。
您的第二个变体有效，因为 B 继承 A，因此您最终仍然将其作为扩展方法调用，但第三个变体则不然。
对于上面的第一个版本无法工作感到抱歉。我将保留它以保持评论的相关性。
Because
DoSomethingtakes a parameter.DoSomething(a)would be legal.Edit
I read the question a bit wrong here.
Since your calling it a a normal static method, and not a extension method, you need to prefic with the class name.
So
A_Ext.DoSomething(a);will work.If you call it like a normal static method, all the same rules apply.
Your second variant works because B inhetits A, and therefore you still end up calling it as an extension method, but the third does not.
sorry about the first version above that does not work. I'll leave it to keep the comment relevant.
扩展方法仍然是静态方法，而不是真正的实例调用。为了使其工作，您需要使用实例方法语法的特定上下文（来自 扩展方法（C# 编程指南）)
因此，虽然通常情况下，两种语法都可以工作，但第二种语法没有显式上下文，并且生成的 IL 似乎无法隐式获取上下文。
Extension methods are still static methods, not true instance calls. In order for this to work you would need specific context using instance method syntax (from Extension Methods (C# Programming Guide))
So while normally, both syntaxes would work, the second is without explicit context, and it would seem that the IL generated can't obtain the context implicitly.
DoSomething需要A的实例来执行任何操作，如果没有限定符，编译器将无法查看您需要调用哪个DoSomething。它不知道要签入您的方法的A_Ext，除非您使用this对其进行限定。DoSomethingrequires an instance ofAto do anything, and without a qualifier, the compiler can't see whichDoSomethingyou need to invoke. It doesn't know to check inA_Extfor your method unless you qualify it withthis.