我的 Declare 声明出了什么问题？
我最近修复了 VB6 应用程序中的一个错误，但我不确定到底出了什么问题。
有问题的部分是 CreateEvent 的错误 API 声明。这是 API Viewer 生成的内容：
Declare Function CreateEvent Lib "kernel32" Alias "CreateEventA"
(lpEventAttributes As SECURITY_ATTRIBUTES, ...) As Long
下一个是错误声明，显然有人不想导入 SECURITY_ATTRIBUTES 结构...
Declare Function CreateEvent Lib "kernel32" Alias "CreateEventA"
(lpEventAttributes As Any, ...) As Long
调用是：
Event = CreateEvent(Nothing, 0, 0, "MyEventName")
此调用在 IDE 中总是，但在编译后的 .exe 中从不。 （CreateEvent 始终返回 0）
我将声明更改为：
Declare Function CreateEvent Lib "kernel32" Alias "CreateEventA"
(ByVal lpEventAttributes As Any, ...) As Long
...并且它起作用了。
现在我有点困惑：
- 为什么使用
SECURITY_ATTRIBUTES时参数是ByRef，而使用AnyByVal代码>? - 为什么错误的声明总是在 IDE 中起作用？
I've recently fixed a bug in a VB6 application, but I'm not sure, what exactly went wrong.
The offending part was a wrong API declaration of CreateEvent. This is, what API Viewer generated:
Declare Function CreateEvent Lib "kernel32" Alias "CreateEventA"
(lpEventAttributes As SECURITY_ATTRIBUTES, ...) As Long
The next one is the wrong declare, obviously someone didn't want to import the SECURITY_ATTRIBUTES structure...
Declare Function CreateEvent Lib "kernel32" Alias "CreateEventA"
(lpEventAttributes As Any, ...) As Long
The call was:
Event = CreateEvent(Nothing, 0, 0, "MyEventName")
This call worked always in the IDE, but never from the compiled .exe. (CreateEvent always returned 0)
I changed the declaration to:
Declare Function CreateEvent Lib "kernel32" Alias "CreateEventA"
(ByVal lpEventAttributes As Any, ...) As Long
... and it worked.
Now I'm a little bit puzzled:
- Why is the parameter
ByRefwhen usingSECURITY_ATTRIBUTESbut must beByValwhen usingAny? - Why did the wrong declare always work in the IDE?
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如果您使用不合格的 As Any 参数，则必须在调用中明确显示。这应该可以解决问题：
我不明白为什么你会在这里使用
Nothing，因为那是一个对象引用，并且调用需要一个指针。ByVal 0&的作用是传递一个空指针——因为它是空的，所以它指向（不）指向什么并不重要。但是传递Nothing ByVal可能会强制ByVal 0&，这就是它起作用的原因。至于为什么它在 IDE 中有效，嗯，IDE 确实对此类事情更加宽容。
If you use an unqualified As Any parameter, you have to be explicit in the Call. This should have fixed the problem:
I can't see why you'd use
Nothinghere, since that's an Object reference and the call is expecting a pointer. WhatByVal 0&does is pass a null pointer -- since it's null it doesn't matter what it's (not) pointing to. But passingNothing ByValprobably forcesByVal 0&, which is why it worked.As to why it worked in the IDE, well, the IDE does tend to be more forgiving about things like this.