可以在C ++/CLI中将“事件"与"nullptr"进行比较吗? [英] Can 'Event' be compared with 'nullptr' in C++/CLI...

问题描述

我正在使用转换工具将C#项目移植到C ++/CLI项目.
在C#项目中，有一个代码部分，其中在如果"条件下将事件"与空"进行比较.

Hi,
I am porting a C# project to C++/CLI project by using a conversion tool.
In C# project has a code section in which ‘Event’ is compared with ‘null’ in an ‘if’ condition.

public event RawFileReadHandler EventRead;
if ( this.EventRead != null )
{
  this.EventRead( this, new RawFileReadArgs( total, current++, file, abort ) );
}

在C ++/CLI中将相同的代码段转换为:

Same code section was converted in C++/CLI as:

public:
    event RawFileReadHandler ^EventRead;
if (this->EventRead != nullptr)
{
  this->EventRead(this, gcnew RawFileReadArgs(total, current++, file, abort));
}

但是，转换后的代码中的此"if"语句会产生编译时错误，因为使用要求成员"成为数据成员".
我想这是不允许在如果"条件下将事件"与"nullptr"进行比较的.

But, this ‘if’ statement in the converted code gives a compile time error as, "Usage requires ''member'' to be a data member".
I guess it is not allowing ‘Event’ to be compared in ‘if’ condition with ‘nullptr’.
Is there any other way by which this ‘Event’ comparison with ‘nullptr’ can be written in C++/CLI?

推荐答案

好问题.答案是在这里类似的问题:检查空委托在C ++.NET中 [ ^ ]

—SA

Good question. The answers were given to a similar question here: Check for null delegate in C++.NET[^]

—SA

在C#中，需要检查null以防止出现空异常.在C ++/CLI中，不需要它.只需删除if语句:如果该事件为空，则什么都不会发生.

现在，如果您想检查事件是否为空，那么SA的链接将有所帮助.

-----

SA给出的链接建议使用私有事件，因此您可以使用GetInvocationList方法及其Length属性(如果为0，则该事件没有附加处理程序).

请参阅以下问题:
http://www.codeproject.com/Messages /1886045/Why-does-GetInvocationList-work-in-Csharp-but-not-.aspx [

In C#, the check for null is required to prevent a null exception. In C++/CLI, it is not needed. Just remove the if statement: if the event is empty, then nothing will happen.

Now if you want to check wether the event is empty or not, then SA''s link will help.

-----

The link given by SA suggests to use a private event so you can use GetInvocationList method and its Length property (if it is 0 then no handler is attached to the event).

Refer to this question:
http://www.codeproject.com/Messages/1886045/Why-does-GetInvocationList-work-in-Csharp-but-not-.aspx[^]
Nish answered that using the GetInvocationList works only if you implement the event by yourself.

So I suppose something like that will work:

Declaring the events:

private:
    // This is your private event.
    System::EventHandler^ privateHandler;

public:
    // This is the public event.
    event System::EventHandler^ PublicHandler
    {
        void add(System::EventHandler^ handler )
        {
            //just attach user handler to your private event
            privateHandler += handler;
        }
        void remove( System::EventHandler^ handler )
        {
            //just detach the user handler from your private handler
            privateHandler -= handler;
        }
        void raise(Object^ sender, EventArgs^ e)
        {
            //raise your private event
            privateHandler(s, e);
        }
    }

检查调用列表:

Checking the invocation list:

if (privateHandler->GetInvocationList()->Length == 0)
{
    //there is no handler attached to this event
}
else
{
    // at least one handler is attached to this event
    // call the handlers or do whatever you want...
    privateEvent(...);
}

我希望这能回答您的问题.

I hope this answers your question.

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