Can 'Event' be compared with 'nullptr' in C++/CLI...

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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 ) );
}

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));
}

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?

Posted 20-Mar-11 23:45pm

Kedar Kumbhar

Updated 20-Mar-11 23:46pm

v2

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Sergey Alexandrovich Kryukov 21-Mar-11 6:07am

Pretty good question, my 5.
--SA

2 solutions

Solution 1

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

—SA

Posted 21-Mar-11 0:06am

Sergey Alexandrovich Kryukov

Comments

Olivier Levrey 21-Mar-11 6:12am

Goog link. A 5. But if the test was there just to prevent a null exception, then all OP needs is to remove it: it is like in C++/CLI the check for null is done internally.

Sergey Alexandrovich Kryukov 21-Mar-11 6:20am

Exactly. After I post it, I was looking for the reference to remember what was answered. The answers were not so good. (My fault, I forgot it and did not check up).
Thank you.
--SA

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Olivier Levrey · Accepted Answer · 2011-03-21T00:11:00

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:

C++

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);
        }
    }

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(...);
}

I hope this answers your question.