delegates

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DiamondDavo - 12 Apr 2007 06:44 GMT

Hi there

I have a delegate that needs to have only 1 function to call. There is a
possibility that the function will be added to the delegate a second time if
the button is clicked twice. Is it "legal" to assign a delegate null before
adding a function to the delegate. eg
del = null;
del += new delfunc();

Thanks
Dave

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Jon Skeet [C# MVP] - 12 Apr 2007 07:57 GMT

> I have a delegate that needs to have only 1 function to call. There is a
> possibility that the function will be added to the delegate a second time if
> the button is clicked twice. Is it "legal" to assign a delegate null before
> adding a function to the delegate. eg
> del = null;
> del += new delfunc();

No need to do that - just do:

del = new delfunc();

However, neither of these will work if instead of a delegate variable,
you're actually dealing with an event, which only has
subscribe/unsubscribe behaviour.

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DiamondDavo - 13 Apr 2007 06:50 GMT

> > I have a delegate that needs to have only 1 function to call. There is a
> > possibility that the function will be added to the delegate a second time if
[quoted text clipped - 10 lines]
> you're actually dealing with an event, which only has
> subscribe/unsubscribe behaviour.

Hi Jon

Thanks for that. It is actually an event. I tried "del = new delfunc();" but
the compiler barfs with only += and -= are valid. Is there a way I can make
sure it is only assigned once?

Thanks
Dave

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Jon Skeet [C# MVP] - 13 Apr 2007 07:31 GMT

<snip>

> Thanks for that. It is actually an event. I tried "del = new
> delfunc();" but the compiler barfs with only += and -= are valid. Is
> there a way I can make sure it is only assigned once?

The best way is to design the problem away - change your design to make
sure that it's only going to be added once in the first place.

Alternatively, keep a flag to say whether or not you've already added
the delegate in question.

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Jon Skeet - <skeet@pobox.com>
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Tim Van Wassenhove - 13 Apr 2007 17:09 GMT

DiamondDavo schreef:

> Hi there
>
> I have a delegate that needs to have only 1 function to call. There is a
> possibility that the function will be added to the delegate a second time if
> the button is clicked twice.

make the event private, and implement accessor methods to do the
verifications...

private event EventHandler bar;

public event EventHandler Bar
{
add
{
// only subscribe if there aren't other subscribers...
if (this.bar.GetInvocationList().Length == 0)
{
this.bar += value;
}
}

remove { this.bar -= value; }
}

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Tim Van Wassenhove <url:http://www.timvw.be/>

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Tim Van Wassenhove - 13 Apr 2007 17:10 GMT

DiamondDavo schreef:

> Hi there
>
[quoted text clipped - 4 lines]
> del = null;
> del += new delfunc();

Wrap the event in a property and implement the add method so that it
verifies

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Tim Van Wassenhove <url:http://www.timvw.be/>

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DiamondDavo - 24 Apr 2007 22:24 GMT

> DiamondDavo schreef:
> > Hi there
[quoted text clipped - 8 lines]
> Wrap the event in a property and implement the add method so that it
> verifies