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.NET Forum / Languages / C# / September 2007Removing events more than once
Hi there, I've tried hunting for this in the C# specs but can't find the relevant info. Does anyone know what the official rule says (or doesn't say) about removing an event more than once. If you apply "+=" to assign an event for instance, can you safely apply "-=" more than once or is this implementation dependent (and possibly dangerous). Thanks very much Hi, > Hi there, > [quoted text clipped - 3 lines] > instance, can you safely apply "-=" more than once or is this > implementation dependent (and possibly dangerous). Thanks very much I haven't read the documentation, but if they used the same pattern than for the rest of the list I can tell you that it does not matter, that it will removed if found and nothing happens if it does not. Hello Ignacio Machin ( .NET/ C# MVP )" machin TA laceupsolutions.com, Yep, exactly. C# Spec. 15.4 Attempting to remove a delegate from an empty list (or to remove a non-existent delegate from a non-empty list) is not an error --- WBR, Michael Nemtsev [.NET/C# MVP] :: blog: http://spaces.live.com/laflour "The greatest danger for most of us is not that our aim is too high and we miss it, but that it is too low and we reach it" (c) Michelangelo I> Hi, I> I> "MikeP" <no_spam@_nospam.com> wrote in message I> news:emLS7CF$HHA.3800@TK2MSFTNGP03.phx.gbl... I> >> Hi there, >> [quoted text clipped - 4 lines] >> than once or is this implementation dependent (and possibly >> dangerous). Thanks very much I> I haven't read the documentation, but if they used the same pattern I> than for the rest of the list I can tell you that it does not matter, I> that it will removed if found and nothing happens if it does not. I> Hello Ignacio Machin ( .NET/ C# MVP )" machin TA laceupsolutions.com, Yep, exactly. C# Spec. 15.4 Attempting to remove a delegate from an empty list (or to remove a non-existent delegate from a non-empty list) is not an error --- WBR, Michael Nemtsev [.NET/C# MVP] :: blog: http://spaces.live.com/laflour "The greatest danger for most of us is not that our aim is too high and we miss it, but that it is too low and we reach it" (c) Michelangelo I> Hi, I> I> "MikeP" <no_spam@_nospam.com> wrote in message I> news:emLS7CF$HHA.3800@TK2MSFTNGP03.phx.gbl... I> >> Hi there, >> [quoted text clipped - 4 lines] >> than once or is this implementation dependent (and possibly >> dangerous). Thanks very much I> I haven't read the documentation, but if they used the same pattern I> than for the rest of the list I can tell you that it does not matter, I> that it will removed if found and nothing happens if it does not. I> > Hello Ignacio Machin ( .NET/ C# MVP )" machin TA laceupsolutions.com, > [quoted text clipped - 3 lines] > Attempting to remove a delegate from an empty list (or to remove a > non-existent delegate from a non-empty list) is not an error Thanks very much to you and the others. It's strange however that the section number you referred to (15.4) is actually 15.3 in the online version of MSDN I'm now looking at. These section numbers don't correspond at all however to the official downloaded spec from Sept/2006 but I think there may be two versions floating around (for the ECMA and ISO perhaps). It's in section 22.3 of the latter document anyway (on page 367 or 383 if you're looking at the Adobe page number). The quote there however is: "Attempting to subtract a delegate from null (or to subtract a non-existent delegate from a non-empty list) is not an error". In any case, I was really asking because sometimes you may want to assign and remove an event in the course of processing a given function. If you remove it in a "finally" clause however, and you jump through that clause because of an exception, it's possible that you'll be removing the event even though it was never assigned (because the exception occurred before that took place). While you could organize things so this doesn't happen, it's just not convenient sometimes, in particular when you're cleaning up different resources (including other events possibly - the code can start to become unwieldy if you introduce multiple "finally" and/or "catch" blocks if you can get away with just one). Anyway, thanks again. > [...] > In any case, I was really asking because sometimes you may want to assign > and remove an event in the course of processing a given function. I agree that there are a number of situations in which removing without knowing whether you've added would be a reasonable and useful design. The one caveat I would stipulate is to take note that you can add the same handler more than once. I haven't personally seen a situation in which this is necessarily a good design, but I'm not willing to say that it could never happen. In that case, removing a handler without knowing specifically that it's been added could result in an incorrect number of subscriptions by that handler to the event. Pete >> In any case, I was really asking because sometimes you may want to assign >> and remove an event in the course of processing a given function. [quoted text clipped - 8 lines] > specifically that it's been added could result in an incorrect number of > subscriptions by that handler to the event. Yes, I'm aware of that but it's not an issue in my case. Actually, this is where I miss C++ (with no intention of starting a religious war). You can simply rely on RAII to clean things up. Mike, As Ignacio says, the behavior is that nothing will happen. Furthermore, looking through the C# spec, I would say it is undefined what the behavior is. However, I would strongly recommend that you keep track of this, as you should know when event handlers need to be added and removed. I would almost classify it as a design bug when you don't (the same way I would classify not knowing when your classes that implement IDisposable need to be disposed of).  Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Hi there, > [quoted text clipped - 3 lines] > instance, can you safely apply "-=" more than once or is this > implementation dependent (and possibly dangerous). Thanks very much Bleh, I didn't see section 15.4 which Michael pointed out. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Mike, > [quoted text clipped - 16 lines] >> for instance, can you safely apply "-=" more than once or is this >> implementation dependent (and possibly dangerous). Thanks very much if u are removing events more than once it will not show any compiletime/runtime error it will work properly . but variable will refenence to null. show before calling uneed to check that event varaible is not null others it will through exceptiin object refence not set event delegatevaraible e; e+= new delegatevaraible (functionname); e-= new delegatevaraible (functionname); e-= new delegatevaraible (functionname); // till now it willwork // now u need to check if(e!=null) { e.invoke(paramete) } > Hi there, > [quoted text clipped - 3 lines] > instance, can you safely apply "-=" more than once or is this implementation > dependent (and possibly dangerous). 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