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.NET Forum / .NET Framework / Performance / July 2005accessing complicated ArrayList members
hi, i'm using ArrayList very often - it's so easy to use and flexible. unfortuntely i run in some trouble with it. generaly speaking this collection is in fact read only when it comes down to storing something complicated - like object or structure. in general many people say that when you change object or structure stored in arraylist then this object (or structure) should be created as new one and put into the arraylist instead of modyfinig existing data. but what about perfomance? ie. i need to store structure that contains bitmap amog some simple values. so it takes some memory. so when i would like to change one field of this structure i am forced to create a new one, with all the data from old one and put it again inside arraylist. as long as this bitmap has few kilobytes it's not such a big deal, but what if the bitmap has 50kb or 500kb? is there a way to do it in efficient way? can i somehow modyfy just one field from my structure stored inside arraylist? generaly speaking this > collection is in fact read only when it comes down to storing something > complicated - like object or structure. where did you get that from? . so it takes some memory. so when i would > like to change one field of this structure i am forced to create a new > one, with all the data from old one and put it again inside arraylist. as > long as this bitmap has few kilobytes it's not such a big deal, Are you writing some sort of loop that iterates the array collection and you need to modify these items? If this is the case, you need to post that part of the code, because from your description, you code is suspect.  SignatureRegards, Alvin Bruney - ASP.NET MVP [Shameless Author Plug] The Microsoft Office Web Components Black Book with .NET Now available @ www.lulu.com/owc, Amazon.com etc > hi, > [quoted text clipped - 15 lines] > is there a way to do it in efficient way? can i somehow modyfy just one > field from my structure stored inside arraylist? >>collection is in fact read only when it comes down to storing something >>complicated - like object or structure. > where did you get that from? well in fact it's just a loose interpretation of some conventions that i red about (on this newsgroup amog others). the problem is that, when i need to get something out of this list i need to unbox it first. after that i get copy of oryginal data, so infact modifying it wont affect data stored inside arraylist. right? >>like to change one field of this structure i am forced to create a new >>one, with all the data from old one and put it again inside arraylist. as >>long as this bitmap has few kilobytes it's not such a big deal, > Are you writing some sort of loop that iterates the array collection and you > need to modify these items? If this is the case, you need to post that part > of the code, because from your description, you code is suspect. yes it is done inside the loop. it looks more or less like this: for (i=0;i<this.imagesListA.Count;i++) { tempImgElement = (ImageElement) this.imagesListA[i]; [...some code...] //img is 'some' rectangle //update ImageElement with new location this.imagesListA[i] = new ImageElement(tempImgElement.Bitmap, tempImgElement.Valid, tempImgElement.FileName, new Rectangle(img.X+this.AutoScrollPosition.X, img.Y+this.AutoScrollPosition.Y, img.Width,img.Height), tempImgElement.State); [...rest of the code...] } in this case i only want to update one field of a structure that looks like this: public struct ImageElement { private Bitmap bitmap; private bool valid; private string fileName; private int height; private int width; private Rectangle imageRect; private ImageState state; [...all remaining stuff...] } Hi, Have a look at this article http://www.c-sharpcorner.com/Code/2003/July/BoxNUnboxSSF.asp If you use Whidbey and framework 2.0, consider also using generics and/or reference types (classes) instead of structures. Peter well generics are realy cool but i'm forced to use 1.1 this article is indead interesting but it shows what i already know. the point is how to minimalize boxing and unbxing problems when dealing with arraylist and complicated data that is stored inside. ie. is there a way to avoid creation of new object/structure when i need to modify only part of it? it would be nice to have something that will work like this: ((myStruct) ArrayList[index]).Field = "new_data"; instead of ArrayList[index] = new myStruct("new_data","old_data2","old_data3"...); > Hi, > Have a look at this article [quoted text clipped - 5 lines] > > Peter > well generics are realy cool but i'm forced to use 1.1 > this article is indead interesting but it shows what i already know. the [quoted text clipped - 6 lines] > instead of > ArrayList[index] = new myStruct("new_data","old_data2","old_data3"...); The best way of doing that is to use a class instead. Your structure looks like it could really do with being a class anyway, to be honest. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet If replying to the group, please do not mail me too > The best way of doing that is to use a class instead. > > Your structure looks like it could really do with being a class anyway, > to be honest. well, infact it was supposed to be very simple struct... but then it grew up alittle :) but anyway is there any difference? when i put something inside araylist it is stored as a "general" object. even if i turn structure into my own class it wont change anythig because i'll still have to handle it the same way when dealing with arraylist. and i don't want to write my own arraylistOfMyOwnType. > > The best way of doing that is to use a class instead. > > [quoted text clipped - 4 lines] > > but anyway is there any difference? Absolutely! Structures are value types, classes are reference types. > when i put something inside araylist it is stored as a "general" object. ... which means that if you're adding a value type, you end up boxing it, and then unboxing it when you cast it after fetching it from the array list. > even if i turn structure into my own > class it wont change anythig because i'll still have to handle it the > same way when dealing with arraylist. and i don't want to write my own > arraylistOfMyOwnType. No, you won't have to handle it the same way. You'll be able to do: ((MyType) ArrayList[index]).Field = "new_data"; And you'll be able to do that precisely because there won't be any boxing/unboxing involved. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet If replying to the group, please do not mail me too > No, you won't have to handle it the same way. You'll be able to do: > > ((MyType) ArrayList[index]).Field = "new_data"; > > And you'll be able to do that precisely because there won't be any > boxing/unboxing involved. oh... thanks! it looks like a'll have to change only keyword "struct" into "class" and all will be fine :) sometimes i think of very simple ways of doing something, then i add lots of aditional functionality and i end up with something that work like it should but gets too complicated. it's just because some sollutions aren't scalable... thanks! ok. that helped alot. i tested class instead of structure and all works great now. thanks. He shouldn't have to bend over backwards to implement this functionality or change structure definitions. Code like this should work foreach(ImageList imList in new Array(ImageListA)) { //change items without exception being thrown imList.changewhateverfieldyouwanthere = "new value"; //rest of code } roughly SignatureRegards, Alvin Bruney - ASP.NET MVP [Shameless Author Plug] The Microsoft Office Web Components Black Book with .NET Now available @ www.lulu.com/owc, Amazon.com etc >> well generics are realy cool but i'm forced to use 1.1 >> this article is indead interesting but it shows what i already know. the [quoted text clipped - 11 lines] > Your structure looks like it could really do with being a class anyway, > to be honest. > He shouldn't have to bend over backwards to implement this functionality or > change structure definitions. But it almost certainly shouldn't be a structure definition in the first place. I can't see that the OP wants value semantics in the first place. The whole problem is because he's got value semantics when he wants reference semantics. > Code like this should work > foreach(ImageList imList in new Array(ImageListA)) [quoted text clipped - 4 lines] > } > roughly What exactly do you mean by "should" here? It cerainly *won't* work if ImageList is a value type. (ImageList isn't actually involved in this thread as far as I can see.) foreach iteration variables are always read-only, and even if they weren't, the value in the ArrayList wouldn't be changed, because it would have to be an unboxed version which would be changed. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet If replying to the group, please do not mail me too Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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