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dotnet performance : Object Equality
Currently I am responsible for writing the C# coding standards document for
a client and I have been doing some investigations with ildasm to establish some additional best practices. When using either the C# equality operator '==' or the object.Equals() methods, which is better? Personally I prefer the object.Equals as it prevents accidental assignement using only a single equals, however the C# '==' is certainly less to write (not that that in itself justifies its use). When you compile the code and examine the resultant IL using ildasm you see that the C# '==' operator is not simply resolved to object.Equals ... as such this begs the question, what is the difference between: a == b call bool [mscorlib /* 23000001 */]System.String/* 01000003 */::op_Equality(string, string) /* 0A000002 */ and: string.Equals(a, b); call bool [mscorlib/* 23000001 */]System.String/* 01000003 */::Equals(string, string) /* 0A000003 */ Is there any performance difference between the two equality methods? Or any other reason to prefer one over the other? Thanks in advance Doug Holland
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Doug Holland <doug.holland@precisionobjects.com> wrote: > Is there any performance difference between the two equality methods? Well, they do two very different things. Unless == has been overridden (which I dislike, apart from a few very special and well-known cases), == just checks whether the two values on either side are the same - and for reference types, those values are references. In other words: object x = (...); object y = (...); if (x==y) checks whether or not x and y are equal references, i.e. whether the values of x and y are references to the same object. if (x.Equals(y)) checks whether or not the object x refers to is equal to the object y refers to, where the class can decide what equality means, so that two distinct objects can be equal. [quoted text, click to view] > Or any other reason to prefer one over the other? It depends whether you want to detect reference identity or object equality. -- Jon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet
Thanks Jon
- Doug [quoted text, click to view] "Jon Skeet [C# MVP]" <skeet@pobox.com> wrote in message news:MPG.1a2e64583cd14d5d989b3a@msnews.microsoft.com... > Doug Holland <doug.holland@precisionobjects.com> wrote: > > Is there any performance difference between the two equality methods? > > Well, they do two very different things. Unless == has been overridden > (which I dislike, apart from a few very special and well-known cases), > == just checks whether the two values on either side are the same - and > for reference types, those values are references. In other words: > > object x = (...); > object y = (...); > > if (x==y) > > checks whether or not x and y are equal references, i.e. whether the > values of x and y are references to the same object. > > if (x.Equals(y)) > > checks whether or not the object x refers to is equal to the object y > refers to, where the class can decide what equality means, so that two > distinct objects can be equal. > > > Or any other reason to prefer one over the other? > > It depends whether you want to detect reference identity or object > equality. > > -- > Jon Skeet - <skeet@pobox.com> > http://www.pobox.com/~skeet > If replying to the group, please do not mail me too
Jon Skeet [C# MVP] <skeet@pobox.com> wrote in
news:MPG.1a2e64583cd14d5d989b3a@msnews.microsoft.com: [quoted text, click to view] > It depends whether you want to detect reference identity or object > equality. > Jon, have you thought about writing up a page for your site that goes into these == and .Equals() issues? I refer people to your parameters page all the time, and I think you'd do a great job on this other often- misunderstood facet of the language.
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Mark <mark@--somewhere--.com> wrote: > Jon Skeet [C# MVP] <skeet@pobox.com> wrote in > news:MPG.1a2e64583cd14d5d989b3a@msnews.microsoft.com: > > > It depends whether you want to detect reference identity or object > > equality. > > > > Jon, have you thought about writing up a page for your site that goes into > these == and .Equals() issues? I refer people to your parameters page all > the time, and I think you'd do a great job on this other often- > misunderstood facet of the language. Good idea. I'll also include it in the FAQ that I've been trying to get round to compiling... (Too many pages to write, too little time :) -- Jon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet
Actually Jon, perhaps you should go with the .NET / Java zeitgeist and start
a blog :) I'd be happy to set you up with a .Text based one (though I imagine MS gives it's MVPs free ones?) {*Really* Off topic: As far as exposure, you can't beat it... I only get 30 unique visitors a day, but I'm #6 on Google for "C# .NET blogs" - right behind Don Box at the moment). Richard -- Veuillez m'excuser, mon Français est très pauvre. Cependant, si vous voyez mauvais C #, c'est mon défaut! [quoted text, click to view] "Jon Skeet [C# MVP]" <skeet@pobox.com> wrote in message news:MPG.1a2f434bf8b7f7e4989b47@msnews.microsoft.com... > Mark <mark@--somewhere--.com> wrote: > > Jon Skeet [C# MVP] <skeet@pobox.com> wrote in > > news:MPG.1a2e64583cd14d5d989b3a@msnews.microsoft.com: > > > > > It depends whether you want to detect reference identity or object > > > equality. > > > > > > > Jon, have you thought about writing up a page for your site that goes into > > these == and .Equals() issues? I refer people to your parameters page all > > the time, and I think you'd do a great job on this other often- > > misunderstood facet of the language. > > Good idea. I'll also include it in the FAQ that I've been trying to get > round to compiling... > > (Too many pages to write, too little time :) > > -- > Jon Skeet - <skeet@pobox.com> > http://www.pobox.com/~skeet > If replying to the group, please do not mail me too
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Richard A. Lowe <chadich@yumspamyumYahoo.com> wrote: > Actually Jon, perhaps you should go with the .NET / Java zeitgeist and start > a blog :) I'd be happy to set you up with a .Text based one (though I > imagine MS gives it's MVPs free ones?) Yes, I think so. Must check. [quoted text, click to view] > {*Really* Off topic: As far as exposure, you can't beat it... I only get 30 > unique visitors a day, but I'm #6 on Google for "C# .NET blogs" - right > behind Don Box at the moment). I've thought about it - and I'll think about it some more. Thing is, I'm always "tweaking" my normal pages, and I don't know how easy that is to do with blogs. Maybe I should investigate them further... -- Jon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet
Guess it depends on the the software... .in .Text it's easy to edit posts
but I imaging there's some very mature JSP open-source for blogging as well. I do like that .Text does automatic trackbacks when you link to other blogs posts and esp. like blogs I can subscribe to via RSS. R. -- Veuillez m'excuser, mon Français est très pauvre. Cependant, si vous voyez mauvais C #, c'est mon défaut! [quoted text, click to view] "Jon Skeet [C# MVP]" <skeet@pobox.com> wrote in message news:MPG.1a2fd6111a27ed08989b4c@msnews.microsoft.com... > Richard A. Lowe <chadich@yumspamyumYahoo.com> wrote: > > Actually Jon, perhaps you should go with the .NET / Java zeitgeist and start > > a blog :) I'd be happy to set you up with a .Text based one (though I > > imagine MS gives it's MVPs free ones?) > > Yes, I think so. Must check. > > > {*Really* Off topic: As far as exposure, you can't beat it... I only get 30 > > unique visitors a day, but I'm #6 on Google for "C# .NET blogs" - right > > behind Don Box at the moment). > > I've thought about it - and I'll think about it some more. Thing is, > I'm always "tweaking" my normal pages, and I don't know how easy that > is to do with blogs. Maybe I should investigate them further... > > -- > Jon Skeet - <skeet@pobox.com> > http://www.pobox.com/~skeet > If replying to the group, please do not mail me too
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