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.NET Forum / Languages / C# / February 2008const and readonly
Hello!! Assume I have this class declaration of MyTest below. I just want to check with you if the two declarations for tal1 and tal2 is almost the same. The only difference is that const is set at compile but readonly can be set at compile time or run time appart from that they are the same. Both is actually a class field because const is always automatically static so for me these two decalations is almost the same appart from my comment above. Do you agree? class MyTest { public static readonly int tal1 = 1; public const int tal2 = 1; ... } //Tony > Assume I have this class declaration of MyTest below. > I just want to check with you if the two declarations for tal1 and tal2 is [quoted text clipped - 13 lines] > ... > } If the constants are only used within the same assembly, that's all correct (const is limited to a select set of types, too). Things are messier when you refer to the constant from a different assembly. Whenever you refer to a const (whether in the same assembly or not) it bakes the value of the const into the *referring* code - instead of actually looking up the value of the field at execution time. That has a significant effect if you build different assemblies at different times. Suppose you have Const.dll and PrintConst.exe which do the obvious things. If Const.dll has a const with the value "First" when you compile PrintConst.exe against it, then you change the const in Const.dll to "Second" and *just* rebuild Const.dll, running PrintConst.exe will still show "First" because the value has been compiled into PrintConst.exe. Does that make sense?  SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk Hello!! I understand but the solution for how .NET framework has solved it when dealing with const between assemblies was not the best choice. //Tony > > Assume I have this class declaration of MyTest below. > > I just want to check with you if the two declarations for tal1 and tal2 is [quoted text clipped - 32 lines] > > Does that make sense? > I understand but the solution for how .NET framework has solved it when > dealing with const between assemblies was not the best choice. Well, it's useful occasionally - and easy to avoid, just by using static readonly instead. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk >> I understand but the solution for how .NET framework has solved it when >> dealing with const between assemblies was not the best choice. > > Well, it's useful occasionally - and easy to avoid, just by using > static readonly instead. Not to split hairs or anything, but if it is constant, why is it changing? :) SignatureLasse Vågsæther Karlsen mailto:lasse@vkarlsen.no http://presentationmode.blogspot.com/ PGP KeyID: 0xBCDEA2E3 > > Well, it's useful occasionally - and easy to avoid, just by using > > static readonly instead. > > Not to split hairs or anything, but if it is constant, why is it > changing? :) Because someone *thought* it was a constant with one value, but that turned out to be wrong. Or because someone used "const" without really thinking about whether or not it was genuinely a constant. I'm not saying it should happen, just that it does and it's worth being aware of the consequences :) SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk 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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