On Thu, 30 May 2002 10:30:07 +0100, Ian Griffiths <[EMAIL PROTECTED]> wrote:
>> >> > someControl.Size.Width * 2; >> > >> >> But the whole statement might _not_ be pointless. >> >> One of the main points of a property is that a write >> >> context you don't get an assignment you get a set >> >> accessor. >> > >> >Ah, but this isn't a write context. >> >> It was in my original question. > >Could you provide the original code fragment then? I don't think I provided one. It all stemmed from a paragraph that was in Petzold book (page 105, 1/3 down, not Richters as I said, sorry): "Don't do this however: Size.Width *= 2; That's setting a property of a property. For reasons beyond the comprehension of people who don't write compilers, it's not allowed." It seems a resonable expectation that if you have a control c that contains a Size property you should be able to write c.Size.Width *= 2; but of course you can't. And there is no work around. It's just not legal. I can see the logic behind this, that c.Size is a get and so the .Width set accessor operates on a copy, exactly as you say. What niggles me is that Petzold seemed to imply there was a deeper reason than this. And as you also point out, the .Size get could return a struct (say) that contained a reference back to c and pretended to be a Size property. In other words it's not completely unimaginable that you might want to do some work to allow users to be able to write: c.Size.Width *= 2; But of course you can't because you can't. I chose a poor example in my post. A better example is where there sub poperties are _not_ dependent on each other. Like Width and Height in a Size. [snip] >But the main thing that worries me about this idea is that this just isn't >how value types usually behave. (With the exception of references, which >are of course stored as value types, and do precisely this. But that's all >they do, and what's more you can't do anything *but* dereference them in >C#...) So technically I don't deny that you could make it work, I just >think it will lead to problems. I would tend to go either with making the >property a reference type, or making its properties read only to prevent the >problem from occurring. I largely agree. However, many property types are already defined to be structs. >> Bt be that as it may, my question is still is there a _fundamental_ >> reason that only compiler writers currently know about (as >> implied by Petzold). > >Well you can always do this: > > Size localCopyOfSize = someControl.Size; > localCopyOfSize.Width *= 2; > But this is not enough. You also have to write it back... Size localCopyOfSize = someControl.Size; localCopyOfSize.Width *= 2; someControl.Size = localCopyofSize; as opposed to someControl.Size.Width *= 2; (which still won't work) Cheers Jon Jagger You can read messages from the DOTNET archive, unsubscribe from DOTNET, or subscribe to other DevelopMentor lists at http://discuss.develop.com.
