>>
>>Interesting. TBH, I too expected the 'as' version to be quicker because of repeated cast in the 'is' version - but a *really* clever optimising compiler might have factored that out?
>
>I don't think an optimizing compiler can do that. It can take the conversion and test outside the loop of course - I have turned optimization off for the test - obviously

>>Anyway, now I'm wondering how the IL offset values relate to execution time - given the total offset for 'is' is 29 and 'as' is 23 this obviously doesn't reflect the true cost when translated to machine code. I wonder how results might vary with different processors ?
>
>The IL offsets do not (or hardly) infuence execution time


>As I said - the AS construct needs only one lookup in the type table(s) whereas AS needs two (IsInst + Cast)
>
>One ild instruction may result in many assembler instructions - eg callVirt, IsInst(ance), cast, ...
>
>
>In optimizing there's something like elimination of common subexpressions - but I don't think that applies here
>There's also dead code elimination - but not applicable here either

>I do not think the optimizer can be that clever - but I may be wrong.

I was working on the assumption that if you could see that a 'as' solution would work safely (and more efficiently) then there is no reason, in theory, why a compiler could not make the same decision :-}

>All in all it's not the ILDasm that is optimized but a pseudo asm, that is later translated into target asm

>
> For more info - it doesn't digest easily - but it helps to sleep
>
>[amazon uk does not have it in stock - but you can find a TableOfContents on amazon.co.uk]
>http://www.amazon.com/Compilers-Principles-Techniques-Tools-2nd/dp/0321486811/ref=sr_1_1?ie=UTF8&s=books&qid=1263979926&sr=8-1

Not sure I want to go there :-}
In the distant past I actually helped write a compiler (targeting a Z80 so that should give a clue as to how long ago it was). In those days the concept was manageable - these days I wouldn't have a clue :-{

This might be interesting tho : http://www.amazon.com/Inside-Microsoft-Net-Il-Assembler/dp/0735615470