c# Casting v As operator

Ennis Ray Lynch Jr

As is for poor programmers. If you know that e always is SomeObject then e should be typed as such using some other method. Otherwise you always have to check the result of the as operation. So then you have the following two scenarios which must always be in case A or case B:

if(e is SomeObject){
SomeObject someObject = (SomeObject)e;
}

or

SomeObject someobject = e as SomeObject;
if(someObject != null){

}

But then in case by I always here the response ... but I know e is always SomeObject. Really then maybe it should be defined as such. The AS operator is designed solely to support developers that don't have a fundamental concept of type. After all, for all of the time I have seen the is operator used with a subsequent cast, checking for null after the AS is a white rhinoceros.

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peterchen

The first when I need the exception, the second when I need the null. Because face it, e is never what you need it to be.

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Pete OHanlon

Thanks Ennis - I'm a poor programmer then because I use as, rather than the double cast, which is doing the same work again. What happens internally with as is that it checks to see if the variable is of the type, and if it is it returns a non-null pointer to that type. With the is operator, you check to see if it is of the type and then you cast it - which still determines internally whether or not it belongs to that type (this is how it throws an InvalidTypeException). In any case where you are using code-discovery, such as IoC, then the as call is more efficient. [Edit]I should add that this relates to our plugin code where the client provides their own logic, and "forgets" to implement the appropriate interfaces.[/Edit]

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modified on Thursday, June 23, 2011 9:30 AM

Pete OHanlon

You have better clients than we do then. You tell them, you must implement this interface in order for this to work, and bam they completely fail to implement the interface.

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Lost User

peterchen wrote:

Because face it, e is never what you need it to be.

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Sahir Shah

Somebody didn't read the subject of the post.

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Jim Crafton

You've been reading java code on the throne again haven't you? :)

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NormDroid

:wtf: are you going on about.

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hairy_hats

I seem to have upset some voters along the way. :-D

hairy_hats

Sahir seems to be comparing C# casts with a handler of equine animals.

GenJerDan

Not that there's anything wrong with that.

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NormDroid

:) Indeed, at this rate he's going to hit an exception that he may not like.

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CPallini

I prefer directly throwing. :)

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Sahir Shah

Well in brief, if you are in the shower and drop the soap and there is an operator mentioned in the subject line around, there is a good chance he might be a hominid. I wouldn't advise you to try and pick up the soap.

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CPallini · modified on Thursday, June 23, 2011 9:30 AM

Well a smart language like C# should provide the construct this way:

if (e is MyObject)
{
// here call MyObject methods on e
}

:rolleyes:

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Daniel Grunwald

I use the (cast) only when I know 'e is always SomeObject'. Which is a rare case - think stuff like (ThisClass)base.MemberwiseClone(). Otherwise, I prefer as + null check over is + cast. It looks cleaner to me, and is also more performant. 'a is T' gets compiled to the same IL as '(a as T) != null', so is + cast ends up casting twice (and last time I checked, the JIT was too dumb to optimize that).

Ennis Ray Lynch Jr

Like I said, my problem is that most developers that use AS do not subsequently check for null. The fact that the .NET framework is written stupidly is a completely other rant. (IMHO AS should use IS and not the other way around).

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Daniel Grunwald

Ennis Ray Lynch, Jr. wrote:

AS should use IS and not the other way around

How that? 'a as B' should be compiled to 'a is B ? (B)a : null'? That would be horribly stupid. Consider how this works on the assembly level: 'is' is a function that takes an object and a type token, performs a type test, returns non-zero if successful, zero otherwise. 'as' is a function that takes an object and a type token, performs a type test, returns the object if successful, null otherwise. Given that objects are non-zero by definition, it makes perfect sense to use the same function for both, and that's exactly what .NET does. What's stupid is that the JIT optimizer is so dumb that this leads to a performance difference visible to the programmer. If the JIT could simply optimize away redundant casts, nobody would have to care what as/is/casts compile to.

Ennis Ray Lynch Jr

What do you think AS is doing now?

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Gary Wheeler

As others have probably pointed out (but I'm too lazy to read the rest of the thread), the two versions do different things. 'A' throws an InvalidCastException if the cast isn't possible. 'B' sets obj to null if the cast doesn't work. Both are valid approaches, depending on the rest of your code.

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