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Does anyone know the real difference between the two ways of type casting in Flex 3?

var myObject1:MyObject = variable as MyObject;
var myObject2:MyObject = MyObject(variable);

I prefer to use the second method because it will throw an Error when type cast fails, whereas the first method will just return null. But are there any other differences? Perhaps any advantages to using the first method?

ketan
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Maurits de Boer
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4 Answers4

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The second type of casting has different behaviour for top level(http://livedocs.adobe.com/flex/2/langref/) types, e.g. Array(obj) does not cast in the straightforward way you describe; it creates a new Array if possible from obj, even if obj is an Array.

I'm sure the times this would cause unexpected behaviour would be rare but I always use "as" for this reason. It means if I do 

int(str)

I know it's a cast in the "attempt to convert" sense of the word not in the "I promise it is" sense.

ref: got some confirmation on this from http://raghuonflex.wordpress.com/2007/07/27/casting-vs-the-as-operator/

susichan
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  • The as method returns null if cast fails.
  • The () method throws and error if the cast fails.

If the value of variable is not compatible with MyObject, myObject1 will contain null and you will be surprised by a null pointer error (1009 : cannot access a property or method of a null object reference.) somewhere later in the program when you try to access it. Where as if you are casting using the MyObject(variable) syntax, you will get a type coercion error (1034 : Type Coercion failed: cannot convert _ to _) at the same line itself - which is more helpful than getting a 1009 somewhere later and wondering what went wrong.

Amarghosh
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  • Yeah, that's what I said in the question. I was wondering if there are any other differences, because I don't really know what the advantage of the 'as' method would be... – Maurits de Boer Nov 26 '09 at 12:29
  • no advantages that I can think of - I always use the second one. – Amarghosh Nov 26 '09 at 12:32
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    There are some side effects: Array(obj) does not cast in the straightforward way you describe; it creates a new Array if possible from obj, even if obj is an Array. I'm sure the times this would cause unexpected behaviour would be rare but I always use "as" for this reason. It means if I do int(str) I know it's a cast in the "rebox" sense of the word not in the "I promise it is" sense. – susichan Nov 26 '09 at 18:56
  • +1 for arrays. You should post it as an answer. That is exactly what OP was asking for - the advantages of `as` casting. – Amarghosh Nov 27 '09 at 04:17
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I think I read somewhere on this site that as is slighty faster than (), but I can't find the question again.

Beside that this question have been asked many times, you will find an more in-depth answer here.

I recently discovered the very useful [] tag when searching on StackOverflow, it allows to only search in questions tagged with the specified tag(s). So you could do a search like [actionscript-3] as vs cast. There are more search tips here: https://stackoverflow.com/search.

And no; the irony in that I can not find the question about performance and write about how to search is not lost on me ;)

Community
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Jacob Poul Richardt
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I think as returns back the base class and not null when the casting fails and () throws an error.

Ashmeet
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