I am bit confused regarding `as` and `as?`.. In t...

# getting-started

[poohbar]

I am bit confused regarding as and as?..

In this example ref as? String the result is null if the ref is not a String or if the ref is null, correct?

I need to cast a reference to String and throw if it is not a String. How can I do that?

[diesieben07]

as casts and throws if impossible, so in your case you want ref as String.

[diesieben07]

as? tries to cast and returns null otherwise, so ref as? String will be null if ref wasn't a String and ref if it was.

[diesieben07]

Whether or not ref was null is only remotely relevant in that null is not a String, so as would throw and as? would give you null (because the cast failed)

[poohbar]

wait so if I ref is null and I do ref as String it succeeds?

[poohbar]

shouldnt it be ref as String??

[diesieben07]

No. It does not succeed, it throws.

[diesieben07]

Because null is not a String

[poohbar]

what if I want to throw a different exception than ClassCast ?

[poohbar]

replace it with if (ref is xyz) ?

[diesieben07]

That, or you can do ref as? String ?: throw FooBarException()

[poohbar]

but my ref is nullable!

[poohbar]

i feel like i am going in circles here 😄

[diesieben07]

If you cast to String it no longer is.

[poohbar]

but i want to allow it to be null

[diesieben07]

Yes, then you need if (ref is String?)

[poohbar]

ok got it

[poohbar]

thanks Take

[diesieben07]

(If you want a custom exception)

[diesieben07]

Ignore that.