swift - why can't I use as. it offers me to use as! but still the app doesn't run -

i using swift 1.2 in xcode 6.4
casting types using "as" in code below:

let playsoundvc:playsoundviewcontroller = segue.destenationviewcontroller playsoundviewcontroller   let data = sender = recordaudio   

but giving me error. says have use "as!" instead of "as" in both lines "force downcast" it. however, when fix , using "as!" app doesn't run. have read made update "as" "as!" now. why app still doesn't run ?

if want downcast (sender has superclass of recordaudio):

let data = sender as! recordaudio // make explicit 

if want upcast (sender has subclass of recordaudio):

let data = sender recordaudio 

if want cast type:

let data = recordaudio(sender) 

but have have correct initializer recordaudio class.

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in swift syntax has changed as swift 1.2

as! failable casts — casts can fail @ runtime expressed new as! operator make potential runtime failure clear readers , maintainers of code. (link)

since as downcast , destinationviewcontroller not only superclass of playsoundviewcontroller. there can more 1 subclass of destinationviewcontroller.

so can crash. therefore have add ! make explicit know doing.

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if not run, maybe destinationviewcontroller not superclass of playsoundviewcontroller.

does playsoundviewcontroller inherit type destinationviewcontroller has?