Downcasting optionals in Swift: as? Type, or as! Type?

Question

Given the following in Swift:

var optionalString: String?
let dict = NSDictionary()

What is the practical difference between the following

Answer 1

Maybe this code example will help someone grok the principle:

var dict = [Int:Any]()
dict[1] = 15

let x = dict[1] as? String
print(x) // nil because dict[1] is an Int

dict[2] = "Yo"

let z = dict[2] as! String?
print(z) // optional("Yo")
let zz = dict[1] as! String // crashes because a forced downcast fails


let m = dict[3] as! String?
print(m) // nil. the forced downcast succeeds, but dict[3] has no value